Category Archives: Expert Opinions

Opportunities and Obstacles for Life on Proxima b

by Prof. Rory Barnes, University of Washington

The discovery of Proxima b is the biggest exoplanet discovery since the discovery of exoplanets. The planet is not much bigger than Earth and resides in the “habitable zone” of the Sun’s nearest stellar neighbor. This planet may represent humanity’s best chance to search for life among the stars. But is Proxima b habitable? Is it inhabited? These questions are impossible to answer at this time because we know so little about the planet. However, we can extrapolate from the worlds of our Solar System, as well as employ theoretical models of galactic, stellar, and planetary evolution, to piece together realistic scenarios for Proxima b’s history. The possibilities are varied and depend on phenomena usually studied by scientists in fields that are considered distinct, but an integrated perspective — an astrobiological perspective — can provide a realistic assessment of the possibility that life could have arisen and survived on the closest exoplanet.

As an astrobiologist and astronomer at the University of Washington, and a member of NASA’s Virtual Planetary Lab, I have investigated the habitability of planets orbiting red dwarfs for years. My research involves building computer models that simulate how planetary interiors and atmospheres evolve, how stars change with time, and how planetary orbits vary. The discovery of Proxima b has me very excited, but being Earth-sized and in the habitable zone are just the first two requirements for a planet to support life, and the list of requirements is much longer for planets orbiting red dwarfs than for stars like our Sun. If Proxima b is in fact habitable, meaning it possesses liquid water or even inhabited, meaning life is currently present, then it will have traversed a very different evolutionary path than Earth. This difference is frustrating, in that it will make our initial interpretations challenging, but also exciting, as it offers the chance to learn how Earth-sized planets evolve in our universe. Whether Proxima b is a sterile wasteland or teeming with life, we are now embarking on an unprecedented era of discovery, one that may finally provide an answer that age-old question “Are we alone?”.

To evaluate the possibility of life on Proxima b, we must begin with the only habitable planet we know, Earth. Life on Earth has established itself in a stunning diversity of habitats, including acidic hot springs, the deepest reaches of the oceans, microscopic channels in sea ice, and the deepest levels of Earth’s crust.  Regardless of how extreme the environment, all life on Earth requires three basic ingredients: energy, nutrients and liquid water. The first two ingredients are very abundant throughout the universe, as is the water molecule. The limiting factor from an astrophysical perspective is that water must be in its liquid phase. The habitable zone is a map of where liquid water could exist on the surfaces of rocky, Earth-like planets, hence its status as the first requirement for a planet to be habitable. Life also requires sufficient time to originate and evolve, but on Earth it has proven resilient to calamities as trivial as a thunderstorm or as traumatic. The variety and tenacity of Earth-bound life encourages astrobiologists to imagine that life can exist not only on Earth-like exoplanets, but also on strange, exotic worlds.

So what to make of Proxima b? It is at least as massive as Earth, and may be several times more massive. Its “year” is just over 11 days and its orbit may be circular or significantly elongated. Its host star is only 12% as massive as our Sun, 0.1% as bright, and it is known to flare. It may be joined to the stars Alpha Centauri A and B, 15,000 astronomical units (AU) away, by their mutual gravitational attraction. All three stars contain substantially more heavy elements than our Sun, but we know very little of the composition of Proxima b, or how it formed. The new data point toward the presence of a second planet orbiting in the system with a period near 200 days, but its existence cannot be proven at this time. These are the facts we have and from them we must deduce whether Proxima b supports life.

Proxima b was detected via the radial velocity method, which does not provide a direct measurement of the planet’s mass, only a minimum mass. So, the first question we’d like to answer is whether the planet’s mass is low enough to be rocky like Earth.  If the planet is much larger, it may be more like Neptune with a thick gaseous envelope. While we don’t know where the dividing line between rocky and gaseous exoplanets is, models of planet formation and analyses of Kepler planets suggest the transition is between 5 and 10 times the mass of Earth. Only about 5% of allowed orbits place Proxima b’s mass above 5 Earth masses, so it is very likely that this planet is in the rocky range.

The next question to ask is if the planet actually formed with water. Water consists of hydrogen and oxygen, the first and third most common elements in the galaxy, so we should expect it to be everywhere. Close to stars, however, where Proxima b resides, water is heated into its vapor phase while planets are forming, and hence it is difficult for planets to capture it. Planets that form at larger distances can gather more water, so if Proxima b formed farther out and moved to its current orbit later, it is more likely to be water-rich. At this time, we don’t know how the planet formed, but three scenarios seem most probable: 1) the planet formed where it is from mostly local material; 2) the planet formed farther out while the gas and dust disk that birthed the planetary system still existed, and forces from that disk drove the planet in to its present orbit; or 3) the planet formed elsewhere and some sort of system-wide instability rearranged the planets and b ultimately arrived in its current orbit. The first method is how Earth and Venus formed, and so Proxima b may or may not possess significant water if it formed in this way. The second method produces planets that are very water-rich because water is more likely to be in its ice phase farther out in the disk and so the forming planet could easily gather it up. The third method is inconclusive as the planet could have come from an interior orbit and formed without water or farther out and be water-rich. We conclude that it is entirely possible that this planet has water, but we cannot be certain.

Next let us consider the clues from the stars themselves. Computer models of the evolution of our galaxy suggest that stars enriched in heavy elements like Proxima cannot form locally (25,000 light-years from the galactic center) as there just aren’t enough heavy elements available. But closer to the galactic center, where star formation has been more vigorous and transpiring for longer, stars like Proxima are possible. Recent work by Dr. Sarah Loebman and colleagues has found that stars in our local solar neighborhood with compositions like Proxima must have formed at least 10,000 light-years closer to the galactic center. It would seem Proxima Centauri has wandered through our galaxy and this history may have played an important role in the evolution of Proxima b.

Computer models of the evolution of the Milky Way galaxy suggest Proxima Centauri has moved outward at least 10,000 light-years from where it formed, shown by the orange circle. The Sun and Earth probably formed near where they orbit today (blue circle), which is where we find Proxima Centauri, too.
Computer models of the evolution of the Milky Way galaxy suggest Proxima Centauri has moved outward at least 10,000 light-years from where it formed, shown by the orange circle. The Sun and Earth probably formed near where they orbit today (blue circle), which is where we find Proxima Centauri, too.

The orbit of Proxima around Alpha Centauri A and B, assuming they are gravitationally connected, is large compared to other multiple star systems. In fact, it is so large that A and B’s hold on Proxima is weak and the effects of the Milky Way galaxy have shaped Proxima’s orbit significantly. The mass of the Milky Way as a whole causes Proxima’s orbit to vary both in shape and orientation continuously. Proxima is also susceptible to gravitational encounters from passing stars that can change its orbit. Recent simulations by Prof. Nate Kaib have found that these two effects can often lead to close passages between the stars in a multiple star system that disrupt their planetary systems. The disruption is often powerful enough to eject planets from the system and completely rearrange the orbits of the planets that remain. New simulations by Russell Deitrick are revealing that this scenario is a real concern for Proxima, too; there is a significant probability that at some point in the past, Proxima swooped in close enough to Alpha Centauri A and B to cause its planetary system to break apart, hurling Proxima b’s siblings into deep space. If such a disruption occurred, Proxima b may not have formed where we find it today because its orbit would have been affected by this disruption.

Even if Proxima is not currently bound to Alpha Cen A and B, it appears to be travelling with them, and it is very likely the stars formed from the same cloud of dust and gas. If they formed together, they should have similar compositions and nearly identical ages. Connecting their ages is important because it is very difficult to measure the ages of low mass stars like Proxima Centauri. Astronomers can estimate the age of Alpha Cen A via asteroseismology, the study of “starquakes.” Stars bigger than the Sun vibrate with large enough amplitudes that brightness fluctuations can be observed, and careful monitoring of the pulsations can reveal a star’s age. Recent work by Dr. Michaël Bazot has found that Alpha Cen A is between 3.5 and 6 billion years old. This range is larger than we would like, but Proxima is certainly old enough to support life, and Proxima b might even be about the same age as Earth!

Next we turn to clues from the Proxima Centauri planetary system. The vast majority of the energy used by life on Earth comes from our Sun, and small stars like Proxima can produce energy for trillions of years. The host star is almost as small as stars come, so for a planet to receive as much stellar energy as Earth, Proxima b must be about 25 times closer in than Earth is from the sun. This distance is where the habitable zone lies. While Proxima is much dimmer than the Sun, it is still a thermonuclear explosion, and, everything else equal, life seems more likely at larger distances. And indeed the close-in orbit does produce numerous obstacles that life on Earth did not have to overcome. These include a long formation time for the star, short and energetic bursts of energy in UV and X-ray light, strong magnetic fields, larger starspots, larger coronal mass ejections, and gravitational tidal effects that cause rotational properties to change and frictional heating in oceans (if they exist) and the rocky interior.

The history of Proxima’s brightness evolution has been slow and complicated. Stellar evolution models all predict that for the first one billion years Proxima slowly dimmed to its current brightness, which implies that for about the first quarter of a billion years, Proxima b’s surface would have been too hot for Earth-like conditions. As Rodrigo Luger and I recently showed, had our modern Earth been placed in such a situation, it would have become a Venus-like world, in a runaway greenhouse state that can destroy all of the planet’s primordial water. This desiccation can occur because the molecular bonds between hydrogen and oxygen in water can be destroyed in the upper atmosphere by radiation from the star, and hydrogen, being the lightest of the elements, can escape the planet’s gravity.  Without hydrogen, there can be no water, and the planet is not habitable. Escaping or avoiding this early runaway greenhouse is the biggest hurdle for Proxima b’s chances for supporting life.

Figure 2: Proxima Centauri’s habitable zone has moved inward since it formed, which may mean that planet b lost its water shortly after it formed, when the system was 1—10 million years old. The habitable zone, shown in blue, doesn’t arrive at the orbit of planet b until almost 200 million years after it formed. This early brightness may be the biggest obstacle for life to have gained a foothold on Proxima Centauri b.
Figure 2: Proxima Centauri’s habitable zone has moved inward since it formed, which may mean that Proxima b lost its water shortly after it formed, when the system was 1—10 million years old. The habitable zone, shown in blue, doesn’t arrive at the orbit of Proxima b until almost 200 million years after it formed. This early brightness may be the biggest obstacle for life to have gained a foothold on Proxima b.

As the star dims, the water destruction process halts, and so total desiccation is not inevitable. If some water remains, the atmosphere may also contain large quantities of oxygen leftover from the water vapor destruction. While having large amounts of water and oxygen may sound like a good recipe for life, it almost certainly is not. Oxygen is one of the most reactive elements, and its presence in the young atmosphere of Proxima b would likely prevent the development of pre-biotic molecules that require conditions with little oxygen to form. Life on Earth formed when no oxygen was present, and photosynthesis ultimately produced enough oxygen for it to become a major component of our atmosphere. Note that the destruction of only some water leads to the rather surprising possibility that the planet could possess oceans and an oxygen-rich atmosphere, but has been unable to support life!

Another intriguing possibility is that Proxima b started out more like Neptune and the early brightness and flaring eroded away a hydrogen-rich atmosphere to reveal a habitable Proxima below. Such a world was investigated by Rodrigo Luger, myself and others, and was found to be a viable pathway to avoid total desiccation. Essentially the hydrogen atmosphere protects the water. If Proxima b formed with about 0.1-1% of its mass in a hydrogen envelope, the planet would lose the hydrogen but not its water, potentially emerging as a habitable world after the star reached its current brightness.

This wide range of possible evolutionary pathways presents a daunting challenge as we imagine using space- and ground-based telescopes to search for life in the atmosphere of Proxima b. Fortunately my colleagues in the Virtual Planetary Lab, Prof. Victoria Meadows, Giada Arney and Edward Schwieterman, have been developing techniques to distinguish the possible states of Proxima b’s atmosphere, whether habitable or not. Nearly all the components of an atmosphere imprint their presence in a spectrum, so with our knowledge of the possible histories of this planet, we can begin to develop instruments and plan observations that pinpoint the critical differences. For example, at high enough pressures, oxygen molecules can momentarily bind to each other and produce an observable feature in a spectrum. Crucially, the pressures required to be detectable are large enough to discriminate between a planet with too much oxygen, and one with just the right amount for life. As we learn more about the planet and the system, we can build a library of possible spectra from which to quantitatively determine how likely it is that life exists on Proxima b.

While the early brightness of the host star is the biggest impediment to life, other issues are also important. One of the original concerns for the habitability of planets orbiting red dwarfs was that they would become “tidally locked”, meaning that one hemisphere permanently faces the host star. This state is similar to the rotation of our Moon, in which the same tidal forces that raise waves in our ocean have caused the Moon to show only one face to Earth. Because it is so close to its star, Proxima b may be in this state, depending on the shape of its orbit. For decades, astronomers were concerned that such a tidally locked planet would be uninhabitable because they believed the atmosphere would freeze and collapse to the surface on the permanently dark side. That possibility is now viewed as very unlikely because winds in the atmosphere will transport energy around the planet and maintain sufficient warmth on the backside to prevent this freeze out. Thus, as far as atmospheric stability is concerned, tidal locking is not a concern for this planet’s potential habitability.

Although tidal locking is not very dangerous for life, it is possible for tides to provide large amounts of energy to the planet’s atmosphere and interior. This energy is often called “tidal heating” and is a result of the deformation of the planet due to changes in the host star’s gravitational force across the planet’s diameter. For example, if the planet is on an elliptical orbit, when it is closer to the star, it feels stronger gravity than when it is farther away. This variation will cause the shape of the planet to change, and this deformation can cause friction between layers in the planet’s interior, producing heat. In extreme cases, tidal heating could trigger the onset of a runaway greenhouse like the one that desiccated Venus, independent of starlight. Proxima b is not likely to be in that state, but the tidal heating could still be very strong, causing continual volcanic eruptions as on Jupiter’s moon Io, and/or raising enormous ocean waves. Based on the information we have now, we don’t know the magnitude of tidal heating, but we must be aware of it and explore its implications.

The host star’s short, high energy bursts, called flares, are also a well known concern for surface life on planets of red dwarfs. Flares are eruptions from small regions of the surfaces of stars that cause brief (hours to days) increases in brightness. Crucially, flares emit blasts of positively-charged protons, which have been shown by Prof. Antigona Segura and colleagues to deplete ozone layers that can protect life from harmful high-energy UV light. Proxima flares far more often than our Sun and Proxima b is much closer to Proxima than Earth is to the Sun, so Proxima b is likely to have been subjected to repeated bombardments. If the atmosphere could develop a robust shield to these eruptions, such as a strong magnetic field that then flaring could be unimportant. Alternatively if it exists under just a few meters of water. Therefore, flares should not be considered fatal for life on Proxima b.

The concern over flaring naturally leads to the question of whether the planet actually does have a protective magnetic field like Earth’s. For years, many scientists were concerned that such magnetic fields would be unlikely on planets like Proxima b because tidal locking would prevent their formation. The thinking went that magnetic fields are generated by electric currents moving in the planetary core, and the movement of charged particles needed to create these currents was caused by planetary rotation. A slowly rotating world might not transport the charged particles in the core rapidly enough to generate a strong enough magnetic field to repel the flares, and hence planets in the habitable zones of M dwarfs have no atmospheres. However, more recent research has shown that planetary magnetic fields are actually supported by convection, a process by which hot material at the center of the core rises, cools, and then returns. Rotation helps, but Dr. Peter Driscoll and I recently calculated that convection is more than sufficient to maintain a strong magnetic field for billions of years on a tidally locked and tidally heated planet. Thus, it is entirely possible that Proxima b has a strong magnetic field and can deflect flares.

So is Proxima b habitable? The short answer is “It’s complicated.” Our observations are few, and what we do know allow for a dizzying array of possibilities. Did Proxima b move halfway across the galaxy? Did it endure a planetary-system-wide instability that launched its sibling planets into deep space and changed its orbit? How did it cope with the early high luminosity of its host star? What is it made of? Did it start out as a Neptune-like planet and then become Earth-like? Has it been relentlessly bombarded with flares and coronal mass ejections? Is it tidally heated into an Io-like (or worse) state? These questions are central to unlocking Proxima’s potential habitability and determining if our nearest galactic neighbor is an inhospitable wasteland, an inhabited planet, or a future home for humanity.

The last point is not as rhetorical as it might seem. Since all life requires an energy source, it stands to reason that, in the long term — by which I mean the loooong term — planets like Proxima b might be the ideal homes for life. Our Sun will burn out in a mere 4 billion years, but Proxima Centauri will burn for 4 trillion more. Moreover, if a “planet c” exists and slightly perturbs b’s orbit, tidal heating could supply modest energy to b’s interior indefinitely, providing the power to maintain a stable atmosphere. If humanity is to survive beyond the lifetime of our Sun, we must leave our Solar System and travel to the stars. If Proxima b is habitable, then it might be an ideal place to move. Perhaps we have just discovered a future home for humanity! But in order to know for sure, we must make many more observations, run many more computer simulations, and, hopefully, send probes to perform the first direct reconnaissance of an exoplanet. The challenges are huge, but Proxima offers a bounty of possibilities that fills me with wonder. Whether habitable or not, Proxima b offers a new glimpse into how planets and life fit into our universe.

Thanks to Victoria Meadows, Edward Schwieterman, Giada Arney, and Peter Kelley.

Editorial note. This is an outreach article based on the scientific report “The habitability of Proxima b I : Evolutionary scenarios”http://adsabs.harvard.edu/abs/2016arXiv160806919B , which was submitted to the Journal Astrobiology on Aug 25th. Proxima’s b putative habitability assessments are crucial to interpret the significance of the detection of Proxima b, design follow-up observations and even reshape instruments and space missions. The Pale Red Dot team contacted two expert groups in advance to provide these early habitability assessments at the time of the announcement. Prof. Rory Barnes led one of the teams. The results from the other team (led by I. Ribas + M. Turbat) are summarized at: http://proximacentauri.info and are also technically explained on two research papers. More studies are surely underway.

PurpleMountain

About the author. Rory Barnes is a professor of astronomy and astrobiology at the University of Washington in Seattle, USA. He obtained his Ph.D. in astronomy from the University of Washington in 2004. After a post-doctoral position at the Lunar and Planetary Laboratory at the University of Arizona in Tucson, he returned to the University of Washington and NASA’s Virtual Planetary Lab in 2009, joining the UW faculty in 2013. He has studied exoplanets through computer models, initially focusing on the orbital dynamics, but has now broadened his investigations to include the roles of the Milky Way galaxy, stellar evolution, atmospheric effects and the thermal and magnetic evolution of terrestrial planet interiors.

Biosignature Gases: A Needle in a Haystack

By Sara Seager, Massachusetts Institute of Technology (MIT)

Somewhere out there, a living, breathing world peacefully orbits its star. Inhabiting the world is a dynamic ecosystem full of thriving bacteria. The life itself has no consciousness or intelligence, but the planet as a whole is an active world, connected through cycles of geophysics, chemistry, and biology in a landscape with liquid water oceans, continents, mountains and volcanoes. We imagine there could be millions or even billions of such planets in our Galaxy. Is Proxima Centauri lucky enough to host such a planet?

To find out whether or not a planet has life, it is not enough to know that the planet is rocky and if the planet orbits in the host star’s habitable zone. We must be able to investigate the planet’s atmosphere. For Proxima Cenaturi, as in most cases, we will need a to use a different telescope than one used to discover the planet (see post by Ignas Snellan). More specifically we want to observe the planet atmosphere to first assess the greenhouse power of the atmosphere to estimate whether or not the surface temperature is suitable for life. Next we want to determine if there are gases that indicate the planet is habitable. Most challenging but most exciting, we want to know if the exoplanet atmosphere contains gases that might indicate if the planet is possibly inhabited.

exoplanet-chemical-signatures
How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. Credits: NASA/JPL-Caltech.

By habitable I mean a planet with surface liquid water oceans, since all life as we know it needs liquid water. But, since oceans are hard to identify from afar, we will look for water vapor as an indicator of water oceans. By possibly inhabited I mean the identification of signs of life by way of biosignature gases—gases that are produced by life and can accumulate in an exoplanet atmosphere to be detectable from afar. Even if we are lucky enough to detect a biosignature gas on a rocky planet orbiting Proxima Centauri, we will not know if the suspect gases are produced by tiny microbes, enormous animals, or intelligent humanoids. We also will not know if the life producing the gases is carbon based or something more exotic. We focus on what life does—life metabolizes and produces byproduct gases, not on what life is.

Oxygen is our most compelling biosignature gas. Oxygen fills Earth’s atmosphere to 20% by volume. But, without plants or photosynthetic bacteria, Earth’s atmosphere would have virtually no oxygen. Oxygen, and its photochemical byproduct ozone, have strong spectral features at a range of wavelengths, accessible with future ground- and space-based telescopes that might be able to study atmospheres of any planets discovered orbiting Proxima Centauri. If we detect oxygen, astronomers and the public alike will be absolutely ecstatic. But does an oxygen detection mean we have found alien life? No. Unfortunately the attribution of oxygen—or any gas—to life is an in depth, complicated, and somewhat subjective process. The reason is that there are many ways oxygen can be produced, and accumulate, in an exoplanet atmosphere that has nothing to do with life. We must be able to rule out all other possibilities of oxygen generation by non-biological processes. Even then, we will only be able to claim a strong suggestion of life detection not a robust detection.

A flurry of recent activity has detailed a number of different oxygen-producing scenarios that are not related to life. Most of the scenarios have to do with a lack of oxygen “sinks”. If oxygen is not destroyed, then even small quantities of oxygen can accumulate over a large amount of time. One of the more compelling oxygen false positive scenarios is related to the ultraviolet (UV) radiation of exoplanet host stars. The UV radiation splits apart molecules in the planet atmosphere, setting off a chain of chemical reactions that produce byproducts that can destroy oxygen. A major player, OH, is nicknamed the “garbage eater of the atmosphere” because of its power to destroy oxygen and other gases. M dwarf stars typically have a high far-ultraviolet radiation flux (< 200 nm) and a lower near-ultraviolet radiation flux (200-300 nm) compared to our Sun. Any exoplanet orbiting an M star will therefore be subject to different photochemistry than Earth’s atmosphere. Specifically, the chains of reactions that produces OH are weaker, owing to strong far-ultraviolet radiation. With a much smaller amount of OH compared to Earth’s atmosphere, abiotic oxygen can accumulate. To identify this false positive scenario we would need to be able to measure Proxima Cenaturi’s far-UV and near-UV radiation. Other oxygen false-positive scenarios include planets with a carbon dioxide-dominated atmosphere but little volcanic emission, an M star that took a very long time to reach a stable hydrogen burning phase, a planet undergoing a transient ocean evaporation from a runaway greenhouse effect, and more. If we are so lucky to find oxygen on Proxima Centauri, we will have a lot of further observations and atmosphere modeling work to do to understand if the oxygen can be attributed to life or if it might be a false positive.

Beyond oxygen, astronomers also consider a wide range of other biosignature gases, including methane, nitrous oxide, dimethyl sulfide, and others. Despite a growing list and detailed studies, I worry that the list of gases may be too limited, or that the types of planets modeled—usually small deviations from an Earth twin—are not broad enough to anticipate the range of what planet types are out there. If Proxima Cenaturi has a rocky planet in its habitable zone, we should do all we can to make sure we don’t miss a sign of life, just because we were too constrained in our thinking.

Life on Earth produces literally thousands of gases. Most are produced in too small quantities to accumulate to any reasonable level in Earth’s atmosphere. In addition, most are produced for highly organism-specific reasons—such as stress and signaling—that appear to be whims of evolution. Some molecules could be produced in larger quantities on another planet and/or accumulate in an exoEarth atmosphere to high levels, depending on the exoEarth ecology and surface and atmosphere chemistry. In other words, there is a possibility that any gas might be a biosignature gas, if it is present in very high quantities in an exoplanet atmosphere and can’t otherwise be explained away.

Motivated by this reasoning, my team spent a few years constructing and curating a list of all molecules that exist in gas form in a planet atmosphere with a similar temperature and pressure to Earth’s. We both combinatorically constructed lists and also exhaustively searched the literature and found about 14,000 molecules. About 2500 of these are hydrocarbons. We plan to work through this list in classes of molecules to understand their atmospheric and surface chemistry, photochemistry, and spectral properties. From this we can select both promising chemical candidates, and promising ways to search the spectrum that could capture the most diverse range of such candidates.

F7.large (1)
Schematic for the concept of considering all small molecules in the search for biosignature gases. The goal is to generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Credit: S. Seager and D. Beckner.

Does this sound like a lot of work for a library of gases even though the study of atmospheres of any planets found to orbit Proxima Cenaturi and others lie a decade or more in the future? It is. But it will take a long time to fully prepare so we don’t miss out on a biosignature gas detection.

Despite an exhuberant realization that the search for and detection of biosignature gases is within reach, there is a long road ahead. Nonetheless the coming decades are opportune for extensive progress in finding and characterizing other Earths, and full of hope for biosignature gas detection. I remain as hopeful as ever as I plan to devote the rest of my career to the search for life on exoplanets.

SeagerPhoto
Professor Seager

About the Author

Sara Seager is an astrophysicist and planetary scientist at MIT. Her science research focuses on theory, computation, and data analysis of exoplanets. Her research has introduced many new ideas to the field of exoplanet characterization, including work that led to the first detection of an exoplanet atmosphere. Professor Seager also works in space instrumentation and space missions for exoplanets, including CubeSats, as a co-I on the MIT-led TESS, a NASA Explorer Mission to be launched in 2017, and chaired the NASA Science and Technology Definition Team for a “Probe-class” Starshade and telescope system for direct imaging discovery and characterization of Earth analogs. Professor Seager was elected to the National Academy of Sciences in 2015, is a 2013 MacArthur Fellow, and in 2012 was named in Time Magazine’s 25 Most Influential in Space.

Terrestrial Planets over the Next Decade

By Don Pollaco, Warwick University, UK

There can be no doubt that NASA’s Kepler mission has been a resounding success. In particular, much of what we know about rocky planets has come from this mission. After saying that, ground-based radial velocity surveys had already indicated the existence of super-Earths—a class of planet not found in our solar system (ignoring Planet 9!), and the first exo-rocky planet discovered was found through the French-ESA CoRoT mission (Corot-7b). The list of “firsts” from Kepler is truly amazing:

  1. Planetary size distribution
  2. The commonality of multi-planet systems
  3. The application of transit timing techniques to derive planetary masses and the recovery of unseen components
  4. The detection and modelling of the first circumbinary systems
  5. The diversity of low mass planets
  6. The evaporation and breakup of small planets

and so, while some results are less good, e.g. estimate of eta-Earth (number of habitable zone planets per star) and the masses of low-mass planets, Kepler’s place in history is assured. To me though, Kepler’s greatest result is really the ubiquity of exoplanets; specifically small planets.

Almost as impressive has been Kepler’s contribution to the proving of stellar asteroseismology. Whilst these techniques had been applied to the Sun and individual stars, Kepler has been used to derive stellar parameters for hundreds of stars at a level never before achieved en masse.

Kepler Small Planets

Kepler has given us a tantalizing first glimpse of the small/rocky planet population and some of the results have been absolutely awesome (Figure 1). For example, masses for the fantastic seven planet Kepler-11 system have been derived through modelling the gravitational perturbations giving rise to the transit time variations, and show these planets are much bigger than expected for their masses—maybe they are mini gas planets or have fluffy extended atmospheres.

Figure1-Massvsradius
Figure 1. The known small planet population in the mass-radius plane (x-axis’ units are Earth masses) compared to different compositions and compared to rocky planets in our solar system There is far more diversity than originally expected.

At the other extreme is Kepler-10c. Kepler-10b (mass 3.33ME, radius 1.47RE, Period 0.84d) was well known as Kepler’s first rocky planet, and spectroscopic observations from the ground with HARPS-N on La Palma not only confirmed this, but also detected the stellar reflex motion from the long period Kepler-10c component. Surprisingly, the mass turned out to be 17.2ME, but the Kepler (2.3RE) radius suggested we were still most likely looking at a massive super-Earth. Given that we struggle to understand the internal structure of the Earth, we are quite mystified to explain that of Kepler-10c. These results and others lead us to believe the small planet population is much more diverse than we originally believed.

When Kepler was being designed, it was generally agreed that there would be little variation of compositions in this population, so that from a measurement of planetary radius its mass could be directly inferred. Consequently, it was assumed that there would be little need for follow up observations. Kepler showed us the need to determine the planetary mass directly.

Radial Velocity Surveys: Masses of Kepler Planets

Since the first discovery of a planet around a Sun-like star (Mayor and Queloz 1995), radial velocity surveys—searching for the reflex motion induced in the star—were often the most efficient discovery technique. Compared to the transit method’s strict requirement on the orbital geometry, radial velocity detection is far more lenient. However, without knowledge of the orbital inclination to our line of sight, all we can determine is the planetary minimum mass. Basically, from radial velocity information alone we can learn about the planetary orbit, but essentially nothing about the planet itself.

Radial velocity information is most useful when it is used alongside transit data. With radius and, most importantly, orbital information coming from the light curve modelling, solutions of the equations of motion can give an accurate planetary mass. Thus, we can get an accurate estimation of the bulk density/composition of a planet. What should be emphasized here is that to derive the planetary mass and radii requires better accuracy in the stellar parameters; in fact, for the best transit light curves knowledge of the host star is often the factor limiting that of the planetary component. The study of exoplanets has led to a renaissance in stellar research and especially the proving of asteroseismology.

The low brightness of the Kepler field stars and the prevalence of small planets is a double whammy for our studies of the masses of small planets—the small reflex motion and lack of stellar photons make mass measurements at best somewhat challenging. So while the Kepler photometry has produced highly accurate relative radii, even the brightest Kepler host stars are challenging targets for radial velocity work. It is ironic that the planets with the most accurate accepted masses are massive planets found from ground-based transit surveys such as SuperWASP or HAT.

As a consequence, researchers have developed our ability to model gravitation perturbations detectable through transit timing variations (as noted earlier) and this is how most Kepler planetary masses have been determined. This has the advantage that they can be derived from the light curve alone and with apparently small errors, but is possible for only a small fraction of the planets. There is still some controversy surrounding the use of masses derived in this way and maybe more importantly in the quoted errors. Maybe this will improve in the future.

However, one of the big lessons from Kepler (and the ongoing K2 surveys of course) is that we need a host star population as bright as possible so we can derive masses, make planetary atmosphere observations, etc. So given this, what does the future hold?

Looking forward—the Transit Roadmap

For exploring the inner parts of solar systems, and in particular the habitable zones, for the next 10–15 years it is likely that transits of bright stars that allow radial velocity observations to be made will dominate (Figure 2). That’s not to say that other techniques and regions of the parameter space will not be important—they will. For example, with SPHERE and GPi we are taking our first steps with dedicated and optimized instruments capable of direct planet detection—at least of luminous, young and massive planets. JWST may also be capable of this. Gaia and various microlensing space missions such as WFIRST (~2025) or EUCLID (2021) will allow us to statistically explore the outer parts of solar systems.

Figure2-TransitRoadmap
Figure 2. The funded transit roadmap showing facilities that are used for detection and bulk characterisation. Some of the facilities here can also be used for atmospheric characterisation.

In terms of transit experiments, we have a crop of ground-based experiments—including the new NGTS, and the re-tasked Kepler K2 surveys. While still at an early stage, NGTS is proving capable of routinely detecting dips which could be due to Neptune-sized objects. Various experiments have been deployed targeting M dwarf stars, where the low intrinsic brightness and small star size mean that ground based photometry would even be capable of detecting Earth-sized planets in orbits of a few days; corresponding to the habitable zones of the feeblest stars.

In general, finding small planets (smaller than Neptune, say) in habitable zones is a difficult task and is best done from space. This not only avoids limitations in photometric accuracy from the Earth’s atmosphere, but also the interruptions caused by the day/night cycles. Even still, as we push to higher and higher accuracies stellar activity becomes a bigger issue with less stars being suitable for radial velocity work. However, understanding stellar activity is an area of much research and there is hope that small radial velocity signals will be detectable against the activity signal in the future. Nonetheless, we are fortunate that both NASA and ESA have recognized the need for new surveys and we have a series of missions that have transit detection at their heart.

CHEOPS is due for launch in 2017 and is ESA first “Small” satellite. This Swiss led mission is designed to look at objects one at a time. CHEOPS has two science drivers:

  1. The follow up of known planets discovered from radial velocity surveys and especially those targets thought likely to transit, and
  2. High accuracy light curves of transits from other surveys, notably NGTS.

So, while CHEOPS is not a survey instrument it will produce extremely accurate photometry of known planets and hence bulk densities. CHEOPS also has many other potential uses such as monitoring transits for timing variations etc.

NASA’s Transiting exoplanet Survey Satellite, TESS, will be launched around the end of 2017. TESS will be orientated into a highly eccentric and inclined orbit which reaches almost to the lunar orbit. For most of the 27 day orbit the satellite will be far from the Earth, enabling accurate photometry. The clever orbit and observation strategy results in sections of the sky being monitored for 27 days before moving to the next section. These sections overlap at the Ecliptic poles and so a small region is monitored for as long as ~1 year. Given this, it is likely that TESS will find many single transiting systems which would benefit from CHEOPS observations.

TESS is aimed at surveying the nearest and brightest stars (mag) and is therefore preferentially examining M dwarfs. These low luminosity stars have habitable zones close in (periods as short as a week or so for the coolest objects). Furthermore, as these stars are quite small the detection of small planets can be achieved easier. Being extremely red objects they are likely to be ideal targets for the JWST and it is likely that the first observations of the atmosphere of a habitable zone planet will come from TESS.

The Future: ESA’s M3 PLATO Mission

Over the years, there has been a succession of transit survey concepts studied by ESA, but in 2014, PLATO was finally selected as the “Medium 3” mission with launch date in 2024. PLATO was designed from the outset to characterize habitable zone rocky planets with Sun-like host stars that are bright enough for observation with the new generation of radial velocity spectrograph’s such as ESPRESSO at ESO’s Very Large Telescope (VLT) in Chile. PLATO is a multi-telescope system which provides a huge field of view (>2,200 square degrees—about 20 times that of Kepler) with excellent sensitivity and it will be stationed in a thermally stable environment at the L2 point, several million kilometers from Earth. While the dynamic range is from 4–13 magnitude, most of the interesting science will be for stars with magnitudes allowing asteroseismic characterization of accurate stellar parameters including their age. Figure 3 shows the predicted rocky planet catch for stars that can be fully characterized through asteroseismology compared to those from Kepler and TESS.

Fugure3-PlanetCatch
Figure 3. Simulations of PLATO transit signal detection performance (in green) for super-Earth planets (less than 2RE) for stars brighter than 11 mag, hence with RV follow-up and host star asteroseismology possible. For comparison, Kepler results are shown (in blue, Fressin et al. 2013) and expected yields for TESS (in red) assuming 27 day observing coverage per field and 2% of the sky observed for 1 year

The time requirement for the ground-based follow up will be almost entirely driven by the smallest, longest period, planets and will represent a significant investment by the astronomical community. For some of the multi-planet systems, masses will also be available from models of any transit timing variations. While PLATO will certainly produce lots of interesting and no doubt unique systems and maybe even moons, rings, etc., the real PLATO reward will be the database of uniformly characterized planetary systems that can be used for future theoretical and observational experiments.

By the end of the next decade, we will have fully characterized hundreds of systems containing rocky planets. Many of these will be bright enough to have their atmospheres examined with the instruments of the day. The database of PLATO systems with known ages will allow us to take the first steps in comparing the observed planet population with theoretical studies, hence throwing light on the important processes that are sculpting the architectures of these systems. In many ways PLATO can be considered the descendant of Kepler and indeed one of the options for PLATO is to revisit the Kepler field to examine the variations in transit timing variations accumulated after a delay of some 15 years.

We live at a very fortunate time. Kepler has opened the window and shown us some of the landscape. The new missions will enable us to make great gains in comparative planetology so that we can understand our place in the Universe.

Dr Don Pollacco, astro-physicist and planet hunter from Queens University, Belfast.

About the author. Don Pollaco is a Professor of Astronomy at Warwick University, UK. He was awarded his PhD in 1990 from St Andrews University. From 1990–1995 he worked as a PDRA and then lecturer at St Andrews and Liverpool John Moores Universities. Between 1995-2000 he was based at the Isaac Newton Group of Telescopes (ING) La Palma. His PhD and postdoctoral studies were concerned with evolved stars and binary systems. From 2000–2012 he worked at Queens University Belfast developing the exoplanet group there. During this period he led the development as PI of the SuperWASP Project and the initial development of NGTS. Since arriving at Warwick in 2012 he has been the Science Coordinator for ESA’s PLATO mission.

 

 

Finding life around our nearest neighbour

Ignas Snellen, Leiden Observatory, Leiden University, The Netherlands

How amazing would it be if we found that there exists a planet, with a mass and temperature just like Earth, around our nearest neighbour—Proxima Centauri. How could we find out whether life exists on such a world?

The  Pale Red Dot project is all about following up a hint that such a planet may exist. This is extremely exciting. Finding such a planet may actually not be such a surprise. Proxima is a faint red dwarf star, seven times smaller than the Sun. From previous observations, for example using the NASA Kepler satellite targeting planets that transit their host star, it is clear that in general red dwarf stars indeed often have small rocky planets. Therefore, Proxima—our nearest neighbour at a distance of 4.2 light-years—could be expected to also host small planets. Since the luminosity of Proxima is one thousand times smaller than that of the Sun, the energy a planet receives from Proxima is that same amount less than it would receive from the Sun. Therefore a planet in an Earth-like orbit would be freezing cold. Astronomers like to think that to host life a planet needs to be able to sustain liquid water. It is crucial for life as we know it as a solvent; making the complex chemical reactions associated with biological activity possible. Indeed, no life on Earth can function without liquid water. For planets around Proxima it means that their orbital distances need to be significantly smaller, only a fraction of the Earth-Sun distance, to be able to sustain life—corresponding to an orbital period of about two weeks, right in the ballpark of the possible planet chased by Pale Red Dot.

Artist's impression of a habitable planet around a red dwarf star (credit ESO/L. Calçada).
Artist’s impression of a habitable planet around a red dwarf star (credit ESO/L. Calçada).

Let us for now assume that the Pale Red Dot observations indeed reveal such a planet. How could we possibly find out there is life on it? Let me first sketch the picture of why this is so difficult. For hundreds of years astronomers have wondered whether there is life on Mars, without a conclusion yet.  Over the last fifty years, Mars research has accelerated at an immense pace. NASA and ESA have sent many probes that dig, drill, and can measure the Mars atmospheric constituencies to high precision—and still we do not know. One has to realize that we will never be able to do this for extrasolar planets. The distances are simply too large. Even Proxima, our nearest neighbour star, is more than two-hundred-thousand times further away than Mars. We simply cannot send probes to Proxima on practical human time scales. Therefore, any information we collect from possible planets orbiting Proxima can only come from remote sensing—observing them using our telescopes on and around Earth.

Is it then entirely hopeless that we could find life? Luckily not! Of course, if the situation is like on Mars, where life may be hiding deep under the surface, or on Jupiter’s moon Europa, where simple life may be deep in its ocean, it will indeed be hopeless. We will never be able to recognize it from a distance. However, the situation is very different for Earth. If an alien civilization would point a telescope at Earth and measure its spectrum, it can see clear signs of biological activity. They can see that molecular oxygen is very abundant in our atmosphere, which is only present due to the organisms that produce oxygen as a waste product (which subsequently animals like us use as an energy source). Oxygen is a very reactive gas, which you do not expect to find in an atmosphere like ours. Indeed it took life on Earth more than a billion years before the oceans were saturated enough that the oxygen they produced ended up in the atmosphere. If all life on Earth would cease to exist, all atmospheric oxygen would disappear on a very short time-scale. Therefore we call oxygen a biomarker gas—it can provide evidence for biological activity on an Earth-like planet. Note however that a lack of oxygen does not mean there is no life. It could still hide under a rock like possibly on Mars.

Also, for more than a billion years there was abundant life on Earth without a significant amount of oxygen in the atmosphere. Anyway, at least we can start somewhere.

So how could we detect oxygen on a planet more than four light years away? Characterizing a planetary atmosphere is significantly more difficult than finding the planet, like is being tried now with the Pale Red Dot project. Most of the techniques used to find planets, including the radial velocity method used for Proxima, are indirect methods. They do not identify one single photon from the planet, but instead indirectly infer the presence of a planet—e.g. using the gravitational pull of the planet on the host star, or the moment a planet transits the disk of its host star blocking a small fraction of starlight that can be observed. This can no longer be the case for atmospheric characterization. We really need to identify photons from the planet as a function of wavelength, and in this way build up a planet spectrum. Although this is very difficult, this can be done, but is so far almost exclusively achieved for hot gas giant planets.

Bird's eye view of ESO's Very Large Telescope which is used for many of the exoplanet characterisation observations (credit: ESO).
Bird’s eye view of ESO’s Very Large Telescope which is used for many exoplanet characterisation observations (credit: J.L. Dauvergne & G. Huedepohl (atacamaphoto.com)/ESO).

Broadly speaking, separating the planet’s light from that of the star can be done in the time, spatial, spectral and polarization domain, or using combinations of these. An example of the time domain is transmission spectroscopy. When a planet, as seen from Earth, moves in front of its host star, a little bit of starlight filters through the planet’s atmosphere, leaving an imprint of absorption from its atmospheric constituencies. A planet can also be eclipsed by its host star, temporarily blocking the planet light, which can be measured. In such a way the thermal emission spectrum or reflected light spectrum can be determined. However, such transit and eclipse observations require that the planet orbit is seen exactly edge-on – which will only be a small probability for the alleged Proxima planet. We have a better chance by trying to separate the planet from the star in the spatial domain. This is called direct imaging, or high-contrast imaging. Using a combination of techniques the starlight can be suppressed by orders of magnitude, leaving the planet signal coming from a slightly different direction unaffected.

Spectrometers coupled with very high contrast imagers on giant telescopes should enable the separation of the light coming from the star and the planet. In this picture, a simulation of the distinct orbital velocity of the planet and the star would allow to achieve higher contrast than using direct imaging alone. Source : Snellen et al. A&A 2015, arXiV
Spectrometers coupled with very high contrast imagers on giant telescopes should enable the separation of the light coming from the star and the planet. In this picture, a simulation of the distinct orbital velocity of the planet and the star would allow us to achieve higher contrast than using direct imaging alone. The signal of the planet image is the faintest dot on the right of the top panel. Source : Snellen et al. A&A 2015, more information in http://home.strw.leidenuniv.nl/~snellen/

Although Proxima is our nearest neighbour, the targeted planet’s orbit will be very close in—only ~1/200,000th of a degree (20 milli-arcseconds) as seen from Earth. The sharpness of a telescope directly scales with its diameter, implying that revealing a planet at such a small angular distance from its host star requires a mirror diameter of at least 25 meters. Even the James Webb Space Telescope, which will be launched in 2018, will not be able to separate this planet from its host star. We would really have to wait until the next generations of ground-based telescopes come online, like the US Thirty Meter Telescope and Giant Magellan Telescope, and the European Extremely Large Telescope. Even with these giants it will be a real challenge—we will have to look through our own atmosphere to detect the atmosphere of the planet. However, we have been practicing just that using giant planets over the last decade, and we are starting to get pretty good at it! We will have to be patient though—do not expect the specific instrumentation needed to detect oxygen to be ready before 2030. A long wait, but the prospects are very exciting!

IgnasSnellenF

About the Author: Ignas Snellen is Professor in astronomy at the University of Leiden in the Netherlands. After concluding his PhD research in Leiden in 1997, he worked for three years as a postdoctoral fellow at the Institute of Astronomy in Cambridge (UK), after which he became an astronomy lecturer at the University of Edinburgh (UK). He returned to Leiden University in 2004. Snellen and his group develop observation and data-reduction techniques for ground-based telescopes, particularly geared to be used for the future extremely large telescopes (ELT). He and his group pioneered studies of molecules in exoplanet atmospheres using high resolution spectroscopy in the near infrared.

Why we are all metal-heads!!

By James Jenkins, Universidad de Chile

Here on planet Earth, we all know what metals are. They are the components of the man-made industrial world around us. From the aluminium tins that contain our fizzy drinks, to the copper cables that distribute power across the globe, all the way up to the giant iron building structures that dominate our city skylines.   However for astronomers, metals are something quite different. They are the atoms that were not created in any great abundance in the Big Bang[1]. This basically means that astronomers call all atoms metals, except Hydrogen and Helium. All these atoms that were previously processed in the hearts of giant stars, or in the intense stellar explosions called Supernova, and continue to be so today. Carl Sagan’s thoughts on metals were poetically portrayed in his famous quote that appeared in the book Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies, were made in the interiors of collapsing stars. We are made of starstuff.”

The abundance of metals, or metallicity, of objects throughout the universe, appears to have a considerable affect on the various properties of those objects. For instance, for Supernovae, the violent explosions that signify the deaths of massive stars, the metallicity of the progenitor star before it collapses to produce the explosion, plays a major role in the determination of what type of supernova the star will eventually become, and therefore, the amount of energy released in the explosion. However, in the exoplanet[2] community, the metallicity of stars took on a whole new meaning in 1997 after the first three exoplanet hosts were studied. It was found that all three of these stars had a much higher fraction of metals by mass in their atmospheres than the Sun does. They were found to be so called, metal-rich stars.

In the subsequent 19 years since this finding, astronomers have discovered that there is a direct correlation between the fraction of iron atoms in a star’s atmosphere and the probability of that star hosting a gas giant planet. In fact, it has been shown that for Sun-like stars with three times the metallicity of the Sun, around 30% have Jupiter-like giant planets orbiting them, whereas only 3% of stars with a mass similar to the Sun host these types of planets. This lower fraction of gas giant planet-hosting stars with lower metallicity continues down to stars with even lower fractions of metals by mass, whereby only 0.3% of stars with one-tenth the metallicity of the Sun host giant planets. This relationship was the first direct connection between the properties of stars that astronomers can measure observationally, and their retinue of planets.

It turns out that such a correlation as the one observed between gas giant planet fraction and stellar metallicity was a direct prediction of the core accretion planet formation process. At the time, the two competing formation channels for planets were core accretion and gravitational collapse. The core accretion formation process is a ‘bottom-up’ mechanism, where the smallest dust particles in the disc of material leftover from the formation of the star, begin to stick together, building up larger and larger pebbles, rocks, and finally planetesimals. The planetesimals can then go on and continue to grow, either by accreting more dusty material from the disc, or by sweeping up the disc’s reservoir of gas. On the other hand, the gravitation collapse channel is a ‘top-down’ mechanism, whereby the disc fragments into smaller pieces of dust and gas due to gravitationally unstable regions in the disc, and with continued collapse over time,  those pieces finally become giant planetary bodies orbiting the central star. In either case, it’s humbling to remember that planet Earth, where life clings to so voraciously, was born out of the stellar garbage left over after the mother Sun was born.

swp
The percentage of stars hosting gas giant planets near to the host star with changing stellar metallicity ([Fe/H]). It can be seen that the stars with the most amount of metals in their atmospheres have a higher chance of growing a giant planet around them. The red curve shows the best-fit model to the data (coloured region) and the equation that describes the model is represented on plot. As discussed in the text, the percentage of stars hosting giant planets increases with the fraction of iron (Fe) over hydrogen (H) in a star’s atmosphere. Credit: Graphic modified from the original published by Fischer & Valenti 2005, ApJ.
Both these mechanisms make key testable predictions, and the fraction of gas giant planets orbiting stars as a function of metallicity was one that came to very different conclusions. Given that the core accretion model starts by sticking the smallest dust particles together to make larger structures, the more dust in the disc, the faster those structures can be built, and the faster those cores can acquire gas from the disc before the disc is lost to interstellar space. Given that the disc is leftover rubbish from the formation of the central star, discs rich in dust should also have formed stars rich in metals, or those with higher metallicities, seemingly in agreement with observations. On the other hand, the gravitational collapse model does not depend on the dust fraction in the disc and so less dusty discs should form as many large planets as dusty discs do, meaning no stellar metallicity bias should be witnessed. In the end, this was the key piece of evidence to suggest that planets are formed through a bottom-up process like core accretion.

With the recent advancement in technology in the laboratory, along with the latest data being fed to astronomers from orbiting space telescopes like Kepler, the picture may not be as clear cut for the population of lower-mass planets, particularly planets with masses similar to that of Neptune or less. It appears that the fraction of stars hosting these low-mass planets, aka. super-Earths, does not increase with increasing metallicity, in the same way as for giant planets. In fact, the opposite might be true, where low-mass planets tend to form predominantly around metal-poor stars.

debris_disk
An artists impression of a metal-rich planet forming disc around a young star. The star is the bright, illuminating source in the center of the disc, and small rocks and planetesimals are beginning to form around the star, due to the large amount of metals that can stick together and form these planet embryos. Credit: NASA/JPL

If the population of super-Earth planets do not follow the same pattern as giant planets, then what does this say about how these planets form? Does this mean they do not form through the same core accretion-type process as giant planets? Well the answer is no, it does not mean that. Remember that core accretion predicts that giant planets will be predominantly found orbiting metal-rich stars because the small cores that grow to become the large planetesimals that are needed to tear the gas away from disc must grow fast enough before the disc is lost. However, low-mass planets that are either core dominated or that don’t require significant amounts of gas have no need to form so quickly. In an environment where there is less metals, cores take longer to form, and therefore they maintain a small mass up until the disc is lost. Therefore, core accretion also predicts that low-mass planets should be more abundant orbiting metal-poor stars than gas giant planets, another win for the core accretion planet formation theory!!

poor
Metallicity of planet host stars against the minimum mass[3] of the detected planets in those systems. Note that this is only for stars hosting planets with estimated masses in agreement with the mass of Neptune or less. The red shaded region is the super metal-rich part of the diagram and it appears to show a lack of planets when compared to the more metal-poor region in the left bottom quadrant of the plot. Credit: Graphic modified from our paper discussing this observational result.

The role that metals have played in the development of the evolving universe and for the emergence of life on planet Earth cannot be understated. Metals are the weaving that so intimately connects us all to the entire cosmos around us. Its ironic that such beauty emerges from such violent beginnings out there in space. As for Proxima Centauri, it is a super metal-rich star, meaning its small proto-planetary disc would have been rich in planet forming material. This at least gives us hope that some of those metals began the chaotic dance that ends with the birth of new planets, and who knows, with such a rich array of metals that could have been there in those early days, are there other life forms on those planets that have been fortunate enough to make use of the universe’s talent for metal production? Maybe the Pale Red Dot project will help us to find out!!

people-jamesjenkins

About the Author. James Jenkins is an assistant professor at the Universidad de Chile based in Santiago, the capital city of Chile. Nestled between the surrounding Andes mountain range atop Cerro Calan, he performs observations of nearby stars to hunt out and better understand extrasolar planets and their host stars. James received his Ph.D from the University of Hertfordshire in 2007, going on to hold a postdoctoral position at Pennsylvania State University, working on the Precision Radial Velocity Spectrograph. At the end of 2008 he found himself working at the Universidad de Chile as a postdoctoral researcher and then as a Fondecyt Fellow between 2010-2013, joining the faculty at the end of 2013. His main research interests include the discovery of planets orbiting metal-rich stars through the Calan-Hertfordshire Extrasolar Planet Search, the search for very low-mass planets in radial velocity data, and planets orbiting giant stars. For his sins, he also studies the magnetic activity of planet host stars, working to better understand how this source of ‘noise’ can be filtered out of planet search data. James is a very active member in the Pale Red Dot project and is one of the editors of palereddot.org.


Notes

[1] The Big Bang is the name given to our current model for the moment of creation of the Universe.

[2] The word ‘exoplanet’ is the disminuative of the words ‘extra-solar planet’, worlds orbiting stars other than the Sun.

[3] Remember that the radial velocity detection method does not give the absolute mass of stellar companions, but instead gives us the ‘minimum mass’, since the inclination of the system to our line of sight is unknown.

Exciting times, big challenges

By Luca Pasquini, European Southern Observatory

What incredible times we are living in — searching for an Earth around another star! Not much more than 20 years ago, I looked at the few astronomers hunting for planets, and I shook my head and thought “they are crazy”. Similar to many other occasions throughout my life, I was to be proven wrong.

Nature, of course, helped those early planet hunters quite a lot, by making Hot Jupiters. These are systems inducing radial velocity variations 10 times larger, and with much shorter periods, than the giant planets of the Solar System, which were the only ones known at that time. I believe that the influence of the discovery of the first confirmed exoplanet 51 Pegasi b by Mayor and Queloz, will be very long lasting indeed. By proving the existence of what was before then only considered likely to some of us, or an obvious hypothesis for others, their work unleashed a wealth of energy to the subject that had been unthinkable before. We are now able to analyse exoplanet atmospheres, and to search for small mass exoplanets in the habitable zones of other stars, planets that can harbour life in the way we know it.

Such a tremendous explosion of results has only been possible because fundamental technological advancements have taken place in the past 20 years, advancements that now allow us to search for solar systems and Earth analogues.

Previous expert opinions have shown us that, in order to detect the planetary radial velocity signals, a lot of effort is needed to filter the stellar ‘noise’ (activity, oscillation, variability), and in order to best do that, one needs very precise spectroscopy and long observing campaigns. This requires proper instrumentation and dedicated telescopes. Several observatories such as the European Southern Observatory (ESO) are well equipped with both.

A HARPS Laser Frequency Comb generated spectrum. The horizontal lines are spectral orders (each has two separated fibres, one fed by starlight, the other by A frequency comb combs). The colour reflects the covered spectral range, from short wavelengths (blue) to long ones (red). Each horizontal line corresponds to a diffraction order and is generated and it is composed of thousands of separated, single emission peaks, whose frequency is known with very high accuracy, and are stable over long times. LFC are the next generation of ‘rulers’ to measure precise radial velocities.
A HARPS Laser Frequency Comb generated spectrum. The horizontal lines are spectral orders (each has two separated fibres, one fed by starlight, the other by A frequency comb combs). The colour reflects the covered spectral range, from short wavelengths (blue) to long ones (red). Each horizontal line corresponds to a diffraction order and is generated and it is composed of thousands of separated, single emission peaks, whose frequency is known with very high accuracy, and are stable over long times. LFC are the next generation of ‘rulers’ to measure precise radial velocities.

Precision is the key word. The radial velocity signal induced by an Earth orbiting around a solar mass star with a period of 1 year, is less than 10 cm s-1. Just to provide some comparison, this implies that we must be able to measure the periodic shift of the stellar radial velocity in the focal plane of a typical high-resolution spectrograph for several orbital periods, over several years with a peak shift of just 2 nanometers (10-9 meters or 0.000000001 meters). In addition to requiring the utmost care in stabilising the instruments in temperature and pressure, reaching such a precision requires a very precise ruler, and for this reason several groups in the world have been engaged for almost a decade in work developing the perfect ruler. Currently the most precise rulers are based on Laser Frequency Combs (LFCs), a technique that led to the Nobel Prize for physics being awarded to T. Hänsch and J. Hall in 2005. The LFC can create a series of precisely equally spaced and stable emission lines for spectrograph calibration, whose frequency is known with high accuracy. The worldwide leader instrument in radial velocity precision is probably HARPS at ESO’s La Silla Observatory in Chile. Up to now it has been using the emission line Thorium-Argon lamps as a ruler, but it has been recently equipped with a prototype Laser Frequency Comb system. The short-term tests of this system indicate that a precision of better than 2 cm s-1 can be reached. Advances in understanding optical fibres and their technology, and getting bigger and better optical detectors have also been vital in obtaining the best performance. Optical CCDs are now very clean devices that can be accurately calibrated.

The Laser Frequency Comb hardware in the HARPS room at the ESO 3.6m telescope at La Silla, it is 'a bit' more complex system than a lamp.
The Laser Frequency Comb hardware in the HARPS room at the ESO 3.6m telescope at La Silla, it is ‘a bit’ more complex system than a lamp.

Great expectations are placed on ESPRESSO, the ‘big brother’ instrument to HARPS, that will be hosted at the ESO Very Large Telescope at Paranal before the end of the year. This instrument will boost the original HARPS precision by one order of magnitude and, in addition, will be used by any of the VLT telescopes, or by the four 8-metre Unit Telescopes together, with a 16 metre telescope equivalent diameter. Often the stars observed are relatively bright, so one could question why large telescopes are needed, but we must realise that high precision requires also a lot of stellar photons, or particles of light, hence the quest for large telescopes. A further step in precision is expected from the high-resolution spectrograph at the 39-meter E-ELT. The CODEX concept was originally conceived to measure Doppler shifts so precise such that we would be able to directly observe the expansion of the Universe and Earth-like planets around solar type stars in their habitable zones. The 25-metre Giant Magellan Telescope is planned to have a similar instrument for its first light.

espresso
The ESPRESSO CCD. With 81 million pixels, this device is the largest monolithic detector ever used in astronomy. Each side is 9 cm long.

Planets are cool compared to stars, and emit most of their light as infrared (IR) radiation, which is invisible to the human eye. Expanding observations to this spectral range is therefore essential, and the development of large IR detectors played a fundamental role: progressing in a few years from arrays of a few thousand pixels, to the most recent 16 million pixel devices, which enabled the construction of efficient high resolution infrared (IR) spectrographs. These spectrographs, such as CRIRES at the VLT, have been used to hunt for planets and to observe exoplanet atmospheres. In addition, the radial velocity signal produced by the rotation of inhomogeneities on the stellar surface can mimic the RV periodic variations induced by a planet, but , while the variations induced by the planet are the same for any measured wavelength of light, they are different in the optical and in the IR for the stellar spots that rotate around the star’s surface. That is why the newest generation of spectrographs have a great interest in the IR, and NIRPS, which will be hosted at the ESO 3.6-metre telescope at La Silla, CARMENES at the Calar Alto Observatory, Spirou at CHFT, and the high resolution spectrograph for the E-ELT, will all have an IR arm.

Even if this contribution focuses on radial velocities and spectroscopy, we should not underestimate the power of imaging. NACO at the VLT has imaged the first exoplanet around a very low mass star, similar to Proxima Centauri, and it is beyond any doubt that images, like the one shown below, have been transformational. More powerful high contrast imagers recently became available, imagers such as SPHERE at the VLT and GPI at GEMINI. These create superb images of planets and proto-planetary discs and are able to detect objects more than one million times fainter than the host star. And the next generation of instruments that will be able to exploit the tremendous potential of the ELTs is already taking shape.

The NACO image of an exoplanet (red dot) close to a very low, cool mass star. Possibly the first image of an exoplanet ever. Credits: ESO
The NACO image of an exoplanet (red dot) close to a very low, cool mass star. Possibly the first image of an exoplanet ever. Credits: ESO

Large surveys are now carried out and images of exoplanets are becoming more and more common. The enabling technology has been adaptive optics, a technique that deforms mirrors to compensate for the atmospheric turbulence and therefore recovers the cleanest image of the telescope, as if it was in space, observing with no turbulent atmosphere to look through. Deformable mirrors with more than 1000 actuators, applying corrections faster than 1000 times per second are needed, and they have been developed either of small size, for instance for SPHERE and GPI, or of large size, and these are directly replacing the secondary mirrors of the telescopes, as has happened at the Large Binocular Telescope and soon will occur at the VLT.

The Adaptive Secondary Mirror of the VLT is 1.12 m in diameter and extremely thin: the glass shell is only 2 mm thick. Being so thin, it can be easily deformed, and 1170 actuators act on the back of the glass to correct in real time the distortions induced by the Earth's atmosphere in the observed images.
The Adaptive Secondary Mirror of the VLT is 1.12 m in diameter and extremely thin: the glass shell is only 2 mm thick. Being so thin, it can be easily deformed, and 1170 actuators act on the back of the glass to correct in real time the distortions induced by the Earth’s atmosphere in the observed images.

In a nutshell, thanks to all these great instruments, exoplanet science has a bright present, and even more promising future.

 

lucaAbout the author. Luca Pasquini is an astronomer, working at ESO, Garching, and since 2013 he has been managing the Paranal Instrumentation Programme. After completing his studies in Firenze (Italy), he moved to become an ESA postdoc at MPE (Germany) in 1986, and then went on to ESO La Silla (Chile), where he was in charge of high resolution spectroscopy and of the 3.6m telescope upgrade. In 1997 he moved to ESO Garching, to work in the instrumentation group there. Before his present position, he has been instrument scientist for the FEROS, HARPS, FLAMES, MUSE, and ESPRESSO spectrographs. His scientific work and interests range from stellar activity and stellar abundances, to search for planets around giant stars and around stars in open clusters, as well as to different applications of precision spectroscopy.

‘Are we there yet?’—The journey to Proxima

By Carole Haswell, The Open University

It feels to me almost as though we are entering a second space age. As a child in 1969 I watched the first humans walk on the moon, courtesy of poor-quality black and white images on TV. When I was a teenager in the 1970s my family rarely missed an episode of Star Trek. While we appreciated the comedic undertones of a Universe in which the aliens were humans wearing make-up and most planets seemed to have breathable air, there was a general feeling that all this might be possible. People would race around the Galaxy in spaceships and have amazing adventures. Or at least people would soon plan journeys similar to the Apollo astronauts’ and travel to explore the Solar System beyond our own Moon. Of course this didn’t happen in the way we young aspiring astronauts anticipated. The Apollo program was curtailed and, more recently, the Space Shuttle too. For decades no-one has travelled as far away as the Apollo astronauts did. The last people to travel beyond low-Earth orbit did so in 1972! Apollo’s moon exploration only lasted for 3 years: a poignant prelude.

Recently, with talk about privately funded missions to Mars, space tourism, and NASA’s development of the Orion spacecraft — designed to take people beyond the moon- the hiatus in human space exploration may be coming to an end. Orion’s first mission, EM-1, is scheduled for 2018.

orion
Orion’s first flight will not have humans aboard, but it paves the way for future missions with astronauts. During this flight, currently designated Exploration Mission-1 (EM-1), the spacecraft will travel thousands of miles beyond the moon over the course of about a three-week mission. Credits: NASA

A few of my generation did become astronauts. The International Space Station has been continuously occupied for over 15 years, and we are learning a lot about how to live in space, including over 20 years experience of growing plants in space. Many of my generation instead turned to astronomical, rather than literal, exploration of the Galaxy. What we have learned has been thrilling. The most exciting developments in my lifetime have been the discovery of the first planets orbiting around stars other the Sun; the now firmly established finding that small planets like Earth are common in our Galaxy; and our developing ability to make detailed measurements of the properties of planets which are so dim and distant that it is impossible to see them because of the overpowering glare of their host star. It is worth noting that the techniques for data gathering and analysis created by astrophysicists for this kind of exploration have also paid dividends in other, more immediately practical, endeavours. Several of my former colleagues and students now apply their expertise in bio-medical research, where the results include new ways of treating diseases such as cancer.

The Pale Red Dot campaign is an important part of our exploration. It focuses on our nearest stellar neighbour, so we will be able to obtain better quality data for Proxima and its planets– if it has any– than we can for any other star of its type. This is because the amount of light we receive from a star depends on the square of its distance as well as its intrinsic luminosity. Most stars in the Galaxy, like Proxima, are significantly less luminous and redder than the Sun. It seems quite likely, therefore, that most of the planets in our Galaxy orbit such dim red stars. So far, planet hunters have focused on the rarer but more luminous Sun-like stars, because they are easier to study. If the Pale Red Dot campaign is successful it will demonstrate that we can successfully measure the properties of planets orbiting stars like Proxima, sometimes called red dwarf stars. Because red dwarfs are so common relative to other stars, most of our near neighbours are red dwarfs. This means that if there are planets outside the Solar System suitable for humans to walk on and breathe the air, the nearest examples are likely to be found orbiting red dwarfs like Proxima. The Pale Red Dot campaign is intimately linked to the potential for human exploration of other worlds.

figure.001
Top: This diagram illustrates the locations of the star systems closest to the sun. The year when the distance to each system was determined is listed after the system’s name. Bottom: Nearest stars in a time range between 20,000 years in the past and 80,000 years in the future.

If the campaign is successful, and discovers a temperate planet orbiting Proxima, it will be some time before humans can visit it, if indeed this ever becomes possible. Proxima is over 4 light years away, which means that even at the speed of light it would take over 4 years to get there. The fastest moving human artefact, Pioneer 10 moving at 132,000 km per hour, would take 34,700 years to travel this distance. At this speed, a thousand generations could live and die before the question “Are we there yet?” has a positive reply. Obviously the key to visiting planets outside the Solar System is being able to travel faster, something which remains in the realm of science fiction, but is beginning to be discussed by scientists too.

Personally, I am content now not to be an astronaut. I’m not even particularly keen on camping, and space travel, for the foreseeable future at least, is likely to be a bit cramped and smelly. But I am very glad to be alive in this amazing era in human history. As I was write this, the first ever detection of gravitational waves has just been announced, opening up a new window which offers a completely different view of the Universe and its contents. Our picture of the Galaxy has become so much more rich and detailed in the last fifty years, and I can’t wait to find out more about the billions of exoplanets in the Galaxy. In particular it will be wonderful to learn more about our nearest neighbouring planetary systems.

15594510978_e6d83bf910_z
Dr. Carole Haswell at The Open University

About the author.

Dr. Carole Haswell spent her early years as an aspiring astronaut in Saltburn-by-the-Sea in northern England. Upon realising this was not a very practical career plan, particularly for a child who needed glasses, she decided to become an astrophysicist. She studied physics and mathematics at Oxford University, and was one of the first generation of female undergraduates at University College, Oxford. She did her postgraduate work at the University of Texas at Austin, where her research focused on plasma astrophysics and accretion in black hole binaries. She had a VIP pass to watch the launch of the Space Shuttle mission which deployed the Hubble Space Telescope. Carole worked as a researcher at the Space Telescope Science Institute in Baltimore, Maryland, and at Columbia University in New York City before returning to the UK to take up a faculty position at the University of Sussex. She moved to The Open University in 1999 and changed her research focus to the exoplanets area. She was an early member of the SuperWASP consortium, which is one of the leading ground-based planet searches and has found over a hundred transiting exoplanets. She wrote an undergraduate textbook ‘Transiting Exoplanets’ published in 2010, and has authored over 120 research papers. She now leads the Dispersed Matter Planet Project, which studies nearby stars and aims to find small rocky exoplanets orbiting very close to them. These planets are losing mass as a result of the stars’ intense radiation and tidal influence. In 2016 she was honoured by the Royal Astronomical Society in a portrait gallery commemorating the centenary of the first female fellows. She had mixed reactions to learning in 2006 that NASA astronaut Nicholas Patrick was born in Saltburn-by-the-Sea; at that time she still aspired to be the first person from Saltburn to fly in space.

Interview with Suzanne Aigrain : On the Search for nearby Earth-like Exoplanets

Interview with Suzanne Aigrain, University of Oxford, by John Strachan

In this interview we ask expert exoplanet researcher Professor Suzanne Aigrain from Oxford University why she is a keen participant in exoplanet research and what her views are on the prospects and difficulties of detecting small exoplanets such as those that may exist around Proxima Centauri.

Can you tell me what first got you interested in astronomy, and exoplanets in particular?

I have a background in physics, and started getting interested in astronomy during internships at the Institute of Astronomy in Cambridge while I was an undergrad. My interest in exoplanet research really came about after I graduated, when I spent 6 months as a trainee at the European Space Agency’s ESTEC research centre in the Netherlands, before starting my PhD. This was an exciting time, as the first transiting exoplanet had just been discovered, and I was given the opportunity to work on this very hot topic. I was hooked, and decided to continue in this field of research for my PhD. 

Detecting small exoplanets using the radial velocity method appears to be very difficult. The debate which you have been involved in concerning the existence or not, of a planet around Alpha Centauri is an example of one such case. Can you give your views on the main difficulties involved in detection of small exoplanets using the radial velocity method?

There are many difficulties which have to be overcome in order for the radial velocity (RV) method to succeed in detecting small exoplanets. Here are a few of the most important:

  1. Instrumental precision: The radial velocity of the star has to be measured accurately enough. Current state-of-the-art RV spectrographs such as HARPS and HARPS-N have accuracy down to 1 m/s, or even slightly below. This is sufficient, with many measurements, to detect Earth-mass planets in the habitable zone of some small stars, such as M dwarfs. However an Earth-mass planet in the habitable zone of a Sun-like star causes a variation of only 10 cm/s, over an entire year… With new calibration techniques such as laser combs, though, the precision of RV spectrographs is still improving, and the next generation of experiments such as ESPRESSO are likely to be able to achieve this precision.
  2. Stellar activity: The apparent RV of a star can vary even if there are no planets orbiting it. For example, most stars have some starspots (the stellar analogue of sunspots)—regions where the magnetic field of the star is particularly strong. Starspots appear darker than the rest of the star’s surface, and as the stars rotate, they will hide a part of the star that is first moving towards the observer, then away. This will lead to an apparent change in RV that could easily be mistaken for a planet. There are all sorts of other effects, mainly due to magnetism and convection (the hot gas inside the star bubbling up to the surface and back down again) that can cause subtle and complex RV variations. To detect small planets despite these, we must model them in detail, and this is a very active area of research today.
  3. Patchy observations: Because our instruments are located on the Earth, we can only observe a given star during the night, when the sky is clear and the star is up in the sky. Additionally, in RV we typically observe one star at a time, so we must chose between the different targets we want to observe. As a result, the observations of each star have many gaps, and this can make it even more difficult to distinguish between planetary and stellar signals.
  4. Comparing models: When we analyse the observations of a given star, we don’t know in advance how many planets it has. To find out, we must try models with different numbers of planets and compare them, and at the same time we must also account for the activity of the star, and the noise of our measurements. This, combined with the patchy nature of the data, makes analysing RV data a very complex, time-consuming and challenging process.
Simplified, schematic sketch of Prof. S. Aigrain's group scheme for the joint modelling of an Doppler time series with ancillary activity diagnostics using a technique called Gaussian Processes. Source : S. Aigrain's group website at http://splox.net
Simplified, schematic sketch of Prof. S. Aigrain’s group scheme for the joint modelling of an Doppler time series with ancillary activity diagnostics using a technique called Gaussian Processes. Source : S. Aigrain’s group website at http://splox.net

These difficulties came in to play in the case of Alpha Centauri B. The observations were dominated by the signal from the companion star Alpha Centauri A (Alpha Centauri is a binary star), and by the activity of the star. There was also a tiny signal with a period of 3.2 days. This signal became stronger after modelling and subtracting the binary and activity contributions—strong enough for the authors of the original study to report the detection of an exoplanet. However, we ran some simulations using synthetic data with the same time sampling, noise properties, and the same kind of activity signal, but no planet, and the 3.2 day signal was still there. That means it couldn’t have come from an exoplanet. We actually think it was an artefact of the time sampling, that just got boosted by the complex modelling needed to remove the activity. Now that we know this sort of thing can happen, we can look out for it in future, hopefully avoiding a repeat of this sort of problem.

Do you think the daily observing of Proxima Centauri during the three months of the Pale Red Dot Campaign will significantly increase the chances of detection?

A daily observing strategy determined in advance will certainly help. This dense set of observations, captured over the next two months, will help with the modelling of the star’s activity signal. We know that Proxima Centauri rotates slowly, so by gathering many observations in a short space of time, we should be less sensitive to the effects of starspots. If any possible planets are found during the new observations, we can then look back at the considerable data set of previous observations, to confirm that they are real.

If the campaign does detect an exoplanet, what characteristics of the exoplanet do you think we will be able to determine from the observations and what follow up observations would you recommend?

If an exoplanet is detected by the radial velocity method some of its orbital parameters will be determined, in particular the period and eccentricity of the exoplanet. We also get a lower limit on the mass of the exoplanet relative to the star. It is a lower limit only because we do not know the inclination of the orbit. If the exoplanet happens to transit across the disk of the star, then we will know that the orbit is edge on, and thus we will obtain the true mass of the planet, as well as its radius relative to the star. Together these give us its mean density, from which we can tell something about its composition (mainly gaseous, mainly rocky, or something in between?). However, only a small fraction of exoplanets transit, so we’d have to be particularly lucky. 

For larger planets, with an extended gas envelope, observing the transits in multiple wavelengths can enable us to probe the composition of its atmosphere. However, for an Earth-like planet, even with the best instruments available, such as the future James Webb Space Telescope (JWST), this kind of measurement may not be possible: its atmosphere may be too thin, and the exoplanet may be too cool to see features in its spectrum during a secondary eclipse (when the planet passes behind the star). We may have to wait until we are able to image the exoplanet directly, by blocking out the light of the star using something like a coronagraph. Some JWST instruments are equipped with a coronagraph, and there is also a project to launch a “starshade” which would act as an external coronagraph for JWST (a project call the New Worlds Explorer). If we were able to isolate the light of the planet, we could then extract its spectrum and learn about the temperature and composition of its atmosphere.

What do you think the impact of finding an Earth-like exoplanet around Proxima Centauri or other stars near to the Earth could be?

This would be a major discovery. It would confirm that such planets must be very common, as we already suspect based on statistical results derived from the Kepler mission: Dressing and Charbonneau (2013) estimated 40-50% of M stars have at least one Earth-sized (up to 1.6 Earth radii) exoplanets in their habitable zone. It would give us an extra impetus to search for more exoplanets in the solar neighbourhood, and to invest in the technology we need to study them in detail. Knowing there are many other worlds potentially like our own in the Galaxy would also change the way we think of our own place in the universe.

What do you think the chances are that, if an exoplanet is detected, it is similar to Earth and that it may harbour life?

If Proxima Centauri hosted any planets substantially more massive than the Earth in its habitable zone, they should have been detected already during previous observations. Therefore, if a new planet is detected as part of the Pale Red Dot campaign, it is likely to be similar in mass to the Earth. Whether such a hypothetical exoplanet might harbour life, though, is very hard to know. Existing models of planet formation, and data from the Kepler satellite and associated RV follow-up, suggest that planets below 1.6 Earth radii are likely to be rocky and to have thin atmospheres, akin to Earth. Larger planets tend to have larger gas atmospheres and may be more like Neptune than the Earth. So if the planet’s mass was small enough, it would probably be rocky. But whether it would also have developed life – that is anyone’s guess! 

The Pale Red Dot Campaign is one of several campaigns aimed at detecting exoplanets. Can you comment on any ongoing campaigns which you are particularly interested in and which may help to detect exoplanets in the solar neighbourhood?

There are very many exoplanet campaigns ongoing or just about to start—too many to list—and many of them are exciting. Two that I am particularly interested in at the moment, are the K2 mission and the TERRA Hunting Experiment (THE) at the Isaac Newton telescope (INT) in La Palma. K2 is the Kepler “Second Light” mission and uses the Kepler space observatory. What particularly interests me about K2 is that it is observing some nearby young open clusters, which are groups of young stars that formed out of the same cloud of gas and dust. This represents our first opportunity to search for young transiting planets and directly learn about their early evolution. THE is an experiment, proposed by Didier Queloz from Cambridge University, which would involve installing a high precision radial velocity spectrograph called HARPS3 (a copy of HARPS and HARPS-North) on the INT, and upgrading the telescope to be fully robotic. This instrument would then be used in a long term campaign (5-10 years) to observe a small number of nearby solar type stars every day. The use of a dedicated instrument over such a long period of time will greatly improve the chances of finding Earth-like exoplanets round these stars. 

Kepler's 2 raw light curves contain a great deal of instrumental effects. Joint analysis of many stars combined with dedicated data-analysis techniques is necessary to enable the detection of the faint signals of transiting planet candidates.
Figure 2. Example of raw light curves as downloaded from K2’s. Kepler’s 2 raw light curves contain a great deal of instrumental effects. Joint analysis of many stars combined with dedicated data-analysis techniques is necessary to enable the detection of the faint signals of transiting planet candidates, which have a characteristic box looking shape. Source : S. Aigrain group website http://splox.net

What do you see as the future for exoplanet detection and characterisation over the next ten years?

The first two purpose-built instruments for direct detection of exoplanets on large telescopes, SPHERE on the Very Large Telescope (VLT) and the Gemini Planet Imager (GPI) on the Gemini telescope, have just started large surveys for young, massive planets on wide orbits around nearby stars. In the next two to three years JWST, TESS (the Transiting Exoplanet Survey Satellite) and ESPRESSO will all come online, improving our ability to detect exoplanets and measure their properties from space and from the ground. The ongoing GAIA astrometric mission should also find many high-mass (>Jupiter-mass) long-period exoplanets in the solar neighbourhood, and the small photometric satellite CHEOPS will search for transits of previously known planets. 

By 2025 the PLATO space mission will search for exoplanets among relatively bright stars with the aim of being able to detect Earth sized planets in the habitable zone around solar-like stars. The European Extremely Large Telescope (E-ELT) with its large aperture will be able to directly detect exoplanets, down to perhaps rocky sized exoplanets, and using high resolution spectroscopes will be able to record a number of their spectra. In the very long term, the ultimate goal is to be able to directly image and take spectra of Earth-like planets around Sun-like stars, and search for signs of biological activity in their atmospheres. This will require a large, space telescope with a state-of-the-art coronagraph, as recently proposed for example in the High Definition Space Telescope (HDST) report.

Photo

About the interviewee. Professor Suzanne Aigrain is head of a research group at the University of Oxford which focuses on the detection and characterisation of exoplanets and their host stars. She was born and educated in France and moved to the UK for her undergraduate studies, where she has remained since, except for a 6-month spell at the European Space Agency’s ESTEC research centre in the Netherlands just after finishing her undergraduate degree at Imperial College London. She completed her PhD on Planetary Transits and Stellar Variability at the University of Cambridge. Since then she has held post doctoral positions at Cambridge and lectureship positions at the Universities of Exeter and Oxford. During this time she has worked as a Co-Investigator or Participating scientist on major collaborations including CoRoT, Kepler, K2, TESS and PLATO. Her research group’s website is www.splox.net and she occasionally tweets as @AirborneGrain.

M dwarf planet search with today\’s spectrographs and tomorrow\’s spectropolarimeters

Proxima Centauri is a star in the main sequence of very low mass; a so-called M dwarf or red dwarf. Let me introduce this population of stars. Red dwarfs are the most common in the solar vicinity and in the Galaxy. They outnumber the stars similar to the Sun by a lot: about 70% of the total stars in the Milky Way are M dwarfs. They are also fainter optically, so that none of them can be seen with the naked eye, even if they are closer than 10 light-years from our solar system. M dwarf stars cover a range in mass from 0.08 to 0.5 solar masses, and are roughly classified from M8 to M0 depending on their spectral properties. In this range, their effective surface temperature grows from 2,300 to 3,500 Kelvin, their radius from 70,000 to 370,000 km and their luminosity from 1/10,000 to 2/100 the solar luminosity. As they consume their hydrogen more slowly than more massive stars, their lifetime is longer and thus, most of our red dwarf neighbors are old stars.

histo
Distribution of stellar types in the Milky Way (in number, not in mass): the red dwarf stars represent the vast majority of stars in the Galaxy and in the solar neighborhood. Credits: Claire Moutou

Searching for exoplanets specifically around red dwarfs has been a trend for a long time. First, because the low stellar luminosity and the proximity to Earth make them good targets for direct imaging, a technique where contrast and angular separation are the limiting factors. Second, for the more favorable conditions which exist for indirect methods: for a given planet mass or radius, the signal expected from the planet is larger when this planet orbits a low-mass, low-radius star. The radial-velocity semi-amplitude expected from a planet orbiting a Proxima-Centaurus like star is 4 times larger than if the same planet, with the same orbital period, orbits a Sun-like star. And third, the habitable zone of a red dwarf corresponds to short orbits of several weeks to months (compared to typically one year for the Sun). So anyone searching for small planets in the habitable zone of a star would search around red dwarfs, just for the sake of boosting the signal over the ever-present instrumental and stellar noise.

This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d may be located inside the habitable zone around its low-mass planet host star. Based on a diagram by Franck Selsis, Univ. of Bordeaux. Credits: ESO

Well, what about the stellar “noise” of red dwarfs, or rather, their magnetic activity? Such subjects are still open to active research and lively debate. Activity is characterized by many related phenomena: spots, magnetic field, rotation, convection, chromospheric flares, granulation, and oscillations… The first—spots—has the most immediate effect on the star: the stellar surface is not homogeneously bright, but spotted with dark features whose lifetime, location, contrast and size may vary strongly from star to star and which rotate at the star spin rate—50 to 100 days is a typical rotation period for an M dwarf. This timescale is, unfortunately, very similar to the period of a planet located in the habitable zone of such a star. This makes room for mistakes. A 50-day rotating star with two main spots separated by 180o would have a radial-velocity curve quite similar to a non-spotted star with a low-mass planet in a 50-day orbit. Such ambiguous configurations have happened in the past and were resolved by additional diagnostics of activity. But were they all found and solved?

Artist impression of an active red star orbited by an exoplanet. This star has a huge stellar spot and several surface eruptions, both phenomena evolving with time: how can we distinguish the tiny planet signal, with variations on similar timescales? Credits D. Aguilar, CfA.

Activity in a more general sense evolves over the lifetime of the star and depends on its mass, which controls the depth of the external convective envelope where many of these mechanisms emerge. The range of M dwarfs contains an interesting boundary where a star can have a partially convective envelope and a radiative core (if it is above 0.35 solar masses) or a fully convective structure (if of lower mass). The dynamo mechanism, the origin of most activity features at the surface of a star, is altered by various convection conditions; magnetic topologies of red dwarfs are strongly different from solar-like topologies. This research, led by Julien Morin and Jean-Francois Donati, is still in their infancy (with about 20 M-dwarf stars for whose magnetic topology is characterized so far), but promises to reveal much about stellar activity phenomena and their impact on exoplanet searches.

I mentioned earlier that none of the red dwarfs, although so close-by, were visible with the naked eye. Isn’t that a bummer for observation? Searches for tiny signals usually crave photons, many photons, as many as possible. So is it feasible to get enough photons from red dwarfs to search for planets anyway? HARPS observes in the optical domain and it is clear from previous results, from Xavier Bonfils and his team, that at least some red dwarfs are bright enough to unveil their super-Earth exoplanets companions. But most of them will be too faint, especially the least massive ones. The peak of their emission lies at longer wavelengths, and it is far more favorable to observe red dwarfs in the near-infrared domain, at wavelengths up to 2-3 microns. This spectral domain offers other challenges, like detector sensitivity, but again, this is not the extreme 10cm/s level that is at stake here, but the 1m/s precision level. New instruments are coming online aiming at this radial-velocity precision in the near-infrared, precisely to explore the vastly unknown world of red dwarf exoplanet companions.

Artist impression of the system  Gl 581, another red dwarf whose exoplanets where unveiled partially by HARPS. Credits: ESO

As I write, on the slope of the Maunakea Observatory in Hawaii, we are awaiting one of these new instruments. SPIRou will be mounted on the Canada-France-Hawaii 3.6m Telescope (CFHT) located on top of Maunakea. It will be the first planet-hunting instrument of CFHT. SPIRou combines the near-infrared domain, necessary for M dwarf studies, with the “radial-velocity-spectropolarimetric” capability of its spectrograph. It will be able in a single glance, to get a precise radial velocity of the observed star, and its instantaneous magnetic imprint. By monitoring stars over their rotation and planets along their revolution, the SPIRou team will be able to disentangle stellar activity and planet signals from the very direct diagnostics of the ultimate culprit, the magnetic field. It is not as easy as it may sound, though. The magnetic field can trigger multiple phenomena at the same time—say, a large rotating starspot and a short-lived chromospheric flare—while the SPIRou measurement will record an average of the stellar surface and lose the important spatial information. We only have spatial information available for our Sun—all other stars are point-like and their mean activity is the only feature we can try to correct for. Regardless, SPIRou will combine the Zeeman effect (distortion of stellar lines due to the magnetic field) with the Doppler effect (spectral shift due to velocity) and enhance the detectability of habitable Earth-like exoplanets. In a few years, it will observe the most nearby red dwarfs and search for their Earth-like companions. Keep tuned!

The Canada France HWawii Telescope.
The Canada-France-Hawaii Telescope located ontop of Maunakea in Hawaii, future host of the SPIRou near-infrared spectropolarimeter. Credits: Claire Moutou.
About the author

cmoutou_claire

Claire Moutou is a resident astronomer at Canada-France-Hawaii Telescope (CFHT) in Hawaii since 2013 and a director of research for the French Centre National de la Recherche Scientifique (CNRS). After a PhD thesis in Paris, she has been working in Observatoire de Haute Provence, European Southern Observatory and Laboratoire d’Astrophysique de Marseille. She has been involved in the search for planets using various methods, from space transits with the European mission CoRoT, to radial velocities with HARPS and SOPHIE, and direct imaging with the ESO/SPHERE instrument. She has explored observationally the interactions between stars and planets in extremely close orbits and the magnetic properties of stars hosting planets. Since 2013, she is the CFHT Observatory Scientist of the SPIRou instrument.

How a Star Can Hide its Earths

Autor: Xavier Dumusque, Observatory of Geneva

Let’s consider that we want to find a planet extremely similar to Earth, meaning that it’s orbiting a star similar to the Sun, with a mass equal to Earth and an orbital period of one year. Let’s also consider that we want to detect this object by measuring the gravitational effect it induces on its host star. I know what you are telling yourself; “No way, the Sun is so massive it doesn’t move!”. You are right, the Sun is extremely massive, in fact 300,000 times more than the Earth, but let’s stick to physics here. The laws of gravity (thank you Newton!) tell us that all object with mass will interact with each other, so the Sun should move. But the question is by how much? Plugging the numbers in, we arrive at a maximum displacement of the Sun’s center of 500 km over a 6-month period. This means that its displacement will be 1,500 times smaller than its radius, and that the maximum velocity the Sun will reach will be only 0.3 km/h (or the velocity of turtle going out for a walk). I agree with you that this is extremely small, but still, it moves!

Displacement of the Sun’s center induced by the gravitational pull of all the planets in the Solar System as a function of time. The Sun moves by about a solar radius, and  the major contributor to this displacement is the most massive planet of the Solar System, Jupiter (CC, Carl Smith’s derivative work)

Let’s now imagine that we build the perfect instrument to measure the tiny effect induced by an Earth twin on its host star. This instrument should therefore be capable of detecting a velocity of 0.3 km/h on a star that is a hundred thousand billion kilometers away. I do not want to go into the details here, but let’s use an analogy to get a feeling of the difficulty we are facing. Imagine that this perfect instrument is a ruler, and you want to measure the width of an object with the same precision that is required to detect an Earth twin. The required precision is 10,000 times smaller than the smallest graduation of the ruler. Not so easy right? With good eyes, you might probably get down to 1/3rd or 1/4th of the graduation, but 1 in 10,000! Today, the best instruments we use are capable of a precision of 1 in 1,000 (see HARPS and HARPS-N). We are therefore capable of detecting a planet ten times more massive than Earth if the host star is similar to the Sun and if its orbital period is one year. At the University of Geneva, where I am working, scientists are now developing a new instrument, called ESPRESSO, that will have the precision required to detect Earth twins.

Let’s now imagine that one year from now ESPRESSO can be used (this is the real timeline), and we start observing several stars to search for Earth-twins. To be confident in a detection, we need to observe at least one full orbital period of a planet, or one year in this case. If these Earth-twins exist, and we are confident there should be a lot of them out there, we should detect a Holy Grail planet before 2020. But—wait a minute!—several things can go wrong here, and I want to just highlight the biggest problem we have nowadays. This big problem is the stars.

Let me try to explain to you how stars can mess everything up. Everything starts with the Doppler effect. A fancy name that physicist like to use, but if you didn’t study physics, you probably do not know what this means, or you heard the name in high school and now it’s forgotten forever. But most of you have already encountered the Doppler effect in real life. One day you were probably walking down the street, when suddenly an ambulance passed by. You could hear the vehicle from far away with its strident siren—you focused on the sound created and could easily hear the pitch of the siren, but once the ambulance passed by you, the pitch changed. Did the driver push a button at the same moment he was passing by you? Probably not. To be sure, you asked other people in the street if they also had the same impression (well, in real life people would have wondered, “who is this weirdo?”, but this is a mental experiment; you can be as weird as you want). And yes, they all confirmed that this phenomenon happened at the moment the ambulance passed them by—confirming that the driver was not playing around. What happened is simply that before overtaking you, the ambulance was moving toward you; whilst after, it was moving away from you. And because sound-waves progress through the air with a limited speed, the difference between the velocity of the ambulance before and after passing by you creates this difference in tonality.

The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star - and so, measure its velocity - one can see if it moves periodically due to the influence of a companion. Image credits : ESO
The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star – and so, measure its velocity – one can see if it moves periodically due to the influence of a companion. Image credits : ESO

Now you know what the Doppler effect is, but what does the ambulance have to do with what we are speaking about here—stars and planets? Well, stars emit light, and because light also has a limited speed (thanks Albert Einstein!), a similar effect will occur. Without entering into too many details, objects emitting light that are moving towards you will appear bluer (or blue-shifted), and objects moving away from you will appear redder (or red-shifted). This Doppler effect is at the origin of the radial-velocity technique used to detect planets. If a star is moving towards you, then away, and continues to do so in a periodic way, this motion is most probably induced by a planet orbiting the star. Another famous example of the use of the Doppler effect in astrophysics is the measurement of the Universe’s expansion. Looking at all the galaxies surrounding us in the Universe, we observe that their light is redder than it should be, therefore all the galaxies in the Universe are moving away from each other; the conclusion being that the Universe is expanding.

I told you that the biggest obstacle to the detection of Earth-twins is the host stars themselves. So let’s come back to this problem. Stars are formed by the contraction of giant molecular clouds, therefore by applying the concept of momentum conservation, you arrive at the conclusion that the stars are rotating around their center, like an ice-skater bringing his arms towards his chest to accelerate his spin. Given that the Sun has a 25 day rotation period and a radius of about 500,000 km, you can do the math and calculate that the rotation velocity of the Sun at its surface is 7,200 km/h. Therefore, looking in detail at the Sun, you will see that the light coming from the approaching limb is bluer than it should be, and the light from the receding limb is redder; do you remember the Doppler effect? So, does this mean that we see half Sun moving forwards and half moving backwards because of the rotation? Yes, it’s exactly what it means, but as the blue and the red shifts are equivalent, the average velocity is zero. This makes sense as the Sun only rotates around its axis, and does not move towards or away from you.

Now, you probably know that the Sun often has dark spots on its surface, so-called sunspots. These sunspots are caused by strong magnetic fields present inside the Sun, that sometimes emerge at the surface. Because they are dark, sunspots can be seen as a mask occulting, or blocking, part of the stellar disc. Therefore, they distort the red- and blue-shift balance; the Sun will appear a little redder (or bluer) and you could mistakenly conclude that it is moving. Considering a large spot on the Sun, that is around 0.1% of the surface area, and a maximum rotational velocity of 7,200 km/h, we arrive at the conclusion that such a sunspot induces a radial velocity effect of 7.2 km/h, which is an order of magnitude larger than the 0.3 km/h required to detect Earth-twins.

Doppler velocity map of the Sun as observed by the MDI instrument on board the SOHO satellite (left image). A black dot was introduced to simulate a sunspot as observed in the solar surface (see  a real sunspot observed by SOHO on the right). The Sun’s rotation produces equivalent blue- and red-shifted hemispheres, but this balance can be broken by a sunspot. On the left image for example, the black dot masks part of the blue shift of the star, so the final flux of the whole star will appear redder than it should be (Credit: SOHO/MDI).

In conclusion, even with an instrument reaching the precision required to detect Earth-twins, perturbing signals induced by stars, such as the effect of sunspots, will significantly complicate their detection. We have been aware of the sunspot problem for nearly 20 years now, and have discovered other stellar effects more recently. Many scientists are trying to understand better these perturbations, and are looking into new techniques to correct for them. I am one of them, and I am convinced that we will manage to solve this critical problem of stellar signals in the coming years.

Xavier2
Dr. Xavier Dumusque.

About the author.

Dr. Xavier Dumusque’s expertise is planet detection taking into account stellar intrinsic signals. Xavier studied Astrophysics at the University of Geneva where he also obtained his PhD in 2012, in collaboration with the University of Porto. After two postdocs at the Harvard-Smithsonian Center for Astrophysics (USA), he came back to the Observatory of Geneva where he is currently working. He is the first author of the Nature article announcing an Earth mass planet orbiting Alpha Centauri B (2012) and of an article presenting the discovery of the Mega-Earth planet orbiting Kepler-10 (2014). Xavier is actively involved in the development of a solar telescope that will help characterize and understand the origin of perturbing signals in the Sun to develop new state-of-the-art techniques to mitigate their impact on the detectability of Earth-twins orbiting other stars. Among the awards he has obtained we highlight the Schläfli Prize for outstanding thesis (Swiss Academy of Science, 2014), the Yale Center for Astronomy and Astrophysics Postdoctoral Prize fellowship (2015), and the Branco Weiss fellowship (2015).