All posts by Pale Red Dot

Equator crossing! Live acquisition of 30-th spectrum with HARPS

We acquired the 30th spectrum with HARPS on February 27th at 8.40 UTC from La Silla. James Silverster was the observer at La Silla and he send life updates of the moment. His team is using HARPS in polarimetric mode to make measurements on stellar magnetic fields. We will have an article at palereddot.org from them soon, so you can see how stellar magnetic fields can be measured in practice.

Here is an extract of the twitter and Facebook live feeds of the event:

08:30 UTC :All green lights at ESO Astronomy La Silla. One more spectrum and we will reach 30/60! Standing by!

LaSillaDimm
Meteo monitor information. Image quality looks really good, seeing <1" (bottom right panel, the seeing tells you how 'point-like' a star would look through a telescope). 1" arcsecond would be awesome in any decent observatory. For the high chilean observatories (La Silla, Paranal, Las Campanas, Gemini-S, etc.), <1" is routinely achieved, which is why the high Chilean mountains are such a good place for astronomy, besides being dry and mostly cloudless.[/caption] 08:45  UTC : @jimmysilvers > @Pale_red_dot Photons are being collected as we speak!

08:46 UTC : @Pale_red_dot > Cool! Send us a picture!!!

08:58 UTC : @jimmysilvers > here you go!

harps_fibre

08:50 UTC : @Pale_red_dot > Yesss, that’s a live image of Proxima’s photons being sucked by HARPS science fibre (dark central hole). Awesome

08:55 UTC :While we wait for the integration to end, how about a stroll and a look at the sky from La Silla? Thx @jimmysilvers

[caption id="attachment_1251" align="alignnone" width="474"]Southern sky at the end of February from La Silla observatory. Credits  : James Silvester Southern sky at the end of February from La Silla observatory. Credits : James Silvester

09:15 UTC : Integration complete. File saved, stored, and shipped to ESO’s HQ at Garching. Thanks James for sharing these moments.

The raw file reaching ESO's archive at Garching marks the end of a successful observation.
The raw file reaching ESO’s archive at Garching marks the end of a successful observation.

09:20 UTC : @Pale_red_dot > success! 50% data collection achieved! #palereddot

Status update Feb 27. One bonus spectrum was obtained on Feb 25.
Status update Feb 27. One bonus spectrum was obtained on Feb 25.

09:30 UTC : Bedtime for the observers. Really high success rate so far. Lots of updates and articles next week! thx for following

The Hunt for Free Floating Planets in Our Galaxy

By Dante Minniti, Universidad Andrés Bello

Extrasolar planets is the name given to all the planetary-mass bodies that orbit stars other than the Sun. So far, many extrasolar planets have been discovered, among them are stars that harbour multiple planets. The confirmed record so far is the Sun with 8 well known planets, but we suspect that there may be many more planetary systems like ours in the Milky Way, and there is evidence that the star HD10180 has 9 planets orbiting it.

But even if the planets form around a star, they can be expelled from their parent system, and so there could be isolated planets out there, wandering the vacuum unattached to a parent star. We call these bodies, free floating planets, the worlds that roam free through our Galaxy.

Why do we suspect that there should be free floating planets out there?  Because of the way that planetary systems likely form and evolve. Many protoplanets form in a protoplanetary disk and then they go through a phase of collisions and rearrangement until the final planetary system is stable. While some coalesce to form giant planets and others fall into the star and are destroyed, we expect that many are expelled from the system completely. In fact, the number of free floating planets in our Galaxy might surpass the number of planets associated with stars. We simply do not know…

It is easier to form planets in the outer parts of a protoplanetary disk, where the objects are more loosely attached to the star and prone to be freed from the gravitational pull of the mother star by close encounters with other planets in the forming disk, or even by close passages with neighbouring stars (not collisions, which should be very rare among stars).

ffloatingearthArtists impression of an Earth-sized free-floating planet roaming through our galaxy.  Credit: J. B. Pullen.

It is also important scientifically to know how many of these objects are out there, in order to test our ideas (theories) of planetary formation. Is it the same number of free floating planets as the number contained in planetary systems? Or are they very rare, say 10 times less? Or are they much more numerous, 10 times more numerous for instance? And also, no less interesting, are small rocky free floating planets more numerous than giant gaseous free floating planets?  The last possibility is sobering, as some theories of Solar System formation predict that many more planets were formed in the early Solar System, the majority of which were expelled through collisions.

We know that there should be free floating planets because a few are being detected in young nearby associations. Very young, recently formed planets, emit light just as stars do for a brief period of time (astronomically speaking a few million years), before they cool and become too faint to detect from our vantage point with current technology. These bodies are very young giant planets, and although faint, they can be detected in the infrared before they are older and their surfaces cool down.

But we think that there should be much more free floating planets out there waiting to be discovered. The main problem to find this putative free floating planetary population is that older planets like Earth are small and dark, they do not emit light for themselves. They are so faint that is is impossible to detect them using images in the optical and near-infrared taken with the largest available telescopes (VLTs, Kecks, Geminis, etc), or even with future telescopes (JWST, GMT, TMT, EELT). Some of these planets may emit in radio wavelengths, but again they would be so faint that they are beyond detection even with our most powerful radio telescopes like Arecibo or ALMA.

We are stuck, we cannot detect these faint free floating planets directly. However it is important to know if there are free floating planets out there, and how numerous they are.

But don´t give up hope yet, astronomers sometimes can find clever solutions to seemingly impossible problems. There is a technique that is indirect, because it cannot image the planets, but can detect the gravitational effect cause by their mass. The father of this idea was Einstein, who else. He predicted that the light from a distant object would be bent when it passes close to a massive object. This is because the massive objects produce a deformation of space and time.  This is called the gravitational microlensing technique.  It measures the brightening of a distant source due to the light bending caused by a massive lens that passes in front of it.
The microlensing measurement simply consists of detecting the change of brightness of a distant object (called the source) by the near-perfect alignment of a massive object (called the lens) along the line of sight of the observer (us, with our telescopes).

vvv_microlensing1An example of a microlensing event that was witnessed by the VVV survey, where the star’s brightness is shown with time.  The event is seen towards the right end of the plot where the quick rise and fall in brightness of the observed background star is due to another object passing by between the star and the observer on Earth.

However, it is not that simple. The timescales of microlensing due to free floating planets should be very short, of the order of hours, as opposed to weeks-months for typical stellar mass objects. This timescale is the critical measurement, but if we are able to measure these short-lasting microlensing brightenings, we could detect planets like Earth, or less massive like Mars or even smaller. This is a very difficult measurement because it essentially requires continuous imaging of tens of millions of stars. You can imagine that these microlensing observations create gigantic databases and technical challenges.

A few groups have been pioneering in this field, discovering thousands of microlensing events so far (OGLE, MOA, MACHO, etc.). However, the short timescale microlensing signals due to our putative free floating planets are lacking. Is it because they do not exist, of because the current experiments are not designed to detect such short timescales? We believe the latter. Even though the evidence for free floating planets is slim at present, we believe that they should be very numerous, and we need to refine out hunting tools to discover them.

Very soon, the K2 mission gives us hope to discover these free floating planets towards the central regions of our Galaxy, the bulge. This extended phase of the Kepler space telescope will observe fields towards the Galactic bulge, where the number of stars is high, with a very frequent and continuous cadence, in order to be able to detect microlensing due to free floating planets.

On the other hand, we have been running a survey of the Milky Way´s bulge over the past few years, called the VVV survey (vvvsurvey.org), using the VISTA near-infrared telescope located at ESO Paranal Observatory. It would be wonderful to do simultaneous near-infrared imaging of the K2 bulge fields in order to characterise the sources. The combination of optical and infrared data is very powerful to measure stellar properties. Knowing the stars parameters (luminosities, distances, reddenings, etc.) would allow us to characterise the rare microlensing event due to a free floating planet, and ultimately to measure the mass of the planet. If many of them are discovered, we could potentially refine the statistics in order to know if the free floating Earths are more numerous than the giant planets, among other things. The K2 bulge observations will be carried out in April-May this year, so stay tuned for the results on free floating planets!

vvv_galaxy2The region of our Milky Way galaxy that has been explored by the VVV survey, including coverage of the galactic center.

Finally, let´s go crazy and discuss life in free floating planets. This is pure speculation but I will contend that these objects are some of the most promising life vessels. As they travel across the Milky Way they can spread the seeds of life throughout the Galaxy. The main difficulty for a free floating planet to sustain life as we know it would be the absence of the main energy source: its parent star. However, when Earth-like planets form they are very hot, and then the igneous rock cools on the surface. The cooling process takes billions of years, depending on the original mass of the planet. For example, our Earth is 4.5 billion years old and it has not cooled down completely yet. Evidence of this are the volcanos throughout our planet´s crust, the plate tectonics, and the fact that seismology indicates that our planet nucleus is still igneous.

This volcanic activity potentially provides a continuous alternative source of energy for life in a free floating planet. This should not be surprising, as geothermal vents deep in the oceans sustain a variety of living organisms. In fact, if Earth would lose the Sun for some reason, the oceans would freeze but only on the surface, deep down they would remain liquid for many billions of years.

This is different for asteroids or minor planets, which can also be roaming through the Galaxy, but which are cold objects. Seeds essentially there have to survive the cold vacuum and hibernate for billions of years in order to be a viable source for spreading life, which is more difficult to sustain. Free floating planets constitute better living vessels, and they could be out there by the millions (or billions!).

For those who love to let the imagination run wild, consider the following speculation. A sufficiently advanced intelligent technological civilisation may chose to free its own planet from the parent star on purpose. This would allow them to cruise free through the Galaxy. In fact, in several billion years more our Sun will start to inflate in order to become a red giant star, engulfing the innermost planets of the Solar System in the process. In order to preserve our planet, we may want to liberate it from our mother star before it becomes a red giant. That is if we have the adequate technology in the future (the energy required to do this is enormous), and if we decide not to lose our Earth, where all Humanity´s initial history has developed. We could then decide to leave our mother Sun, becoming a free floating living Earth. It would be either this, or let Earth roast completely, because the Sun will inexorably become a red giant. To travel through our Milky Way in our own Earth is a dream future for an astronomer. And I don´t know about you, but I would like to preserve our heritage in this way…

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About the Author. Dante Minniti is a Full Professor at the Universidad Andrés Bello in Chile, and Adjunct Scholar for the Vatican Observatory, Italy. He did his undergraduate studies in Astronomy at the Universidad Nacional de Córdoba (Argentina), and obtained his PhD in 1993 at the University of Arizona (USA).  He was a Postdoctoral Fellow of the European Southern Observatory (Germany) in 1993-1996, and a Lawrence Livermore National Laboratory Postdoctoral Fellow (USA) in 1996-1998.  Dante has been a member of the MACHO Team since 1996, and leader of the VVV Science Team since 2006. He was awarded the John Simon Guggenheim Fellowship Prize in 2005, and the Scopus Prize in Physics and Astronomy in 2008. In 2012 he was appointed a Member of the National Academy of Sciences of Argentina.  His broad research interests are: Extrasolar Planets, Astrobiology, Globular Clusters, Stellar Populations, Stellar Evolution, Gravitational Microlensing, Galaxy Formation, and Galactic Structure.  He has authored more than 300 refereed publications, that have accumulated more than 12000 citations in the scientific literature to date, whilst also authoring the recent books “Mundos Lejanos”, “Vistas de la Galaxia”, and “Nuevos Mundos”.  He is the deputy-PI of the “Millennium Institute of Astrophysics (MAS)”.

What is a planet?

Detections of planets orbiting nearby stars naturally require establishing what is meant by a “planet” and how do we know a planet when we see one? This question is not as easy to answer as it is to pose. To understand the difficulties, it is useful to examine the answer to: “Is Pluto a planet?”—a simple question without a simple answer.

Astronomers like categorising objects of astronomical and astrophysical interest, giving them labels such as “planets”, “stars”, “asteroids” and many more. According to the decision of the International Astronomical Union (IAU), Pluto is classified as a “dwarf planet” because it is not the “gravitationally dominant body” on its orbit. Setting aside for a moment what that actually means, it is, however, pretty obvious based on the recent imagery from the NASAs New Horizons mission that Pluto is certainly a world in its own right – something that we could be very tempting to call a planet regardless of what the IAU has decided.

According to the IAU, a planet is defined as follows:

  1.   A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

  2.   A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

  3.   All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

Although Pluto is in orbit around the Sun and has sufficient self-gravity to assume a roughly spherical shape through hydrostatic equilibrium, it has not cleared the neighbourhood of its orbit by gravitationally tugging smaller objects out of its residential area in the Solar System. IAU therefore decided to classify Pluto as a dwarf planet rather than a full-blown member of the set of planets in the Solar System.

The situation gets more complicated when remembering that Jupiter, for instance, has also failed to clear the neighbourhood around its orbit, as there are thousands of trojan asteroids at and around Jupiter’s orbit. Yet, nobody disagrees whether we should classify Jupiter as a planet or not.

But with respect to extra-solar planets, the IAUs resolution is not valid – it has only been designed to be valid when classifying celestial bodies in the Solar System.

Astronomers consider extra-solar objects that orbit stars other than the Sun to be planets if they (1) are large enough to have reached hydrostatic equilibria, and (2) small enough such that they cannot sustain nuclear fusion in their cores and thus cannot be considered stars or even brown dwarfs. But that is only where the problems begin—it is not at all trivial to determine how large the objects orbiting other stars are when their very presence is difficult to observe.

It is reasonably straightforward to conclude that an object transiting a nearby star, such as the extraordinary haul of worlds found by the Kepler spacecraft, is large enough to have reached hydrostatic equilibrium if it is in fact large enough to be seen blocking the light coming from the stellar surface. But what about the larger objects that are comparable in size to Jupiter? Because the planetary transits can only reveal their radii in relation to their host stars, it cannot be known whether some of them are in fact more than roughly 13 times more massive than the Jupiter, which is sufficient for the fusion of deuterium into helium in their cores. Such objects would then be classified as brown dwarfs rather than planets.

The situation is even more complicated, sometimes frustratingly so, when observing exoplanets with the Doppler spectroscopy technique applied in the Pale Red Dot campaign. Because this technique can only be used to reveal the lower limit for the planetary masses, it is impossible to tell whether any individual discovery actually corresponds to a genuine planet rather than a small star or a brown dwarf even though, on statistical grounds, the vast majority of them are certainly small enough to be considered planets.

But whether extra-solar objects of suitable size to be classified as planets have cleared the neighborhoods of their orbits is beyond our observational capabilities. It is also less than certain what the possible free-floating planetary sized objects should be called as they do not revolve around stars of any kind.

It is quite possible that a general definition of a planet proves as elusive as that of a “continent” that no geographer dares to define—nor are they even interested in doing so. Similarly, biologists cannot produce a general definition for “life” but more often than not simply say that “they know whether it is alive or not when they see it”, which can be seen as an attempt to brush the problem under the carpet. It is probably a human trait to attempt classifying things into rigid categories even when nature has cynically decided that there is simply a continuum of objects and that any and all classifications are thus only subjective opinions without any deeper meanings. In such cases, the definitions do not help in understanding nature any better—and may even hinder scientific developments by providing a biased frame of reference.

And Pluto, as it – in my opinion – certainly is a world in its own right, deserves to be called a planet regardless of any subjective definitions individuals might consider appropriate. Although that might not be acceptable for all, one thing is clear. If an object resembling Pluto was found orbiting a star other than the Sun, I believe it would be called a planet.

Mikko Tuomi
Mikko Tuomi

About the author. M. Tuomi is working as an astronomer at the University of Hertfordshire, UK. His research interests include detection and characterization of low-mass planets and planetary systems around nearby stars, development of statistical models or Doppler spectroscopy data to understand variability caused by astrophysical and instrumental effects, study of the dynamical properties of tightly packed planetary systems, and exploring the
statistical properties of small planets orbiting the stars in the Solar
neighborhood. He has also worked as an environmental scientist at the Finnish Environment Institute. M. Tuomi is one of the editors of palereddot.org, and can be blamed for first spotting tentative evidence of ‘The signal’ in archival UVES and HARPS data.

‘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.

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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.

Inspiring the public with exoplanet discoveries

As astronomers we have a fantastic subject for sharing with a non-specialist audience. It is not only of wide-scale general appeal but there are regular discoveries which capture the public’s imagination. Over the last 20 years exoplanet research has delivered some of the most exciting and inspiring results.

In 1995 the first exoplanet orbiting a Sun-like star was discovered, 51 Pegasi b. It was discovered just before I went to University to study a degree in Astrophysics, so I have a close attachment to it. It provided a fantastic starting point for me to talk about science in social settings (something which is often seen as anathema). My very first public talk was in 1996 about this new science of exoplanets.

Press Coverage

The first impression many of the public receive about a scientific discovery is through press coverage. This is a particularly important in keeping public interest about a long lasting or extended research project or even space mission.

NASA’s Kepler was highly successful in attracting media attention as was prolific at finding exoplanets. Their policy of releasing information about planetary candidates before they had confirmed if there really was the signature of an exoplanet in the data, may have helped this. In any case, Kepler is by far the most successful mission at finding exoplanets, boasting over 1000 discoveries (at time of writing).

Some of the most inspired exoplanet PR are the Exoplanet Tourist Bureau pictures produced by NASA’s Jet Propulsion Laboratory (which is on the verge of brand advertising). This was a very clever piece of art which not only made visually appealing pictures but ones which very simply captured a key piece of science that was known about each planet. Although not strictly related to any mission, the Exoplanet Tourist Bureau’s first 2 picture-postcards were of Kepler planets.

Recent art poster from the 'Exoplanet travel Bureau', an outreach campaign by NASA's JPL. In this poster, all planets contending to be the first ever detected exoplanet are featured. Credits : NASA/JPL
Recent art poster from the ‘Exoplanet travel Bureau’, an outreach campaign by NASA’s JPL. In this poster, all planets contending to be the first ever detected exoplanet are featured. Credits : NASA/JPL

Pitching your story to the press is tricky, as the discovery of the very first exoplanet shows. Many people think the first exoplanet was 51 Peg b but it was actually a planet orbiting around a neutron star (a dense, invisible object left over from the violent death of massive star and quite unlike our Sun), discovered in 1992. Perhaps this was too much uncertainty in the discovery, maybe the physics was not presented in a tangible way, or maybe even the name of the planet, PSR B1257+12 B, was just too obscure. For whatever reason this mysterious heavenly body is rarely attributed with the title of First Exoplanet Discovered.

Recently the International Astronomical Union (which has, amongst other things, responsibility of the naming of heavenly bodies) launched a competition to give many exoplanets and exoplanetary systems new names – Name ExoWorlds. The public could make proposals for renaming of selected exoplanets. The results were announced in December 2015 and PSR B1257+12 B has been renamed Poltergeist – an invisible entity which creates physical disturbances (representing the effect the planet has on it’s host star).

Citizen Science

After attracting some attention from the public, what happens next? As a scientist who is passionate about education and engagement, I want that audience to take their new-found interest further.

The rise of citizen science has lowered the barriers for involving anyone in scientific discovery. The approach usually requires participants to do a repetitive task which a computer finds difficult but the human brain finds easy. Crowd sourcing scientific measurement taking in this way has been pioneered by The Zooniverse, who have a project where participants are invited to search through Kepler data which had been rejected by the automatic NASA planet finding software.

During the BBC Stargazing Live TV programmes in January of 2012, the value of this method was spectacularly proved. Zooniverse partnered with BBC to launch a public campaign which resulted in over a million independent measurements and the discovery of an exoplanet – PH1b or its formal, canonical name – Kepler-64b.

Personally, I define “citizen science” slightly differently as “a large scale remotely accessible science investigation performed by non-specialists, which trains them in data analysis and also the subject area”. My definition gives citizen science more of an educational angle. This was the thinking behind a citizen science project I made, along with my then colleague at LCOGT, Stuart Lowe, called Agent Exoplanet.

'Agent exoplanet' is an example of a citizen science effort. http://lcogt.net/agentexoplanet/
‘Agent exoplanet’ is an example of a citizen science effort. http://lcogt.net/agentexoplanet/

The primary aims of Agent Exoplanet are to analyse real scientific data taken with the LCOGT network, combine your measurements with other citizen scientists, display this result and understand the science it is showing. We wanted to provide a self-contained, easily accessible platform, to maximise participation by a non-specialist audience. All of the analysis tools are built into it, from graphing data points, model fitting and make the measurements directly on the astronomical data files. While the participants will not discover new exoplanets with Agent Exoplanet, they will have a better understanding of the scientific process and about exoplanet research. You can try it yourself too!

Visualising the data

An important component of an sharing any scientific discovery is to make the information easily understandable. One of the most powerful ways to do that is with well thought out graphics.

Example for infographic for exoplanet visualization. Detection methods of all known exoplanets was made by Stuart Lowe. Source : The infographic book of space, http://cosmos-book.github.io/
Example for infographic for exoplanet visualization. Detection methods of all known exoplanets was made by Stuart Lowe. Source : The infographic book of space, http://cosmos-book.github.io/

An eye-catching and interactive visualisation of the sizes and detection methods of all known exoplanets was made by Stuart Lowe and Chris North for Cosmos: the infographic book of space. The different detection methods are colour coded and the you can explore some basic properties of each exoplanet by clicking on them.

One of the most beautiful and poetic visualisations of exoplanets I have seen was made by Alex Parker. It shows what all the currently known exoplanets would look like (nearly 2300 – many of these were just candidates when Alex made the video) orbiting around the same star, in this case the Sun. All the planets and orbits are scaled so their relative sizes and distances are appropriate for this model. They range in size from 1/3 to 83 times the radius of Earth. It is quite mesmerising.

Worlds: The Kepler Planet Candidates from Alex Parker on Vimeo.

In a week where astronomy has been in the news so much, with the momentous discovery of gravitational waves, it has shown to me that there is a huge public appetite for accessible science stories. Fortunately, for the last 20 years, exoplanets have provided a steady and varied diet of exciting discoveries, from Earth 2.0, to a diamond planet to the discovery of Tatooine. For a long time to come, I believed we will be surprised by the continued variety of exoplanets and strange solar systems the Universe provides us to study.

Eduard Gomez inside the TARDIS. It is a device from the BBC's 'Dr. Who' TV show which is both a time-machine and a starship; but it looks like a classic British telephone booth from the outside.
Eduard Gomez inside the TARDIS. It is a device from the BBC’s ‘Dr. Who’ TV show which is both a time-machine and a starship; but it looks like a classic British telephone booth from the outside.

About the author.  Edward Gomez is a professional astronomer and Education Director of LCOGT.net and honourary lecturer/adjunct faculty in the School of Physics and Astronomy. As part of his role with LCOGT he investigates novel ways to engage the public in astronomy. This has taken the form of creating citizen science projects like Agent Exoplanet, interactive educational web apps like Star in a Box, and online community events like Show Me Stars. The global education hub for LCOGT is based in Cardiff University In addition, he is part of the Schools Engagement Team and assist with the outreach of the Universit, which is funded by the Welsh Government‘s National Science Academy to run the programme Universe in the Classroom, inspiring children and teachers with Universe in a Box kits and stellar role models, across Wales. Universe in the Classrom is run in partnership with the international project Universe Awareness. Eduard is co-chair the IAU task force for children and schools, under the guidance of the Office of Astronomy for Development (OAD). Our aim is to help people in astronomically developing countries to engage with and inspire children and teachers. He also regularly appears on the BBC radio wales programmes, Science Cafe and Eleri Sion Show. He have served as guest judge for the national Debating Matters competition Currently he is working at how we make the LCOGT network accessible to the general public and what tools we need to make the most of its potential, and using the power of astronomical images to inspire people who would not normally be interested in science. He loves music and plays the lute.

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.

Cool Stars with a Magnetic Personality

by Élodie Hébrard (York University) and Rakesh Yadav (Harvard-Smithonian CfA).

There is an invisible force active on the Sun which is due to its magnetic field. You may know that magnetic fields can be produced by electrical currents. The Sun’s plasma is a highly charged fluid. Due to the combined effect of large-scale ordered motions induced by the Sun’s rotation and the chaotic boiling of the plasma, there is an ample supply of electrical currents on the Sun to sustain the magnetic fields. This process of field generation by fluid motions is known as the Dynamo mechanism.

Magnetic fields are responsible for producing very violent events called X-ray flares on the Sun. The energy which is stored in the magnetic field as tension gets released in the form of energetic electromagnetic radiation (ultra violet and X-rays). These flares also accelerate the plasma in the nearby region, which is ejected with very high velocities away from the Sun. These are known as Coronal Mass Ejections (CMEs). The energetic radiation and the CMEs form a dangerous partnership as they can gradually erode away the atmosphere (crucial for developing life) of a closely orbiting planet. You can learn more about X-ray flares in this short video.

Lets move away from our solar system and discuss the Pale Red Dot. Stars which are substantially cooler than the Sun are usually referred to as “red dwarfs” or “low-mass stars”. Proxima Centauri is one such star. But do not be fooled by their “coolness”! Astronomers have been looking at such stars for decades now—it turns out these stars are very active. In fact, they generate many more X-ray flares and CMEs than the Sun. Due to the high levels of violent events on these stars, the planets orbiting around them might encounter much more hostile environments than the planets in our own solar system. Such high activity is due to the presence of a magnetic field which is much stronger than what our Sun can produce. The high activity also makes it rather tricky to find Earth-like planets around these stars. Hey, wait a minute, how do we know that these stars have strong magnetic fields?

The best way to measure the magnetic field of a star is to use the subtle effects it induces on the light it emits. Indeed, if a star has a magnetic field, its spectrum is affected: the different spectral lines split into several components, and each component has its own polarisation (it means that the magnetic field changes the vibrational properties of the light). This splitting effect is called the Zeeman effect. Measuring polarisation in spectral lines allows a reliable measurement of the stellar magnetic field, as explained in the short video below.

So now we know how we are able to measure a magnetic field, the next natural step is to reconstruct the map of the stellar magnetic field in order to know what it looks like: a dipolar field? a toroidal field?… To do that, we use the stellar rotation! More specifically, analysing the circular polarisation in spectral lines at different times as the star rotates, we get step by step a full 2D image of the magnetic field at the surface of the star. To carry out this exercise, astronomers use a method called Zeeman-Doppler Imaging (ZDI)—based on techniques developed for medical imaging! The following animations summarise the principles behind ZDI.

As the star rotates, an Earth-based observer sees the magnetic spot under different viewing angles, and moving at different projected velocities (upper panel). The level of circular polarisation measured in a spectral line evolves consequently (lower panel). The case of a radially oriented field (as depicted by the red arrows) is depicted here. Credit: J.-F. Donati.
Same as the previous figure, but here with the case of an azimuthally oriented field (as depicted by the red arrows) depicted. As opposed to the radial field situation the circular polarisation signature flips sign. This allows ZDI to disentangle between field orientations. Credit: J.-F. Donati.

Dark starspots (similar to sunspots) are a visible consequence of
the magnetic field activity of a star. As presented in Xavier Dumusque’s article, these spots induce distortions in the spectral line profiles (because of the Doppler effect), that induce radial velocity (RV) shift. Moreover, as the star rotates and the spot is carried across the visible disc, this distortion travels through the line profile (see figure below). Therefore, collecting data at different rotation phases allows us to unveil how the bright features are distributed on the stellar surface, exactly as for the magnetic field. In this case the method is simply called Doppler-Imaging.

As the star rotates, an Earth-based observer sees the dark starspot at different locations on the visible stellar disc (upper panel). This results in characteristic distortions in stellar spectral line profiles that induce an apparent radial velocity (RV) shift (lower panel). Such RV shifts can mimic the signal of a planet or completely hide the presence of a genuine planet. Credit: J.-F. Donati.

Instruments developed to gather simultaneously both the spectrum and its polarisation are called spectropolarimeters. The most used are ESPaDOnS atop the Mauna Kea in Hawaii, NARVAL atop the Pic-du-Midi in France, and HARPS-pol at La Silla observatory in Chile.

The telescopes hosting the three high-resolution spectropolarimeters designed for studies of stellar magnetic fields. From left to right: Canada-France-Hawaii Telescope, Maunakea, Hawaii, USA; ESO 3.6m Telescope, La Silla Observatory, Chile; Télescope Bernard Lyot, Pic-du-midi Observatory, France. Credits: J-C Cuillandre/E Hébrard/OMP.

What can we do with these measurements? First, as stellar spots plague the planet detection from radial velocity measurements, we can use the map of the spot distribution to infer the induced RV. Although new, this method holds tremendous promise in being able to filter out the stellar signal, and thus to regain the power of diagnosing the potential presence of orbiting planets. Second, if our final goal is to detect a habitable Earth-like planet around cool stars, characterising the planetary environment is of prime importance. Indeed, the reduced temperatures of cool stars move their habitable zone closer in than around Sun-like stars. Earth-like planets orbiting such stars would experience a stronger stellar magnetic pressure, exposing the planet’s atmosphere to erosion by the stellar wind and CMEs. Therefore there is an interest for estimating the stellar magnetic environment surrounding these planets. From the reconstruction of large-scale magnetic field topologies with ZDI, one can extrapolate the field outwards (see V374 Peg figure below) and ultimately it will allow a more thorough characterisation of detected planets, and a better assessment of the suitability of a planet for hosting life. Finally, the observed large-scale magnetic properties can be useful to better understand the stellar interior and the magnetic field generation.

Magnetic field lines of the active red dwarf V374 Peg, extending in space above the surface of the star. The surface magnetic field has been mapped with ZDI, serving as a basis for the extrapolation to the whole magnetosphere. The simple dipole, magnet-like structure of the field is very obvious. Field lines forming loops above the surface are shown in white, while field lines open to the interstellar medium are shown in blue. Credit: MM Jardine & J-F Donati.

So far we have discussed what we know about red dwarf stars observationally. Let’s go into some details about the latest theoretical models which try to explain why these stars have such strong magnetic fields. We will now discuss a recent supercomputer simulation which tried to mimic what happens in red dwarf stars.

In computer simulations a star is considered to be a perfect sphere of hot plasma which rotates around an axis. To model the plasma flows, we assume that it follows the Navier-Stokes equation—which basically tells us that the change in the momentum of a tiny fluid packet is proportional to the sum of various forces acting on it. The behaviour of the magnetic field is governed by Maxwell’s equations (under the so-called MHD approximation). Furthermore, there are other equations of importance which describe the energy conservation and the thermodynamical state of the fluid (temperature, pressure, etc). These equations are then solved using sophisticated numerical algorithms (codes with 10s of thousands of lines) which are run on some of the world’s largest supercomputers.

Hydra570
The HYDRA supercomputer at the Max Planck Computing and Data Facility in Garching bei München, Germany. In total there are ~83,000 cores with a main memory of 280 TB and a peak performance of about 1.7 PetaFlop/s. Credit: Max Planck Society.

If we model conditions, which are similar to those found in red dwarf stars, the simulation produces many properties similar to what we actually observe. The magnetic field resulting from this simulation is depicted in the figure below. The field lines are coming out of the visible north pole of the “star”. This is due to a large region of magnetic field with one polarity (shown with yellow shades). A similar behaviour occurs in the south pole which is not visible in the image. Along with large regions with similar polarity, there are smaller regions containing both polarities of the magnetic field (close-by yellow and blue shades), scattered almost all over the surface. These “bipolar” regions are necessary to generate twisted and stretched field lines which lead to X-ray flares and CMEs. In fact, the bipolar “active” regions on this “star” are much more numerous than what we see on the Sun. By extension, this model then predicts that the red dwarf stars should generate many more X-ray flares. The strength of the magnetic field in the image is also typically about several kiloGauss, at least ten times stronger than the Sun’s typical magnetic field.

Magnetic field simulation
Numerical simulation aimed at studying magnetic field generation in a red dwarf star. The two magnetic polarities are depicted in yellow and blue. The cyan-color pipe shows the rotation axis. Credit: Rakesh Yadav.

To sum up, this “star in a computer” is able to self-consistently produce a very strong magnetic field and predicts that these stars should be much more active than the Sun. We have made some progress in the sense that this simulated star satisfies some observational constraints. The next step is to use the predictions from this simulation and test them using more detailed observations. The Pale Red Dot project is one such step.

About the authors

Elodie Hébrard

Élodie Hébrard graduated her PhD in astrophysics in 2015 at the Institut de Recherches en Astrophysique et Planétologie of the University of Toulouse (France). She studies the use of the Zeeman-Doppler Imaging technique to characterise stellar activity and magnetic fields, ultimately designing new approaches to filter out the activity-induced  radial velocity signals that  mimic those due to planets. Élodie is now a postdoctoral fellow at the Department of Physics and Astronomy of the University of York (Canada).

Rakesh Yadav

Rakesh Yadav is a theoretical astrophysicist who uses supercomputers to understand how planets and stars produce their magnetic fields. He finished his BSc and MSc (physics) in 2011 at the Indian Institute of Technology Kanpur, India. He moved to Germany in 2012 to pursue a PhD in computational astrophysics at the Max Planck Institut für Sonnensystemforschung and the University of Göttingen. After finishing his PhD in 2015 he joined the Harvard-Smithsonian Center for Astrophysics as a Post Doctoral scientist.

‘The Signal’

The campaign is up and running! As of February 9th 2016, we have only lost two nights on HARPS due to weather, which means we have 15 good qualityspectra ready for processing. On the photometric follow-up side, the LCOGT telescopes have been obtaining good data (photometry in UBV bands), and the ASH2 telescope has already accumulated 19 nights of good photometry as well (visual and red colours). The BOOTES station is suffering technical difficulties that we hope to sort out soon. Fortunately, the ASH2 telescope (the last one to join the effort, but the most successful photometer so far!) offered a degree of redundancy that saved the day! We will introduce all the observatories involved in forthcoming posts.

Feb8
Status update of the observations. We recently reached 25% of the HARPS data, and two photometric follow-up observatories have been operating nicely over the same period. We expect more downtime due to weather, but the survey goals will probably be achieved if we can hit 80% of the planned observations.

Now that we also had the opportunity to read about the Doppler method and how stellar activity can mimic the presence of a planet, let’s talk about what we are trying to achieve here. Analysis of previous campaigns show that a possible smooth signal was observable when monitoring the star at moderately high cadence; but we must remain cautious because stellar activity can produce the same kind of variability. As described in the article by Paul Gilster, Proxima has been monitored for small planets before. The most exhaustive works include the UVES/ESO survey for rocky planets around M-dwarfs, conducted between 2000 and 2009; the searches with HARPS by the Geneva team; and recently obtained data from our own high cadence program with HARPS, called Cool Tiny Beats(2013-2014). Here are some of the technical details for those of you who are interested in them…

The VLT/UVES Doppler data, and possible signals in it

The UVES Doppler measurements were published in Kuester & Endl 2008. In previous posts (eg. see Figure 1), we have seen that if we have a planet we should see an oscillatory motion over time. These measurements didn’t look much like that (Figure 2). Still, the velocities of UVES seemed to be not completely random.

Example of measurements (in red) overplotted on the expected Doppler signal caused by an exoplanet on the Star. Changes in the velocity of the Sun-like star 51 Peg used by M. Mayor and D. Queloz to infer the presence of a gas-giant planet in a short period orbit around the star.
Figure 1. Example of measurements (in red) overplotted on the expected Doppler signal caused by an exoplanet orbiting a star. Changes in the velocity of the Sun-like star 51 Peg, used by M. Mayor and D. Queloz were used to  infer the presence of a gas-giant planet in a short period orbit around the star.
Doppler measurements of Proxima from UVES. No clear sinusoid can be spotted by eye, which already rules out the presence of long period gas giants around the star.
Figure 2. Doppler measurements of Proxima from UVES. No clear sinusoid can be spotted by eye, which already rules out the presence of long period gas giants around the star.

Stars are only visible for a few months of a year, so that could be the smoking gun of a planet with a period similar to Earth that we happen to be sampling at more or less random moments of the orbit. Kuester & Endl 2008 had reasons to suspect that this variability was indeed caused by activity, or even some unknown instrumental effect. Once that possible signal was removed by fitting a sinusoid to it, very little remained in the residuals besides apparently random noise at the 2-3 m/s level. The Doppler signal of a planet is stronger if the planet is closer to the star (as in the Solar System where Mercury takes less than three months to circle the Sun, the motion of close-in planets is faster). So, while no clear signal could be extracted from these measurements, the data did tell the researchers that no large planets were orbiting the star with periods shorter than few hundred days.

Limits to the minimum mass of planets orbiting Proxima. Concerning the 'Habitable Zone' (here marked in green between 4 and 15 days, but new models suggest it extends to periods as long as 27 days), planets down to 3 Earth masses (minimum mass) were ruled out by the data.
Figure 3. Limits to the minimum mass of planets orbiting Proxima. The ‘Habitable Zone’  is marked in green between 4 and 15 days, but new models suggest it extends to periods as long as 27 days. Planets down to 3 Earth masses (minimum mass) were ruled out by the data. Extracted from Endl, M.; Kürster, M. 2008 A&A

The minimum masses of planets ruled out by UVES are illustrated in Figure 3. Let us note that we say ‘minimum mass’ of the planet because the Doppler method only measures the motion along our line of sight. Even in that case, statistical arguments indicate that it is highly unlikely to find any planet less than ~5 times the mass of the Earth in its habitable zone, with other techniques. With this upper limit set, the UVES program stopped observing Proxima and another handful of M-dwarf stars at the end of 2008.

The HARPS/Geneva team observations of Proxima prior 2012

During the same years, Proxima was observed about 25 times with HARPS. While the star did show variability at the 2-3 m/s level, it also showed evidence of activity in occasional flaring events, and some excess of radiation coming from its chromosphere. In any case, the measurements were consistent with those of the UVES survey in the sense that no obvious signal was detectable above ~2 m/s. In 2013, the star was again observed in the extended HARPS survey for M-dwarfs led by the ex-Geneva astronomer X. Bonfils, now based in Grenoble, but no report has appeared on significant period variability so far. So these campaigns led to no convincing evidence of a signal.

Doppler velocity measurements by X.Bonfils and his team taken between 2002 and 2009 with HARPS. Source : Bonfils et al. 2013 A&A, available via arXiv.
Doppler velocity measurements by X.Bonfils and his team taken between 2002 and 2009 with HARPS. Source : Bonfils et al. 2013 A&A, available via arXiv.

The ‘HARPS – Cool Tiny Beats’ observations (2013-2014)

In 2013, the same team as  the Pale Red Dot campaign started a programme to measure radial velocities at high cadence (focused on a small sample of very nearby M-dwarfs) to hunt for short period planets, pulsations and understand the connection of stellar activity with apparent Doppler signals. Proxima was a natural target for the survey, which was executed in two runs of 12 nights each (May 2013-Jan 2014). As opposed to the rest of the stars in the sample, the radial velocity measurements of Proxima were smoothly varying over both observing runs. Unfortunately, given the length of both runs, the strict periodicity of the variability could not be verified. Worse than this, the long term Doppler variability found by the UVES survey was still present but seems rather unpredictable, meaning that combining the data from years ago did not help much in confirming it. This is when Pale Red Dot was conceived…

Doppler measurements of Proxima obtained in 12 consecutive nights in May 2013, suggestive of smooth variability on the timescale between 10 and 20 days. The origin of this 'signal' is what the Pale Red Dot campaign is trying to figure out. Image credits : G.Anglada-Escude.
The top panel contains Doppler measurements of Proxima obtained in 12 consecutive nights in May 2013 which are suggestive of smooth variability on the timescale between 10 and 20 days. The lower panel is what we call a ‘periodogram’, which is a mathematical tool to identify possible periodicities in the data. Because the observing run was limited to ~12 days, we cannot really constrain the putative period with this data only. The Pale Red Dot campaign is trying to figure out if this is a strictly periodic signal feature by observing the star ~60 nights in a row, thus covering several cycles of the putative signal, and comparing the variability with simultaneous photometry. Image credits: G.Anglada-Escude.

So while we are convinced there is a signal in the Doppler measurements of Proxima, previous data do not allow to confirm its presence and clarify its origin. The long term variability of Proxima spoiled our attempts to combine data from previous observations so we needed a dedicated campaign.

In summary

Combination of UVES and HARPS data at different cadences suggest that the star is showing a smoothly varying Doppler signal. Since the UVES survey set an upper limit between 2-3 Earth masses and if the signal is not activity induced, it must correspond to a planet smaller than that (between 1-2 Earth masses>). The signal might well be caused by stellar activity, which should be quasi-periodic as opposed to the strict periodicity of the orbital motion of a putative planet. So this is what we want to figure out! If you really want to learn more, feel free to any member of the Pale Red Dot team!

What is the exact plan?

We are following Proxima Centauri for about two months. If the planet is there we should see the velocity of the star going up and down between 3 and 5 times depending on the precise period. Simultaneously, we are monitoring Proxima with telescopes from LCOGT.net, the ASH2 Atacama telescope and the BOOTES network. The contiguous and regular sampling of the observations together with quasi-simultaneous photometry should allow us to model its long term variability better and, hopefully, confirm whether the Doppler signal is caused by a planet or not. If it isn’t…  we will move on and keep searching for #palereddots around other nearby stars…

Thanks for following!

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.

The Las Cumbres Global Telescope Network

by Wayne Rosing, LCOGT Founder and Chief Technologist

I founded and incorporated Las Cumbres Global Telescope Network in 1992, but the notion of a global ring of telescopes, connected with the internet and using CCD detectors, first occurred to me in 1983. Others were discussing similar ideas. The plan was to build a global telescope (singular) dedicated to time domain astronomy. With six observing sites spread around the world, such an instrument could observe any single target continuously over many days,and could also interrupt planned operations to take data on new interesting transient objects, no matter where they might appear in the night sky.

My old world atlas has circles dating back as far as 1984, marking roughly where the LCOGT telescopes are located today. The aim was to have uniform sky coverage, pole to pole. That implied placing telescopes at roughly plus and minus thirty degrees latitude. In the South, the placement of continents pretty much determined the longitude choices. In the North, the objective was to be four hours East or West of each of the Southern sites. Then the result would be that for some periods of the year, provided the weather is clear, three sites would simultaneously be in the dark and able to image astronomical targets in the equatorial plane.

The LCOGT network

From the beginning of the project a key element was the notion of a near real-time scheduling process that would optimize the system’s choice of targets to achieve the best possible science at any given time while taking into account local conditions at the sites and scientific constraints. An additional requirement was that the entire system had to be robotic, not requiring human night assistants or remote real-time observers.

In 2005 LCOGT set about implementing this vision. By the end of 2005, LCOGT had acquired the two 2-meter Faulkes Telescopes (the FTs), located in Haleakala on Maui, Hawaii, and at Siding Spring, Australia. Although these are larger than the 1-meter telescopes that form the bulk of the current network, they add the capability of doing daily followup of fainter objects from both the Northern and Southern hemispheres.

Fading sunlight, a young crescent Moon, and brilliant Venus shared the western sky in this view of 2005’s final sunset from the top of Mount Haleakala, on Maui, Hawaii. Also known as the Sacred House of the Sun, Haleakala, is Maui’s dormant volcano. At 10,000 feet the summit is an ideal site for astronomical observatories, and this scene also features the silhouette of the northern hemisphere Faulkes Telescope. – image Credit & Copyright: Rob Ratkowski

During the next two years the foundation was developed for the global network we have today; Efforts were concentrated on upgrading the FTs, developing a software system to tie the network together, and on engineering two rings of telescopes, one on each hemisphere. LCOGT has since deployed nine 1-meter telescopes at four sites (in Chile, South Africa, Australia, and Texas), and plans to add another pair of telescopes in Tibet early in 2017. We still have the possibility of deploying three more 1-meter facilities if funding permits. In addition, seven 0.4-meter telescopes have been deployed at four sites (including the Canary Islands).

Three domes at night. Chile.

A side effect of this distributed and redundant capability is that, each year, LCOGT has tens of thousands of hours of observing time on its instrument. LCOGT scientists and partners can plan long-term key projects which may require telescope-years of observing time. There are three basic types of measurements LCOGT telescopes acquire: First, images of small patches of the sky. From these images one can derive celestial coordinates, relative brightness, and sometimes the rate of motion for stars, asteroids, near-earth objects, satellites and galaxies. Second, by taking images in multiple color bands astronomers can do photometric data reductions to classify each object based on general color characteristics. For example, blue stars are hotter than red ones. Third, spectroscopic measurements spread incoming starlight out into detailed color bins — from about 1000 colors over the visual spectrum to as many as 50000 resolved colors. Emission and absorption by specific atomic elements can be measured; chemical element abundances, stellar temperature and even local gravity can then be determined.

Current key projects on the network include:

1) The Next Generation Sample of Supernovae (PI: Andy Howell) – Using photometric and spectroscopic observations of both Type Ia and core collapse supernovae we can learn about their characteristics in a statistically significant sample. These types of exploding stars are used as astronomical ‘standard candles’, a way of measuring the distance to the far edge of the visible Universe.

2) Exploring Cool Planets beyond the Snowline (PI: Rachel Street) – Follow-up observations of microlensing events are employed to detect and study planets in the outer parts of planetary systems around distant stars. This project relies on the Einstein deflection of light by the gravity of one star to magnify, or ‘lens’, a star-planet combination found near the center of our galaxy 20,000 light years away.

3) Echo Mapping of AGN Accretion Flows (PI: Keith Horne) – By monitoring photometrically and spectroscopically a diverse sample of active galactic nuclei we can determine their physical characteristics using the reverberation mapping technique. Basically as matter falls into a large central black hole, energy is emitted and then is reflected in outer areas of the galaxy. Reconstruction of this data can give insight into the structure of galaxies.

As the first science projects matured, we complemented our imaging capabilities with low-resolution high-throughput spectrometers for the two FTs. Thanks to an NSF grant we are also building a network of high-resolution spectrometers which will be deployed as a global ring, each instrument being coupled to a pair of 1-meter telescopes. This capability will be used to validate and characterize new extrasolar planets found by space- and ground-based survey telescopes, and to advance astrophysics by studying pulsations, rotation, and the magnetic activity of stars.

NRES Prototype in the LCOGT lab

It has been a decade-long effort to build LCOGT into what it is today. The vast majority of the funding to do so has come from private sources. We are grateful for support for various parts of the effort from the University of St. Andrews on behalf of Scottish Universities Physics Alliance (SUPA), the National Science Foundation, NASA, the Qatar National Research Fund (QNRF) on behalf of Qatar Environment and Energy Research Institute, and the Dill Faulkes Educational Trust. Our site partners are the Australian National University, the Cerro Tololo Inter-American Observatory (CTIO), the Institute for Astronomy in Hawaii (IfA), the Instituto de Astrofisica de Canarias (IAC), the University of Texas at Austin McDonald Observatory, the Ali Astronomical Observatory of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), and the South African Astronomical Observatory (SAAO).

We are grateful too for the dedicated effort provided by over 150 individuals who have built LCOGT from a dream into a scientific institution of which we can all be proud.

Wayne Rosing, January 2016.

Wayne Rosing

About the author. Rosing was an engineering manager at Digital Equipment Corporation (DEC) and Data General in the 1970s. He became a director of engineering at Apple Computer in 1980. There he led the Apple Lisa project, the forerunner to the Macintosh. He then went on to work at Sun Microsystems in 1985. After managing hardware development for products such as the SPARCstation, he became manager of Sun Microsystems Laboratories in 1990.[1] From 1992 through 1996 he headed the spin-off First Person, which developed the Java Platform. He was then chief technology officer at Caere Corporation, which developed the optical character recognition product OmniPage.

Rosing served as vice president of engineering at Google from January 2001 to May 2005. In May 2005 he was appointed a senior fellow in mathematical and physical sciences at the University of California, Davis, and continued to serve as an advisor to Google.

As a hobby throughout his career, Rosing built telescopes, telescope control systems, and ground telescope mirrors. At Davis, Rosing consulted on the Large Synoptic Survey Telescope project.