Category Archives: Expert Insights

Proxima b, where did it come from?

by Dr. Gavin Coleman, Queen Mary University of London

The recent discovery of Proxima b has not only excited much of the public, but also scores of scientists who are attempting to explain its many different aspects. At the time of writing, there are already 8 papers discussing a wide variety of topics concerning Proxima b: ranging from its potential habitability and the impact of flares, to how to characterise the planet’s atmosphere with NASA’s new James Webb Space Telescope. Whilst these pieces of work are important in looking at the present state of the planet, (well the state of the planet 4.25 years ago) one important question from its past needs to be answered. How did it form and evolve into the planet that is detected today? Knowing how and where it formed can give valuable insights into its composition and atmospheric properties, whilst understanding its evolution can give hints at to what else, if anything, should be expected to be discovered orbiting Proxima Centauri. But before we examine the specific case of Proxima b, it is useful to understand just what planets form from and how they do it in a very general case.

Before a star system is born, the entirety of its material is held in a protostellar nebula in the form of gas and dust; this includes the parent star (composed of mostly hydrogen and helium), along with all of the planets, asteroids and dust particles (which are mostly heavy elements such as carbon, oxygen, etc). The nebula then collapses in on itself to form a protostar surrounded by a protoplanetary disc. This gas and dust disc then accretes on to the protostar over a period between 1 and 10 Myr, and this is the time and location that we expect planet formation to take place. The dust in the disc begins to settle to the middle of the disc, whilst simultaneously clumping together and coagulating into larger pebbles, and eventually into asteroid sized bodies. We call these bodies planetesimals. As they continue orbiting the star, they interact with other planetesimals – occasionally colliding and forming larger planetesimals – until they eventually become planetary-sized objects (what we call protoplanets) similar to the terrestrial planets today (Mercury, Venus, Earth, Mars). If these protoplanets can accrete enough material they may become massive enough to hold a substantial atmosphere and then grow into gas giants similar to Jupiter and Saturn. This all occurs during the lifetime of the protoplanetary disc, so that the only objects surviving once the disc has accreted on to the star are the star itself, any surviving planets, and all the remaining asteroids, pebbles and dust.

Figure 1: Diagram showing the formation of a star and planetary system from a protostellar nebula. Image Credit: Tom Greene (2001)
Figure 1: Diagram showing the formation of a star and planetary system from a protostellar nebula. Image Credit: Tom Greene (2001)

Now this is a very general view of planet formation; what happens when we begin to examine the specific case of Proxima b? When looking at individual systems, there are many different scenarios that branch off of the general case described above. I will now explain these scenarios and their implications for the composition and structure of Proxima b, and also the multiplicity of the Proxima Centauri system – that is whether we should expect more planets to be discovered.

Scenario (i) in situ planet formation: It is thought that the terrestrial planets in the Solar System formed near their current orbits from a group of smaller protoplanets embedded within swarms of even smaller planetesimals in a gas-free environment, after the end of the disc lifetime. When using this scenario to form Proxima b, it is assumed that enough solid material in the form of protoplanets and planetesimals has been able to form and migrate to the general location of Proxima b before the end of the disc lifetime. To then form Proxima b, the protoplanets and planetesimals undergo significant gravitational interactions with each other, resulting in numerous collisions, allowing the protoplanets to grow. Over a long period, typically a million years, the majority of the planetesimals and smaller protoplanets are accreted by the more massive protoplanets, leaving only a few protoplanets remaining. The initial mass contained in the reservoir of protoplanets and planetesimals, will determine the mass of the few remaining objects. Typically 2–3 of the survivors contain about 80–90% of the total mass, with the remaining mass in a number of much smaller objects. The important thing to note here is that with this scenario there are always multiple planets formed. This is important for Proxima b; if Proxima b formed through this method, and is the most massive of the survivors in its local area, then we should expect to find more planets orbiting Proxima Centauri in the future.

When thinking about what kind of world Proxima b will be like if it formed this way (i.e. will it be a dry rocky planet or a water-rich ocean-world), we have to know what kind of material the planet formed from. With this scenario it formed from material that was located inside the snowline, so there will be little water or volatile material within its composition. The only way for water to get onto the planet is if it was transported into the planets local vicinity. If there was a massive object located out past the snowline, then it could scatter smaller asteroids and planetesimals into Proxima b’s vicinity, delivering water and other volatiles (similar to the Late Heavy Bombardment on Earth). If this did happen then Proxima b would still be predominantly rocky but would also contain a modest amount of water.

Figure 2: Diagram showing the formation of Earth from 2 protoplanets - Proto-Earth and Theia. Image Credit: Sean Raymond
Figure 2: Diagram showing the formation of Earth from 2 protoplanets – Proto-Earth and Theia. Image Credit: Sean Raymond

Scenario (ii) Multiple migrating embryos: Where the scenario above took place in a gas free environment, this one looks at Proxima b forming during the gas disc lifetime. To form Proxima b during the gas disc lifetime, it is necessary to do so far from the star, at distances greater than 1 AU. Why do they need to form so far out? Planets embedded in protoplanetary discs are expected to undergo significant inward migration. This is due to the planet’s gravity slightly perturbing the gas in its local vicinity, exchanging orbital energy with it, and consequently changing its orbit. As these perturbations are not symmetrical (i.e. the outer disc perturbations are different in strength than the inner perturbations), this results in a net torque acting on the planet, forcing it to migrate either inward or outward. Normally, the gas exterior to the planet’s orbit tugs on the planet more than that interior to its orbit, resulting in the planet losing orbital energy. This is important as the time it takes for protoplanets to migrate all the way in close to the star is typically comparable or shorter than the disc lifetime. This is why it is not possible for Proxima b to form close to the star whilst the gas disc is around, it will migrate into Proxima Centauri and be engulfed, leaving no planet.

To form Proxima b at the same time as the gas disc is accreting on to the star, it has to form far from the star in the outer disc. To do this, we assume that initially a number of smaller protoplanets have been able to form far out in the disc (with semi-major axes greater than 1AU). As the gas disc evolves, these protoplanets migrate inwards and begin to interact with each other, occasionally colliding, forming more massive protoplanets. As the gas disc comes to the end of its lifetime, the remaining planets have now all migrated to the local vicinity that Proxima b is observed today. Typically only a handful of planets survive the migration and accretion process, as the less massive protoplanets have collided with the more massive ones. Similar to scenario (i), 2-3 of the surviving planets contain the majority of the mass in the system. These planets would be comparable to Proxima b. A small number of less massive planets would also be found in the vicinity.

Now this outcome seems very similar to the one found above in scenario (i), so how can we tell the difference? Both scenarios state that there should be more planets orbiting Proxima Centauri. But what about the water content? In scenario (i), all the planets formed inside the snowline, so they should be dry, but in this scenario, the planets formed far outside of the snowline. This would result in them containing large amounts of water and volatiles, and as the planets migrate interior to the snowline, they have enough gravity to hold on to it. So here Proxima b would be extremely water- and volatile-rich, and could represent an ocean world, one of many in the Proxima Centauri system.

Figure 3: The evolution of multiple protoplanets (black dots) migrating in to form Proxima b. Planets inside the grey region are analogous to a Proxima b analogue. Proxima b is represented by the black cross.
Figure 3: The evolution of multiple protoplanets (black dots) migrating in to form Proxima b. Planets inside the grey region are analogous to a Proxima b analogue. Proxima b is represented by the black cross.

Scenario (iii) Formation as a single planet: This scenario is similar to scenario (ii). It again attempts to form Proxima b during the gas disc lifetime; but in this case only one protoplanet, a young Proxima b if you like, is used. This protoplanet is surrounded by swarms of planetesimals and/or pebbles, and as it migrates inwards towards the star, is able to accrete any planetesimals or pebbles it collides with. Over the gas disc lifetime, the protoplanet is able to migrate all the way in towards the star, and depending on the amount of planetesimals/pebbles it accretes, its mass can be as small as Mars (0.1 Earth masses) or as massive as a super-Earth (3–10 Earth Masses). It can also have a similar mass to the observed Proxima b (~1.3 Earth masses).

When this scenario produces a planet comparable to Proxima b, it is always alone. Since the planet migrated from the outer disc, outside the snowline, any planet similar to Proxima b would be abundant in water and volatile material. It would again be an ocean-world, but in contrast to scenario (ii) it would be the lone planet around Proxima Centauri. If future observations  showed that Proxima b does indeed have a significant amount of water and volatile material, and has no sibling planets, this scenario would be supported as a viable formation history for the planet.

So where did Proxima b come from? Did it form within the protoplanetary disc far away from the star and migrate all the way in to its current location, or did it perhaps form in situ after the gas disc has accreted on to the star? Did it form with multiple other planets or is it all alone? Does it have a significant amount of water or is it bone-dry? Though the above formation scenarios allow all of these possibilities, it is still not clear which one is most likely. Only with future observations will we be able to determine which scenario is the most probable path of evolution for Proxima b. But what if we look further than Proxima b? What does knowing the formation history of Proxima b tell us about planet formation in the galaxy as a whole? Should we be expecting to find more Proxima b-like planets orbiting in other stars’ habitable zones, or was Proxima b a very lucky find? Only time, and more research, will tell. Either way, knowing where Proxima b came from and how it came to be what it is today will be important in informing us what to expect from the rest of the stars in the galaxy, and whether we could eventually find a habitable Earth-like planet teeming with life…

Editorial note. This is an outreach article based on the scientific report “Exploring plausible formation scenarios for the planet candidate orbiting Proxima Centauri”,


About the Author. Gavin Coleman is a post-doctoral researcher at Queen Mary University of London (UK). He recently finished his PhD examining the formation of planetary systems, and focusing on the numerous factors that influence the diversity in their architectures. He is member of the Pale Red Dot collaboration and an active editor of this blog.

Planetary System Dynamics

By Francisco J. Pozuelos, Instituto de Astrofísica de Andalucía (IAA-CSIC)

The ultimate goal of exoplanetary research is to place ourselves in the universe. Are we just the result of normal evolution? That is to say, does life tend to appear almost everywhere, which means that the emergence of intelligent life is just a matter of time. Or, on the other hand, are we unique? Something that just happens in a few places in the vast universe? This question has haunted the humankind since consciousness and for the first time in the history we are close to answering it. We are living in exciting times.

When the next generation of telescopes and instruments point to the sky, we will be able to observe planetary systems as never before. Super-Earths, exotic planets, planetary systems under extreme conditions… We do not know what we are going to find, but for sure it’s going to be surprising.

Artistic impression of the PLATO spacecraft searching for exotic exoplanetary systems. Credits: DLR (Susanne Pieth).

However, it is necessary to take into account that this new technology just offers us a picture frozen in time. To understand what we observe it is mandatory to develop dynamic studies on the order of the life time of the system, from a few million to billions of years. This is possible thanks to the great advances in computational science in the last decades, which allows us to investigate what causes the planetary systems to become as we see them today and how they are going to evolve in the future. It is also necessary to understand that we need to study the planetary system as a whole, taking into account other planets, planetesimal disks, even the evolution of the host star. Here we comment on a few examples of planetary dynamics which will help us to increase our knowledge about formation and evolution:

  • Planet-Planet interaction and Migrations. It seems that multi-planet systems tend to have more circular orbits. This fact decreases the influence of planets on each other, resulting them being stable for very long periods of time. On the other hand, those planetary systems with planets in eccentric orbits generate a chaotic and unstable scenario where the bodies can collide and even be thrown out of the system. In addition, during the first steps of evolution after the formation process, planets can suffer so-called “migrations”. Due to this mechanism planets can evolve to outer or inner orbits; such a scenario can explain the existence of hot Jupiters.
Simulation showing the evolution of the Solar System. Left:  early configuration of the outer planets and planetesimal belt before the Jupiter and Saturn 2:1 resonance . Center:  scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue). Right:  final configuration after ejection of planetesimals by planets. Credits:  R. Gomes et al.
  • Tidal interactions. Some of the observational techniques used to detect exoplanets are more sensitive to those planets whose orbits are close to the host star. These planets will experience significant tidal forces as a result of this proximity. The relevance of the tides in the evolution of planets in close-in orbits was apparent with the discovery of 51 Peg b, whose semi-major axis was established as only 5% of the Sun-Earth distance. Since then, the tidal interaction between host stars and close-in planets is considered to be the cause of many effects. For example, these tidal forces are generally expected to lead to the alignment of their rotation axes, synchronization of their rotation and orbital periods, a reduction in orbital ellipticity (tidal circularisation), an accompanying reduction in semi-major axis, and a conversion of orbital energy into tidal heating of the planet. This effect of tidal heating in rocky or terrestrial planets and exo-satellites may have significant implications for habitability. For example, in our Solar System, the cool satellite Europa is a rocky body covered by 150 km of water ice crust, for which the tidal heating may maintain a subsurface water ocean. Or, in the case of the Jovian satellite Io, where the extreme violence of the tides provoke intense global volcanism and rapid resurfacing, ruling out any possibility for habitability. Thus, a correct treatment of the tidal interactions is necessary to determine if the planet was/is/will be habitable, and for how long. Of special interest will be those planets in close-in orbits, classified as terrestrial planets and hosted by M stars, where the habitable zone is expected to be in the region where the tides are acting.
Evolution of the semimajor axis (a), eccentricity (e), and distance of a synthetic planetary system composed by a Jupiter-like and Earth-like planets in presence of tidal interactions. Credits: Francisco J. Pozuelos.
  • Debris Disk-Planet interactions. Debris disks, with a qualitative similarity to the main asteroid belt and Kuiper belt in the Solar System, have been observed in various exoplanetary systems. These debris disks are composed of second-generation material, and their presence implies the existence of a significant planetesimal population. The impact range of these minor bodies and the planets is especially interesting for those planetary systems with planets on the habitable zone. First it is assumed that they are an important source of water and organics once the formation process is finished. On the other hand, a large impact removes any possibility for habitability. This fact was understood with the breathtaking impact of Shoemaker-Levy 9 with Jupiter in 1994, when the impact of two objects in the Solar System was observed for first time.

All these studies will complement the information obtained from telescopes giving us a better idea of how planetary systems evolve. We are going to be able to determine how rare the Solar System and our planet are.


About the author

Francisco J. Pozuelos is a postdoctoral researcher at Instituto de Astrofísica de Andalucía, Spain. He finished his PhD in 2014 about the relationship between comets activity and their dynamical evolution. These studies provide unique clues to determine how the Solar System was formed, and how the gravitational interactions between minor bodies and planets have affected the current Solar System configuration. Since 2014 Pozuelos is collaborating with PLATO 2.0-ESPAÑA at the University of Granada (Spain) where he is developing computational codes to study the evolution of planetary systems, taking into account processes as such as stellar evolution, tidal interactions, and the stellar wind.

Magnetic Open Cluster Stars of a Peculiar Kind observed with HARPS-POL

by James Silvester, Uppsala University

Magnetic fields play a fundamental role in the atmospheric physics of a significant fraction of stars on the Hertzsprung–Russell diagram. The magnetic fields of the chemically peculiar magnetic A and B type stars (Ap/Bp) have quite different characteristics than, for example, cooler stars like the Sun. In these magnetic Ap/Bp stars the large-scale surface magnetic field is static on time-scales of at least many decades, and appears to be “frozen” into a rigidly rotating atmosphere. The magnetic field is globally organised, permeating the entire stellar surface, with a high field strength (typically of a few hundreds up to a few tens of thousands of gauss—by comparison the sun has a polar field strength of one to two gauss). These stars are so called “chemically peculiar” as a result of having peculiar abundances (amounts) of certain chemical elements compared to what is seen in the Sun or other solar type stars.

Shows a comparison between a solar abundance model spectrum (red dashes) and typical peculiar abundance spectrum (solid green) for a 13000 K star.
This figure shows a comparison between a solar abundance model spectrum (red dashes, T=5800 K) and typical peculiar abundance spectrum for a hotter star (solid green, T=13000 K).

It is thought that the presence of this magnetic field strongly influences energy and mass transport, and results in strong chemical abundance non-uniformities within the atmosphere. These uniformities can take the form of large abundance structures in certain layers in the atmosphere.

Originally the magnetic field geometries of these chemically peculiar Ap/BP stars were modelled in the context of a simple dipole field (think of a bar magnet stuck in the star). However, with the acquisition of increasingly sophisticated data, it has become clear that the large-scale field topologies exhibit important differences from a simple pure dipole model. Through the advent of high-resolution circular and linear polarisation spectroscopy we have found the presence of strong, small-scale complex field structures, which were completely unexpected based on earlier modelling.

How do we measure magnetic fields of Ap/Bp Stars

In 1897 Dutch physicist Pieter Zeeman discovered that in the presence of an external magnetic field; light is polarised circularly if viewed parallel to the direction of the magnetic field and is plane (or linearly) polarised if viewed perpendicular to the magnetic field. In addition, spectral lines in the presence of such a field can be split into discrete levels—the so called Zeeman effect—where the strength of the magnetic field is proportional to the width of the splitting within the spectral lines.

The figure below illustrates how both a spectral line and how the linear and circular polarisation signature can change in the presence of a strong magnetic field, in the case of when the field is either perpendicular or parallel to the line of sight of the observer.

Spectral line shapes in different polarization states.
Spectral line shapes in different polarization states.

We use these effects to study the magnetic field of stars with the aid of a spectropolarimeter. The form of the polarised light we receive tells us about the direction of the field on the surface of the star with respect to the observer, and the level of Zeeman splitting within a given spectral line or set of lines allows us to determine the strength of the magnetic field.

In cases where the polarisation signal is too weak to effectively measure in individual spectral lines,  we can use line averaging techniques to improve the signal by averaging all the lines in the spectrum showing polarisation signatures into one line.
The recent advances in tomographic imaging techniques and the new generation of spectropolarimeters such as ESPaDOnS (at the CFHT), NARVAL (at the TBL, Pic du Midi) and HARPS-pol (ESO 3.6 at La Silla) offer the opportunity to improve our understanding of the magnetic field of Ap/Bp stars by allowing us to map the magnetic field and chemical surface structure in quite some detail.  (For the finer details of how we map magnetic fields see the great article by Élodie Hébrard and Rakesh Yadav).

The extended magnetic field topology map for the Ap star HD 32633, the number in the right hand corner is the phase of rotation.
The extended magnetic field topology map for the Ap star HD 32633, the number in the right hand corner is the phase of rotation.

Even though we are able to very successfully map the magnetic fields of Ap/Bp stars, there are however some questions that remain open.  Notably the origin of these magnetic fields is not fully understood and importantly neither is the evolution of such magnetic fields and the atmospheric chemical structures with time. This is where Ap/Bp stars in open clusters comes in.

The Current Project – Observations of Cluster Ap/Bp Stars

The question of the evolution of the magnetic field and chemical surface structures in Ap/Bp stars can be investigated by studying these stars in open clusters.  An open cluster is a group of gravitationally bound stars, and whilst it is very difficult to get an precise age for an individual star, it is however possible to get a more precise age for an open cluster, because you have an ensemble of stars which are thought to be of a similar age. Therefore if you can confirm that a star is a true member of a cluster,  you have a much more reliable age for that star than if it was an individual star.

By studying the magnetic fields of stars in different open clusters with different ages, we can in essence look at Ap/Bp stars at different stages of evolution.  This allows us to investigate if the magnetic field complexity, or the form the magnetic field takes, varies as a function of age,  e.g does the magnetic field structure of an Ap/Bp star evolve with time?

The cluster NGC 6475 / M7. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Image credits : ESO
The cluster NGC 6475 / M7. Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla Observatory in Chile. Image credits : ESO

Our team, including astronomers based in Sweden, Canada and France, has begun to obtain observations of cluster Ap/Bp stars using the HARPSpol spectropolarimeter. By measuring the circular polarisation of these magnetic Ap/Bp stars and by obtaining measurements at all phases of rotation, we will be able to create magnetic and surface chemical maps for all the stars we observe.   It is hoped that the resulting maps from our target stars will give us insight into how the magnetic field geometries and chemical surface structures of Ap/Bp stars vary with age.   Having more information about the evolution of the magnetic field will also provide a powerful constraint for stellar evolution models.


About the author. James Silvester is a postdoctoral researcher at Uppsala university in Sweden. He completed his BSc in Astrophysics at the University of Hertfordshire in 2004 and then moved to Canada to do an MSc (2007) and his PhD (2014) at Queen’s University in Kingston, Ontario.  The main focus of his research is to understand the magnetic fields of intermediate mass stars. James and his team use HARPS combined with a recently installed polarimeter (also known as HARPS-POL) to measure magnetic fields. His program overlapped with Pale Red Dot, and he was one of the observers that helped obtaining those Proxima spectra by the end of each night. Astrophotography is also among his hobbies.

Magnetic Fields: those troublemakers!

By Rim Fares, Osservatorio Astrofisico di Catania, Italy

I am sure that you are already convinced of this statement if you have followed the previous contributions to this blog. But let’s make sure things are clear: isn’t everything relative? Magnetic fields do trick planet hunters, but their study gives us insights into stars and planetary environments, which help us better understand exoplanetary worlds.

So why do we worry about magnetic fields if we want to detect Earth-like planets?

They can mimic a planet’s signature and make our goal of detecting Earth-like planets much tougher.

In cool stars (don’t be mislead by this name; these stars still have surface temperatures up to 7,000 K), magnetic fields manifest on the stellar surface in different ways. They can emerge in the form of dark spots. These regions have strong magnetic fields (up to a few thousand Gauss in the solar case). As Xavier Dumusque explained in his contribution, the effect of these spots can be greater than the effect of an Earth-like planet on the radial velocity variations of a star.

This is not everything; stars have large-scale magnetic fields too. While spots are small areas on the surface relative to the size of the star (but can be the size of a planet!), large-scale fields are distributed on the scale of the star itself. The star can, for example, act as a huge dipole. This dipolar field contributes to the large-scale field.

As Elodie Hebrard and Rakesh Yadav told us in their contribution, stars have magnetic personalities. Extensive studies of large-scale fields show that stars with similar masses and rotation periods seem to have magnetic fields that share similar properties (field strength, configuration). To detect very small planets, we need to understand the contribution of the large-scale field to the radial velocity variations of a star. But to make things complicated, stars can have magnetic cycles. Which means that both small-scale (e.g. spots’ emergence) and large-scale fields vary with time.

Cristina Rodriguez told us in her contribution that stars beat by waves’ propagation inside the stars. These beats can mimic planetary signals. Over a cycle, the magnetic field changes, and so does the music of the star.

But magnetic fields are not as evil as you think. Have you ever dreamt of watching an aurora, wondered about the ice age, the safety of astronauts? Well, there is a magnetic field contribution to all that. They play an important role from the birth of a star to its death (they affect stellar rotation, stellar wind, mass loss of the star, …). They contribute to the interactions between stars and their surrounding planets. Magnetic fields of planets protect them from stellar winds and are probably important for habitability.

Figure 1. Artist impression of the interaction between the Solar wind and the earth’s magnetosphere. The magnetic field of the Earth (presented to the right) form a magnetosphere that protects us from the Solar wind (coming from the Sun – to the left). A bow shock, highlighted in purple, form from such interactions. Credit: NASA/ESO/SOHO

While it is a tough job to detect Earth-like planets, it is much easier to detect massive planets that are very close to their stars (because they produce a much stronger effect on the radial velocity of the star, or on the light-curve when they are transiting). These planets are called hot-Jupiters, because like Jupiter they are very massive, but more than 50 times closer than Jupiter is to the Sun, so they are pretty hot. Their discovery was a big surprise to the community, because we do not have such planets in our own solar system (the massive gaseous planets are the furthest from the Sun). It raised questions about how they form, and how they migrate from further distances to very close to the star. At these distances, being massive, these planets interact with their stars through tidal interactions (like the Earth-Moon case, and its effect on the oceans’ tides). The large-scale magnetic field of the star and the stellar wind also interact with these planets (remember the auroras I talked about? They result from the interaction between the solar wind and our atmosphere, and the amazing colours they produce are due to the molecules we have in the upper atmosphere).

These interactions affect the planet but they might also affect the star. In fact, some observations suggest there are spots on the stellar surface that follow the planet on its orbit, instead of rotating with the star. When we observed a sample of stars that have hot-Jupiters, we discovered the first polarity flip of the magnetic field for a star other than the Sun. This star is Tau Bootes, a star visible to the naked eye in the Bootes constellation. 

Figure 2. Artist impression of the system Tau Bootes. The star has a large-scale field that flips on yearly basis. The exoplanet is very close to its parent star. Credit: Karen Teramura, University of Hawaii Institute for Astronomy

Imagine this star being a huge dipole (Figure 2), the north-south poles flip every year. This discovery was very surprising because the Sun flips polarity every 11 years. Tau Bootes’ flips are very fast compared to the Sun. When we discovered that, we though that it might be due to the tidal interactions with its very massive planet. This star and the planet are synchronised, which means that the star rotates with the same period of the planet on its orbit. This system was the only one observed with such a characteristic, and it was the only one for which a fast magnetic flip is detected. Coincidence or causality? We are still puzzled about that. We did observe polarity flips in other stars more recently, but for now Tau Bootes is the only star observed with regular flips over a long time period.

Back to the planet—imagine it close to the star, and bombarded by the stellar wind. It can interact with this wind and produce signatures at different wavelength (that’s cool because if you observe the same system with different instruments, you can have the whole picture of the physics that is happening up there). It can for example produce radio emission and signatures in the UV that can be due to a bow shock formation, or to the evaporation of the atmosphere.

We know that these hot-Jupiters are not habitable, so why do we care? Because they are easy to observe and characterize, but also because they give us hints to our future studies on habitable planets. Actually, Earth-like planets we are trying to detect in the habitable zone around cool red dwarfs are subject to a more aggressive environment than that of our own Earth. Red dwarfs can have strong magnetic field, and frequent ejections of particles toward the planet. The planet needs to be protected from the stellar wind in order to keep its atmosphere and be habitable. Consider the example of a volcano. Here in Catania where I currently work, we have an amazing view of Mount ETNA, the highest active volcano in Europe. The soil is fertile and farmers are happy. But the volcano is active, it can erupt and lava flows can destroy villages (luckily though, the flows are usually slow). The stellar ejections can have similar effect as volcanoes on earth, the exoplanet needs to get a protection from them: a planetary magnetic field (that will help deviate the stellar particles from hitting the planet). For now, we cannot detect planetary magnetic fields directly, we have to use indirect techniques, for example detecting a bow shock in the ultra-violet. If we study the stellar magnetic field and wind, and detect these bow shock (or radio emissions), we can calculate the magnetic field of the planet. Currently, we are trying to detect these effects on hot-Jupiters. This will shed light on the strength of the planetary magnetic fields.

The coming years will see many space and ground based instruments, and we are all excited about getting new data and making new discoveries. Among these instruments, SPIROU-–a spectropolarimeter for the study of Earth-like planets in the habitable zones—will help us study the magnetic field while having very good radial velocity precision. SPIROU also has another goal: helping to understand planetary formation and migration by observing young stars and the hot-Jupiters that are formed around them. Exciting times ahead!


About the author. Rim Fares is a research fellow at the Osservatorio Astrofisico di Catania, Sicily, since February 2015. Her main research interests are cool stars’ magnetism, the interactions between exoplanets and their cool stars, and exoplanets’ environment. Before moving to Italy, she was a research fellow at the University of St Andrews, Scotland. She has a PhD from Paul Sabatier University, Toulouse, France.


Big projects to unveil the birth, life, and death of a Pale Red Dot

By Jorge Lillo Box, European Southern Observatory (ESO)

Like a person, planets are born, evolve rapidly in the early stages of their lives, and spend most of their time interacting with others in their surroundings (in this case stars, other planets, comets, asteroids, etc.). At last, they die in a joint evolution with the system where they lived. The large crop of extrasolar planets discovered to date (around two thousand) is providing valuable information about how exactly these processes take place. But there are still many open questions that are key to understanding the whole picture. Starting from a planet’s birth and finishing with its death, I will briefly review some big projects and facilities aimed at answering these crucial questions.

Planets are byproducts of stellar formation. Stars are formed after the collapse of a molecular cloud. The result of this process is a massive object (the star) surrounded by a circumstellar disc composed of gas and dust. This disc is indeed the incubator where planets will be formed. However, the exact mechanism of planet formation is still a mystery, with different theories trying to explain the process and to conjugate theory and the observations. A property that seems to have a key role in planet formation is the amount of gas and its lifetime in the disc. Also, the amount of warm water, and in general the chemical contents available in the disc will determine the type of planets that can be formed. Two key projects using data from the Herschel mission of the European Space Agency (ESA) aim at characterizing these parameters in the different stages of planet formation: GASPS and DUNES. They are producing impressive results with the detection of warm water vapor in these protoplanetary discs, a crucial ingredient linked with planet formation and the development of life. Additionally, the observations carried out by the ALMA radio-interferometer (Chile) are shedding light on the formation process, detecting protoplanets in the first stages of their live as well as possible signatures of multi-planetary formation.

ALMA image of the protoplanetary disk around HL Tauri. The image shows clear dark rings that could be due to the presence of forming planets, although other theories (not involving planets) have also been proposed to explain them. Credits: ALMA (ESO/NAOJ/NRAO).

During the process of forming a planet within a protoplanetary disc, other minor objects are also created. Moons, comets, asteroids, and minor planets are also by-products of this process and could play a key role in the subsequent evolution of the planetary system, as well as being crucial for the development and support of life. For example, we know that tidal forces induced by the Moon on our Earth have an important role in transporting heat from the equator to the poles, contributing to the climate patterns of our planet. Similarly, in other extrasolar systems, these objects may exist and play similar or even more important roles. Additionally, the properties of their orbits (inclinations, eccentricities, etc.) are a direct consequence of the dynamical evolution of the system during its first stages. Hence, the detection of minor bodies in extrasolar systems will contribute to our knowledge on planet formation and evolution, and potentially the evolution of life. The HET and TROY projects aim at detecting these minor objects. First, the HET project has the challenging goal of detecting the first exomoons—natural satellites orbiting around known planets. They have obtained different candidates, although no confirmation has been published as for today. The TROY project aims at detecting exotrojan planets—bodies co-orbiting with known extrasolar planets in the stability points of their orbit. In the Solar System, we know that a cloud of trojans inhabits the Lagrangian points L4 and L5 of Jupiter’s orbit. Indeed, even our Earth has a long-term 300-meter diameter object co-orbiting with us.

Illustration of the minor bodies in the inner part of the Solar System, including Jupiter trojans and the main asteroid belt. These objects are byproducts of planet formation and have key information about that process. Detecting them in extrasolar systems may help us to understand the early evolution of planetary systems. Credits: NASA (Creative Commons).

On the other side, important space missions are discovering large amounts of exoplanets with which we can start doing statistics of planet populations. For instance, thanks to NASA’s Kepler mission, we now know that solar-like stars in our Galaxy harbor on average 0.77 earth-size planets. This is a crucial discovery since it tells us that earths are more or less commonly formed in the Universe. In the forthcoming years, new space missions such as TESS, CHEOPS, PLATO, Gaia, or JWST will each contribute to improve these numbers by detecting, and characterizing, extrasolar systems in other niches (for example, long-period planets).

The end of the story, as it happens in our own lives, is tragic. After several billion years, the host star exhausts its internal fuel (hydrogen) and starts to contract, while the external layers expand, making the star several times bigger than it was during its adult phase—becoming a red giant. The consequence of this process for the surrounding planets can be traumatic and catastrophic, possibly being engulfed by the star after a spiraling in-fall. But some of them can still survive. Determining the conditions for a planet to be engulfed by its host star is still a matter of debate. It is important to note that this process will also take place in our Solar System (although in several billion years). Hence, it is crucial to understand how planets die, and under which conditions, to know the future and the expiration date of our Earth. Several projects like EXPRESS, TAPAS or JOTA are currently looking for planets orbiting giant stars in order to contrast the theoretical predictions with actual data to shed light on the end of planetary systems.

An artist’s impression of HD 189733 b showing rapid evaporation of the atmosphere. Credits: NASA’s Goddard Space Flight Center (Creative Commons license).

A great technological and scientific effort is being put in to the study of all these processes. Understanding how planets are formed, evolve, and interact with other bodies in the system along their lives and finally finish their lives, is crucial to understanding our own world. The Pale Red Dot project is contributing to this by trying to detect the closest planetary system that we can find, a cousin of our own Solar System. Who knows what surprises this work will  bring? Just a few months left to get the answer…

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Dr. Jorge Lillo-Box

About the author.

Dr. Jorge Lillo-Box is a fellow at the European Southern Observatory (Santiago de Chile). Jorge studied Physics at the Complutense University and University of La Laguna. Afterwards he moved to the Astrobiology Center (INTA-CSIC, Madrid) where he got his PhD in 2015. Since last year he is settled in Chile where, if he is not pointing to a star at the Paranal Observatory, he would be delving into the study of the evolution of planetary systems in the last stages of their lives and in the detection of minor bodies through the TROY project. Among the several planets he has discovered in different niches, we highlight the first planet transiting a giant star or the closest planet to a host star ascending the Red Giant Branch, Kepler-91b.

Planetary transits: how can one measure the mass, size, density, and atmospheric composition of a planet one cannot even see?

Finding transiting planets

Our Solar System is shaped like a rather flat disk: all the major components are very close (to within a few degrees) to some imaginary “average” plane. Indeed, most (but not all) planetary systems are flat, so we can imagine other planetary systems as roughly disk-shaped. Relative to our own Solar system some of these flat systems will be seen “face on”, some will be “edge on”, and most will be somewhere in between.

One result of this randomness is that only a small minority of these systems will be oriented in such a way that their orbital plane is close to our line of sight (i.e. “edge-on”)—but this serendipity has multiple wonderful results indeed! Planets in such a system will seem to pass, or transit, in front of their host star once every orbit—as seen from earth—blocking a small fraction of the host star’s light for a short while in a “mini Solar eclipse”. This, as we will see below, allow us to both detect new exoplanets and learn a great deal about them.

Now, since stars appear like (very small) luminous disks, to a good approximation if half of the stellar disk is blocked by something opaque then we will see that star’s light diminish by half—this is what exoplanetary transits are all about. Firstly, we may ask: how small or large will this “mini eclipse” be? It is useful to have some examples: a Jupiter-like planet is about 10% the diameter of the Sun, so it can block about 1% of the Sun’s light (since the area of a circle is proportional to its diameter squared). Reversing this relation, if an astronomer finds a 1% deep transit-like dimming of a far-away star, he/she will conclude that the transiting object is approximately 10% the diameter of the host star—without ever measuring the size of the planet by an image that actually resolves the planet (obtaining such an image will require an unrealistically large telescope with a primary mirror about a kilometer in diameter).

Now, a 1% dimming of a star is something one can measure from the ground—so ~15 years ago multiple surveys started looking for such large planets (scale of Jupiter in size)—monitoring millions of stars every few minutes for months, and even years, on end. Why millions? Because not all stars are bright enough or stable enough to allow the detection of a 1% transit, and only a fraction of them actually host a large planet close-in to its host star (a so-called “Hot Jupiter”), and only a fraction of those are aligned close to our line of sight… so one has to start with millions of stars to find a handful of transiting planets. Processing this large amount of data requires sophisticated algorithms and a large degree of automation, and so quite some time passed before astronomers found out how to do this the right way. Today, a few hundreds of these ground-based detected transiting planets are known.

Top: As planets revolve around their star they exhibit phases akin to the phases of the Moon, since we see variable parts of their day or night sides throughout their orbit. This picture, however, is not what one can see with a telescope. Bottom: the observed flux (=brightness) of a star hosting a transiting planet. We will see different amount of flux depending on the part of the planetary orbit: when the planet goes behind the star we will see just the star alone. At all other phases we will see a both the star and the planet , and the the latter will also block some of the star's light during transit.
Top: As planets revolve around their star they exhibit phases akin to the phases of the Moon, since we see variable parts of their day or night sides throughout their orbit. This picture, however, is not what one can see with a telescope. Bottom: the observed flux (=brightness) of a star hosting a transiting planet. We will see a different amount of flux depending on the part of the planetary orbit: when the planet goes behind the star we will see just the star alone. At all other phases we will see both the star and the planet, and the latter will also block some of the star’s light during transit.

The space revolution

Unfortunately, detecting transiting planets much smaller than Jupiter is very difficult from the ground, if not impossible: an Earth-sized planet is a about tenth the diameter of Jupiter, or 1/100 the diameter of the Sun, so if can only impart a transit which is about 0.01% deep, much too small to be detected from ground-based observatories, which have to combat things like day/night cycles, weather, atmosphere, variable temperatures, etc.—all of which make precise measurements difficult. Fortunately, space-based observatories simply avoid all of these disturbances, so even by simply placing the exact same equipment in space one immediately gets much more precise measurements due to the very stable environment available in space—with no day/night cycles, no weather, no atmosphere etc. Indeed, NASA’s Kepler mission did more than that: the instrument aboard the Kepler spacecraft was highly optimized to the precise measurement of stellar brightness that does allow the detection of small Earth-like planets despite their small transit depth and long period. This, in turn completely revolutionized the exoplanets field.

Before Kepler about 300 exoplanets were known from all transit and radial velocity surveys combined, while Kepler alone found (as of today) over 1,000 confirmed planets, with >3,000 more candidates still waiting confirmation. Importantly, the kinds of planets Kepler found were very different from the giant planets usually found by other transit and radial velocity surveys; it found that most planets are small, with the most common type of planet only 2-3 times the diameter of the Earth. In fact, so many planets were found that it is now understood that most stars have at least one planet, and likely multiple planets, around them.

More than just detecting planets

Indeed, finding lots of planets is a lot of fun, but there is more—much more—to transiting planets than that. For starters, by combining the mass (from instruments like HARPS that measure precise radial velocity) and radius (from transits), one can easily calculate the planet’s mean density, which in turn can tell us something about the bulk composition of the planet. It was found that planets are indeed far more diverse than anyone had ever suspected before—with planets having densities spanning about two orders of magnitude; some planets are almost all high-density iron while others are less dense than Styrofoam, virtually devoid of solid material and probably all made of light elements like hydrogen and helium.

One of the early successes of the transit method stemmed from the understanding that the top layers of the planetary atmosphere are rarefied enough so they are not completely opaque. This means that as the planet transits the host star, some of the stellar light that passes through these layers is not completely absorbed and actually makes it through to us, and now it carries some information about the conditions that exist in these upper layers of the planetary atmosphere in its spectrum. By subtracting the on- and off- transit spectra one can isolate this faint planetary contribution to the overall light coming from the star, and this indeed allowed the detection of multiple interesting molecules on these far-off worlds, for example: carbon monoxide, carbon dioxide, water, hydrogen, methane and more. In the future the detection of “biomarkers”—evidence of extraterrestrial life—will be attempted using this technique.

The observed global temperature map of exoplanet HD 189733 b, the first of its kind, as inferred from the different phases of this remarkable hot Jupiter. Temperatures range from about 650 degrees Celsius on the coldest part, to 930 degrees Celsius on the hottest. Note that this hottest spot on the planet is not directly in the middle, i.e. it is not just
The observed global temperature map of exoplanet HD 189733 b, the first of its kind, as inferred from the different phases of this remarkable hot Jupiter. Temperatures range from about 650 degrees Celsius on the coldest part, to 930 degrees Celsius on the hottest. Note that this hottest spot on the planet is not directly in the middle, i.e. it is not just “below” the star or at “high noon”—but rather shifted to the East.

Moreover, by observing these planets at longer wavelengths, in the infrared, one can observe light emitted from the planet (vs. just reflected from—or refracted by—it) and thus permit us to measure the planet’s temperature on the day side. Some planets also allow the creation of rudimentary temperature maps (!) for them, finding that the hottest point on the day-side of the planet is not exactly facing the star—but shifted from it by strong, persistent jet streams. At that point one can start talking about the planet’s energy budget and global weather patterns, cloud formation and circulation,… characterizing the planet, and not just detecting it.

In some other cases “transit timing variations” were identified: the transits of some pairs of planets are not quite periodic but they appear to transit slightly earlier or later than they “should”, and in sync; when one was late, the other was early. These perturbations are cause by the gravitational interaction between different planets in the systems: both are pulling on each other, disturbing their regular rhythms. This allows us to reverse the problem and ask: “if I do see these irregularities, what kind of masses can cause them?”, the answer effectively finds the mass of the planet(s) in the system without ever using the expensive resources like HARPS—but for free—just by carefully analyzing the times of transit.

By studying yet finer details of the transit light curve—some of which have been detected and some are still too difficult to find even with current space-based data—many more things can be inferred about the planet: from planetary rings, from planetary oblateness to a measure of its rigidity, to planetary rotation rate and mean wind speed—all are derivable (in principle) from transit light curves.

We are thus able to sometimes measure the size, density, composition, dynamical interaction and much more for transiting planets, and I remind you: all that for a planet we can’t resolve or even directly see, and only because by some pure chance they happen to cross the face of their host star as seen from Earth.

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About the author. Aviv Ofir is a Dean’s Postdoctoral Fellow at the Weizmann Institute of Science’s Planetary Sciences department. Prior to arriving at Weizmann, he was a Minerva Postdoctoral Fellow at Göttingen University in Germany. He has been a member of the science team of the HATNet ground-based transit survey, as well as the space-based CoRoT and Kepler transit surveys and the Cool Tiny Beats radial velocity collaboration. Dr. Ofir has extensive experience analyzing large photometric datasets, and invented a number of techniques for the efficient detection of faint signals in these data.

Stars beat!

By Cristina Rodríguez-López, Instituto de Astrofísica de Andalucía

Excuse me? Stars what? Yes, stars do beat! Even our own Sun! You may think that stars are immobile, comfortably and quietly burning their nuclear fuels up there in the sky, but the reality we have got to know through observations is quite different. Stars ‘breath’ and ‘contort’ and ‘wriggle’ in a variety of ways, altering their radii and surface temperature which produce periodic changes in their luminosity and surface velocity that we detect with our telescopes and instruments. These effects are known by the technical name of pulsations, or oscillations, and the technique that tries to extract all possible information from these pulsating stars is known as Asteroseismology, or, in the specific case of our Sun, Helioseismology.
The naming responds to the techniques used, analogous to the Earth-seismology, that extract information from seismic waves propagating in the interior of our planet to derive the composition and stratification of the Earth. Asteroseismology analyses the frequencies of oscillation of the light coming from the stars, which are a fingerprint of the star’s interior chemical composition and structure, as well as of other fundamental parameters such as its mass, density and age.

So, what are these pulsations? Pulsations are, strictly speaking, pressure and gravity waves (usually referred to as p- and g-modes) that propagate in the interior of the star; you can think about them as sound and maritime waves respectively, this meaning that changes in the pressure and buoyancy are responsible for them. P- and g-modes propagate at different depths inside the star (see Fig. 1), allowing to probe the different regions just by measuring their frequency on the surface, that would be, otherwise, inaccessible.

Figure 1. Schematic view of two different pulsation modes propagating inside a star. P-mode are shallower and reflected on the surface at higher frequencies than g-modes that travel deeper to the core of the star. Credit: By Tosaka – Own work, CC BY-SA 3.0,

The fact that some of these oscillations are sound waves, is why, in a more poetic way, we talk about the ‘music of the stars’. Sound waves resonate inside the stars, in much the same way as in a musical instrument, and although their frequencies are too low for us, humans, to hear -and sound does not propagate in vacuum-, some scientists have amplified them, so that we can actually hear how they would sound. The experience is, however, not the pleasantest, as star’s sound waves are not harmonic.

But how do pulsations originate? Pulsations originate from the displacement of the plasma inside the star. The radiation originating from nuclear fusion in the star’s core or layers try to find its way up to the surface, but in doing so, there are some stones in its path: one are the so called partial ionization zones of chemical elements. In these layers, the star’s opacity increases with increasing pressure, holding the radiation coming from the interior and pushing the upper layers, which then become more transparent, releasing the radiation and falling in afterwards, starting the cycle again, as if it was an inhaling-exhaling process, in the same way as an engine works. If the excitation produced in these regions is larger than the damping produced in all other ones, then the star will likely show oscillations in its surface. This is called the kappa-mechanism and it is responsible for the pulsations in several types of variable stars such as RR Lyrae, delta-Scutis, beta Cephei, Hot Subdwarfs or White Dwarfs. In stars that have an inner radiative zone and an outer convective zone, the radiative flux can be blocked and then released at the interphase -or tachocline- of these two layers in a cyclic way, like it is the case for gamma-Doradus variables. Finally, in these partially convective stars, such as our Sun, an stochastic excitation of pulsations, due to the turbulent convective motions in the surface of the star -as it happens when we heat water on a pot- can also be produced.

Do all stars pulsate? And does Proxima pulsate? That’s a good question! To answer it, we may take a look at the Hertzsprung-Russell pulsating diagram (Fig. 2) which locates all types of known pulsating variables, in terms of their temperature and luminosity. The diagonal dashed line that traverses it locates the stars in the main sequence (MS), where they burn hydrogen in the core. Some of the delta Scuti are pre-MS stars, while giants like Cepheids or Mira lie above the MS and subdwarfs and white dwarfs, like GW Vir, sdBV and DAV, that sit below it, are all post-MS more evolved stars. So, it seems that all stars no matter what their mass or temperature, or their evolutionary stage, have the potential to pulsate, although beware that not all stars located there do! that is, these instability strips are not pure, but populated also by ‘normal’ non-pulsating stars.
In the case of Proxima, and more generally for M dwarfs, we do not know it yet. Pulsations have been theoretically predicted, but not yet detected observationally, although we are looking for them! We can only be sure that, if oscillations exist, they will be of very low amplitude, of the order of millionths of a magnitude if we monitor the incoming amount of light, or less than 1m/s if we measure how much the surface of the star rises and falls. If oscillations had larger amplitude than these limits, we would have already detected them.

Figure 2. Hertzsprung-Russell pulsating diagram. M dwarfs are located at the bottom of the Main Sequence diagonal line in orangish-red colour. Locations of different pulsating star classes are labeled and encompassed by striped ellipses. Credit:

So, explain me then how do you detect pulsations? The frequencies of the oscillations, or inversely, their periods, may vary between only a few minutes, to hours or a few days. For M dwarfs, like Proxima, the range is predicted to vary most likely between only 20 minutes and 3 hours. To detect these oscillations, we use two techniques also used in the search for exoplanets: photometry and spectroscopy. Photometry measures the amount of light coming from the star over a period of time: if the star is non-variable, its light-curve, or light received as a function of time, will be constant, whereas it will periodically vary if it is a pulsating star (see Fig. 3). The same technique is used by satellite space missions dedicated to search for planetary transits, such as KEPLER and CoRoT, and future PLATO, that measure the diminishing of the light of a star when an orbiting planet traverses our line-of sight. Spectroscopy, through the radial velocity method, measures the periodic velocity changes in the spectral lines of the star caused by the rising and drop of the stellar surface due to the oscillations, just as velocity changes are caused by the gravitational tug of a planet orbiting the star; this is the same type of observations we are carrying out for Proxima with HARPS spectrograph.

Figure 3. Light-curves of variable stars in cluster M1. Credit:

Then, what role do pulsations play in the discovery of exoplanets? First, we have to be sure that pulsations are not mistaken for a planet, as sometimes, they can mimic them; also if the star is pulsating, the planet search observations have to be designed such as to minimise the influence of the pulsations in the data acquisition, or to a posteriori correct the data from them, before the signature of a planet can be assessed. However, most interestingly, when a pulsating star is a planet host, we can very precisely derive its mass, radius and age, and this is an unique and precious way to pin down the physical parameters of the planet and its evolutionary history.
So, let’s hope that we find lots of planets surrounding pulsating stars!


About the Author. Cristina Rodríguez López is a post-doctoral researcher at the Instituto de Astrofísica de Andalucía (CSIC, Spain) within the group ‘Low-mass stars and exoplanets and associated instrumentation’. Her research interests include asteroseismology of low-mass stars in different stages of evolution and the search for exoplanets. Previously, she received her PhD from the University of Vigo and was a post-doctoral researcher at the Laboratoire d’Astrophysique de Toulouse-Tarbes (now IRAP, France) and a visiting scientist at the Department of Physics and Astronomy of the University of Delaware (USA). She is responsible for the scientific workpackage ‘Asteroseismology of M dwarf stars’ within the CARMENES Consortium, which brings together 11 Spanish and German Institutions around the scientific exploitation of the CARMENES spectrograph, a new planet hunter focused on discovering Earth-like planets around M dwarf stars, much the same objective pursued by the Pale Red Dot project, of which she is also a co-investigator and editor.

Directly Imaging Exoplanets

By Sasha Hinkley, Astrophysics Group, University of Exeter


It is now clear that planetary systems around stars other than our own are common, and these extrasolar planetary systems display a stunning diversity of architectures, orbital geometries, and planetary sizes. Yet experiments such as NASA’s Kepler mission, which search for the periodic dimming due to planets transiting the face of their host star in edge-on orbits, rely on indirect detection techniques in which the presence of planetary mass companions is only inferred based on variations in the host starlight. However, astronomers are now able to directly image extrasolar planetary systems using a combination of new technologies described below. Indeed, this technique has now returned a large handful of directly imaged planetary mass companions at wide orbital separations. Just as the discovery of the “Hot Jupiter” 51 Peg b orbiting its host star at a small fraction of an Astronomical Unit (AU) came as a complete surprise in 1995, so have the recent discoveries of planetary mass companions orbiting stars at much wider (20–600 AU) orbital separations using direct imaging. While the indirect techniques such as precision Doppler and transit monitoring work to resolve the signal of an exoplanet in time, the technique of exoplanet direct imaging depends on separating the light from a faint exoplanet from its extremely bright host star directly in an image.

However, as might be expected, imaging these extremely faint planets orbiting their host star is extraordinarily challenging: even young planets, still glowing from the residual heat of their formation, may be a million times fainter than their host stars. The task is analogous to observing a firefly fluttering around a bright searchlight. Now imagine trying to gather an image of this firefly while working in London with the searchlight located in Paris! This analogy accurately describes the sensitivity and resolution necessary for astronomers to uncover the faint light from these exoplanets.

To accomplish this task, astronomers must first correct for aberrations and fluctuations in the incoming light due to turbulence in the Earth’s atmosphere. Just as the turbulent air above a stretch of hot concrete causes the image of a distant object to contort and vary in time, so too does the Earth’s atmosphere affect the signals from distant planets. Those of us working in the field of exoplanet direct imaging use a technique called Adaptive Optics (“AO”, see below). By spatially separating the light of a faint exoplanet from that of its very bright host star, the direct imaging technique can directly resolve the nearby environments of stars, and is thus sensitive to wide companions (tens or hundreds of AUs), probing planetary architectures out of reach of the indirect transit and Doppler techniques.

Image showing separation of an object from its parent star by 2 arc seconds. Taken using adaptive difference imaging (ADI) at the Keck telescope in Hawaii.
An image obtained from the W.M. Keck Observatory showing a candidate planetary mass companion orbiting a nearby star. The light from the host star, located at the lower left, is being controlled using Adaptive Optics, and has been blocked by an opaque coronagraphic mask. The image of a faint candidate companion can be seen at the upper right.

How it works: AO & Coronagraphy

AO refers to a suite of customized optical components installed in a telescope, typically “downstream” from the primary and secondary mirrors (although several observatories are now building AO systems directly into the secondary mirrors to reduce the number of reflective surfaces). The task of this machinery is two-fold. First, it must use a “wavefront sensor” to sense the extent to which the turbulence in the Earth’s atmosphere has affected the incoming stellar light. Next, to correct for this distorted incoming starlight, AO systems use a “deformable mirror”, which is a flexible mirror that is continuously reshaped by mechanical actuators gently pushing and pulling the mirror hundreds or thousands of times per second. Once the stellar light has been carefully controlled using AO, astronomers use a “coronagraph”, a collection of opaque optical masks, to block out the host star light, similar to the moon passing in front of the sun during a solar eclipse. This combination of AO and coronagraphy provides the contrast needed to image faint exoplanets orbiting close to nearby stars.

SPHERE is the extreme adaptive optics system and coronagraphic facility at the ESO Very Large Telescope. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of extra-solar planetary systems at optical and near-infrared wavelengths.
SPHERE is the extreme adaptive optics system and coronagraphic facility at the ESO Very Large Telescope. Its primary science goal is imaging, low-resolution spectroscopic and polarimetric characterization of extra-solar planetary systems at optical and near-infrared wavelengths. Here the SPHERE instrument is shown shortly after it was installed on ESO’s VLT Unit Telescope 3. The telescope fills most of the picture and the instrument itself is the black box visible at the lower right. Credit: ESO/J. Girard (

New Instruments: GPI and SPHERE

Until recently, the handful of direct images of exoplanets, with orbits even wider than the outermost planets of our own solar system, were obtained with instrumentation not specifically dedicated to this task. Rather, they were obtained using instruments using some kind of custom setting or configuration allowing these extremely challenging observations to happen. Recently, however, several instruments have now been deployed that are dedicated exclusively to the task of obtaining images and spectroscopy of extrasolar planets. In addition to ongoing and planned suites of instrumentation at the Palomar and Subaru telescopes, the most notable projects are the Gemini Planet Imager (GPI), and the Spectro-Polarimetric High-Contrast Exoplanet REsearch (SPHERE) instrument, deployed at the Gemini South telescopes and ESO Very Large Telescopes in Chile, respectively. These instruments have embarked on surveys of hundreds of nearby stars that astronomers speculate could be ripe for hosting wide separation exoplanets.

This very detailed new artist’s rendering shows the European Extremely Large Telescope (E-ELT) in its dome on Cerro Armazones, close to ESO’s Paranal Observatory in northern Chile. The design shown here is close to the final one, but some small changes are expected.
This very detailed new artist’s rendering shows the European Extremely Large Telescope (E-ELT) in its dome on Cerro Armazones, close to ESO’s Paranal Observatory in northern Chile. The design shown here is close to the final one, but some small changes are expected.

The Next Generation of 30–40 m Telescopes

At the same time, construction is now underway for the European Extremely Large Telescope (“E-ELT”) at Cerro Armazones, Chile. This telescope will have a mirror with a diameter of 39 meters: a collecting area and sensitivity several times greater than any current telescope. Further, it will be equipped with an ensemble of instruments designed to address a range of astronomical goals. Among the most promising for the direct imaging of extrasolar planets is the Mid-infrared ELT Imager and Spectrograph (“METIS”). The superior contrast and sensitivity of METIS operating on a 39 m telescope will allow exoplanet imaging on orbital scales comparable to our own earth for nearby stars, and METIS will be sensitive to massive planets at slightly further orbital separations from stars residing in the nearest associations of young stars. Even further, conservative calculations suggest METIS will image 20–30 (currently known) planets initially detected by the Doppler methods, as well as a few small—potentially rocky—planets around very nearby stars. When METIS achieves first light in roughly 2025, numerous more Earth-like planets will have been identified, requiring follow up observations with the next generation of 30–40 m class telescopes for more detailed characterization

Direct High Resolution Spectroscopy:

While the images of exoplanets returned by the current and upcoming generation of instruments are particularly exciting, perhaps the greatest legacy of these instruments will be delivered by their ability to provide spectroscopy of the exoplanets. Unlike the transit technique, which fundamentally detects grazing stellar radiation after it has interacted with an exoplanet, by spatially separating the light of the host star and the extremely faint planet, the direct imaging method gives access to photons emitted directly from the exoplanet atmospheres. Obtaining photons directly from the surface of exoplanets will allow astronomers to gather all of the detailed information that has been retrieved for stars using high-resolution spectroscopy (e.g. chemical abundances, compositions, and thermodynamic conditions). This direct spectroscopy will allow unambiguous interpretation of the spectra. As such, the future of comparative exoplanetary science lies in the technique of exoplanet direct imaging.

Sasha Hinkley image 


Sasha Hinkley is a permanent member of Staff in the Astrophysics Group at the University of Exeter in the UK. Prior to arriving in the UK, Dr. Hinkley was a NASA Sagan Fellow as well as a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow at the California Institute of Technology in Pasadena. He has been involved with development of instrumentation for exoplanet direct imaging at Palomar Observatory, and is an active user of both the W.M. Keck Observatory on Mauna Kea, as well as the ESO SPHERE instrument at Cerro Paranal, Chile.

Worlds of Rock and Ice

by Yiannis Tsapras, Zentrum für Astronomie in Heidelberg and

“The worlds are formed when atoms fall into the void and are entangled with one another; and from their motion as they increase in bulk arises the substance of the stars.” – Leucippus of Elea (5th century BCE)

What new knowledge may be gained by studying exoplanets? There is a direct syllogistic link between this fledgling field of inquiry and our progressive comprehension of the complex set of processes that we collectively define as life. Our understanding of life is derived from observations of its diverse manifestations on our home planet,  all of which are inextricably linked,  sharing a common biological origin – a single genesis. The discovery of a ‘second genesis’, of life elsewhere in the universe that does not rely on ribonucleic acid (RNA) and deoxyribonucleic acid (DNA),  has profound repercussions not only for science but for every field of human inquiry.  A top-down approach to addressing this issue directly is that of the Search for
ExtraTerrestrial Intelligence (SETI) project.  It began in the ’70s and has looked for signs of transmissions from alien civilizations ever since. An alternative approach involves a gradual exploration from the bottom-up, where new scientific knowledge may be gained with each intermediate step.

Planets form in gaseous disks surrounding young stars, but the exact physical processes that drive their formation and evolution are not yet fully understood.

We know that these disks last for a few million years and that planet embryos can migrate while they are still embedded in them. Some of these planetary embryos migrate into their host stars,  while others survive and grow throughout this process but end up far from where they first started forming.  How are these planets distributed around their host stars and are there similarities with our own system or is the Solar system in some ways unique? The presence of liquid water on a planet’s surface is not a guarantee for the existence of life,  but it is a necessary ingredient for the development of life as we know it.  Liquid water can only exist at a certain range of distances from the host star, thehabitable zoneWhat fraction of planets reside within the habitable zone of their host stars, are their orbits generally stable or unstable, and what are their physical characteristics? Finding these planets and examining their distribution is our first goal, both observationally and theoretically.  We seek to study their diversity and understand the processes that drive their formation and evolution.

Searches for extrasolar planets have already yielded such a bountiful, and extraordinarily diverse, harvest that it has been necessary to overhaul the planet formation model originally put forth to explain the Solar system. These discoveries include planets orbiting Pulsars, “Hot Jupiters“—gas giants that orbit their stars every few days, planets that have been ejected from their systems and are now floating alone in the darkness, and the first few Earth-sized planets orbiting far enough from their stars that any water on their surface would be in liquid form.

Astronomers use a variety of methods to find these planets and each method is sensitive to a different domain of the planet distribution.

The distribution of known exoplanets as a function of mass vs. semi-major axis (normalized to the location of the snow line). Planets discovered by radial velocity (black triangles), transits (red circles), direct imaging (blue squares), pulsar timing (yellow stars) and microlensing (green pentagons) are shown. The planets of the solar system (yellow circles) are also depicted for comparison. The region of sensitivity of each method is also indicated.
The distribution of known exoplanets as a function of mass vs. semi-major axis (normalized to the location of the snow line). Planets discovered by radial velocity (black triangles), transits
(red circles), direct imaging (blue squares), pulsar timing (yellow stars) and microlensing (green pentagons) are shown. The planets of the solar system (yellow circles) are also depicted for comparison. The region of sensitivity of each method is also indicated.

By studying the emerging patterns in this distribution, and testing the theoretical predictions with real data,  we develop our understanding of how planets form and how their orbits evolve. There is,  however,  a certain type of planet that we still know very little about; Cold rocky planets,  far enough from their stars that any water on their surface would have turned to ice. Theory predicts that at the early stages of planet formation,  proto-planetary embryos in this region are likely to form cores of ice and rock and,  provided the conditions are right, gradually grow by accreting material from the surrounding gaseous disk, transforming them into gas giant planets. However,  if there is little gas available,  or if it gets depleted quickly,  they do not grow very large. These planets are almost undetectable with transit or radial velocity searches, but are exactly the type of planet microlensing searches are designed to find.

Einstein predicted that any massive star will act as a gravitational lens,  bending the light rays from any background star that passes behind it,  as seen from the Earth,  and distorting its apparent shape. The effect of lensing at cosmological distances is practically observed as multiple distorted images of the background (source) star around the edge of the gravitational influence of the lensing star. In microlensing the angular distances between the images generated by the lensing effect are of the order of micro-arcseconds, and the images cannot be individually resolved with current technology.  What is actually observed during microlensing events is an increase in the brightness of the source star as the lens appears to move closer to it on the plane of the sky, followed by a gradual dimming back to its normal brightness as the lens moves away.

The relative proper motions between the stars in the Galaxy produce microlensing events that last for a few weeks up to several months. If the lens is a planetary system,  there is a chance that the planets may also perturb the light coming from the source star resulting in short-lived but intense anomalous features on the event lightcurve that reveal the presence of the planet. These anomalies typically last for a few days in the case of Jupiter-mass planets and only for a few hours for Earth-mass planets.  Since the presence of the lensing object can only be inferred by its gravitational effects and not the light it produces, the method can detect planets around stars that are extremely faint, even around brown dwarfs and black holes.

Lightcurve of microlensing event OGLE-2005-BLG-390 showing a 12-hour planetary anomaly attributed to a ~5.5 Earth-mass planet.
Lightcurve of microlensing event OGLE-2005-BLG-390 showing a 12-hour planetary anomaly attributed to a ~5.5 Earth-mass planet.

Microlensing is a rare phenomenon. In our own Galaxy, only about one in a million stars will undergo microlensing at any given time. Yet modern microlensing surveys (OGLE,  MOA), monitoring the
extremely crowded fields of the Galactic Bulge,  announce ~2,000 such events every year.  A small subset of these events is selected for monitoring by follow-up teams (RoboNet,  μFun,  MiNDSTeP, PLANET) and intensive observations commence to look for planetary deviations.

Early results from microlensing searches at intermediate orbital radii (1-10AU) indicate that ice and gas giant planets are a relatively common feature around M-dwarf stars. These searches have also found a number of quite massive planetary and brown dwarf companions orbiting low-mass stars, several planets with masses close to that of the Earth, systems with multiple planets, and possible first evidence of an exomoon. Over the next few years, ground-based searches will be complemented by space missions, which will allow for a much clearer understanding of the planet distribution at and beyond the icy regions of planetary systems.


About the Author. Currently holding a research position at the Zentrum für Astronomie in Heidelberg,  Yiannis previously sated his scientific curiosity working for the LCOGT institute (2007-2014). During the 2012-2014 academic period he was also a Teaching Fellow at the School of Physics and Astronomy, Queen Mary University where he taught Physics for the Science and Engineering Foundation Programme and a MSc course on Extrasolar Planets and Astrophysical Discs. Prior to that, he held a research position at the ARI in Liverpool. He received a PhD in Astronomy from the University of St Andrews in 2003 and a MSc in Radioastronomy from the University of Manchester in 1999.  His  primary research interests lie in the field of exoplanet science where he uses the complementary techniques of microlensing and transits to search for planets orbiting distant stars. He is a founding member of the RoboNet project which uses the LCOGT telescope network to detect exo-planets by microlensing. Besides exoplanets, he has a keen interest in robotic astronomy, ongoing dark-matter surveys, CMB cosmology and science outreach programs.

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 (, 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)”.