Category Archives: Exoplanet Discoveries

The Mysteries of a Circumstellar Spiral and the London Fog

A few days ago we posted a new paper on arXiv on the spectacular spiral in the HD 100453 system. The new paper, led by Steward astronomy graduate student Kevin Wagner, settles the question of the origin of the enigmatic spiral arms.

A near-infrared color-composite image of the young binary star system, in which the gravitational pull of the fainter, lower-mass companion (seen on the left) drives two beautiful spiral arms in the gas-rich disk of the primary star. From Wagner et al. 2018


This rare two-armed spiral structure was discovered by Kevin – as a first-year graduate student – back in 2015 (and covered in a popular press release) using the SPHERE extreme adaptive optics system on the Very Large Telescope in Chile, in observations carried out in our Scorpion Survey. Strikingly, the spiral arms are about 40 times as long as the Earth-Sun separation — that is, longer than the Neptune-Sun separation in the solar system.

The spiral arms are not the only features in the system: they connect to a well-defined inner dust ring, within which there is much less dust (making this a “transition disk”). At about the location where the arms connect to the ring two darker regions are visible, which may or may not be important.

But interpreting light scattered from fine dust is always a difficult undertaking:

What do we really see on the image? Which structures are real and which are only tricks of light?

In Monet’s paintings of foggy London, tricks of light hide most of the buildings and we can recognize the city only from a few well-kown silhouettes: shapes and colors are altered and details are missing. Still, the presence of the fog itself and the change of lighting across the image in different versions of his The Houses of Parliament tell us about the scene as much as the few recognizable figures.

In The Houses of Parliament (Effect of Fog, 1903) Claude Monet isolated and transformed the well-known silhouette of the Parliament through the effects of the thick London fog. The soot particles that polluted London’s air were similar in size and in their effects on visibility to the particles astronomers often refer to as dust grains, although their origins and compositions differ.

Following Kevin’s discovery, the HD100453 spiral arms emerged as an immediate mystery: what process drives these structures? Explaining their presence is particularly important, because one-armed spirals are seen in an increasing number of disks and a few others now also sport two-armed disks. The arms may mark planets hidden by the dust — just like the buildings of London in Monet’s mysterious fog — or may just be a trick of light? It was proposed that shadows from in the inner, faster-orbiting disks may be projected to the outer disk, creating the illusion of structures (like the larger dark gaps where the spiral arms join the rings, and elsewhere), where none exists. Or, such shadows may also change the temperature of the outer disk (it will be cooler in the shadows), which would in turn change the overall structure of the disk, giving rise to a spiral arms as changes propagate in the disk — translating a shadow to a physical evolution in the disk.

The problem was that no one knew enough about the system to distinguish real geometry from tricks of light — multiple different proposed models could explain the arms seen in scattered light equally well, but differed in their assumptions of the underlying system architecture.

In our new study Kevin has used old and new observations of the system to constrain the orbit of the companion (a red dwarf star labeled B) over the past fourteen years.






Although the fourteen years is only a small fraction of the star’s orbit around the center of mass in the system, it was sufficient to constrain the plane of its orbit, which was previously unknown. Kevin then used another technique to better constrain the plane of the disk itself: as you can’t quite trust scattered light images to determine where the structures (mass) is hidden, he used longer wavelength observations from the ALMA radio interferometer array to figure out how inclined is the disk. ALMA can see through the fine dust grains and can pick up where more of the mass is located: although the images do not provide fine details, they can tell us how the disk structure is oriented.

The results are exciting: star B and the disk are orbiting in the same or nearly identical planes.

Now, with the known orientation Ruobing Dong could carry out hydrodynamical simulations of how the star and the disk would interact – even if started without spiral arms, these quickly form in the simulations and follow the morphology of the scattered light images extremely well.

These results show that no unseen planets are required to drive the spiral arms. They also provide a possible template that we may follow when interpreting other systems with spiral arms. Particularly compelling are the ones in which there is no known companion star – now, equipped with a model that reproduces a well-understood system we may be able to pinpoint the planets hiding in these disks: just like we know that the buildings of London are hiding in Monet’s paintings.

Exoplanet Postdoc Position Open

I am glad to announce a postdoctoral opportunity within the EOS/NExSS project, in my group at Steward Observatory in Tucson. We are excited to connect and compare planet formation models and their predictions to exoplanet populations; we are looking for a postdoctoral researcher with expertise in planet formation, exoplanet population studies, and/or statistical assessment of exoplanet surveys,

The review of the applications will begin on February 9, 2018.

Interested? Please see the AAS ad for more details:

Contact me with any questions (!

Transit Spectroscopy, Biosignature Searches, and the Myth of Perfect Stars

Can we detect atmospheric biosignatures in the next two decades? Only if we can meet a major, newly-recognized challenge to our studies of exoplanet atmospheric composition.

Over the past years the Hubble Space Telescope has proven to be our most powerful tool to probe the atmospheres of transiting exoplanets: the comparison of spectra taken before and during the transits can reveal the compositions of the atmospheres. Exciting discoveries included condensate clouds, hazes, extremely efficient scatterers, molecules (water, methane, and carbon-dioxide), and atoms (sodium and potassium). Some of the most ambitious research programs are pushing this technology to levels never envisioned previously as they reach spectacularly precise measurements on increasingly small planets. This technique may even allow the detection of water vapor in the habitable zone earth-sized planets in the TRAPPIST-1 system with HST and additional gases with JWST.

But underpinning these measurements is an approximation that is called in question now – in fact, one that we show is wrong for typical systems.

Astrophysicists focusing on exoplanets often assume that the planet host stars are perfect. As we show in our new paper, led by Steward Observatory graduate student Ben Rackham, this assumption is underpinning high-precision transit spectroscopy. In reality, stellar heterogeneity contaminates exoplanet transmission spectra (HST and JWST) and — unless we figure out how to correct for this effect — it will greatly limit our ability to search for biosignatures in the next two decades.


Because transit measurements are relative measurements stellar spectra cancels out – in the first order approximation:  the difference of a spectrum taken before transit and during transit will provide the transmission spectrum of the exoplanet atmosphere.


It is tempting to think that this approximation holds with infinite precision: in fact, the majority of the transit spectroscopy papers in the literature simply adopt this approximation without further considerations. This may be OK for the typical, less precise measurements, but remains a dangerous assumption for high-precision studies and for all but the least active stars.

In reality, the stellar spectrum does not cancel out in transmission spectroscopy, because the first-order approximation described above confusingly equates two different light sources: the stellar disk (the spectrum of which is observed before the transit) and the actual light source, which is just a very small fraction of the stellar disk — the projection of the transit chord onto the stellar photosphere (see figure).

Mercury’s transit in front of the Sun illustrates that even quiet stars are heterogeneous on the fine length scale of exoplanets.

Stars are not perfect: in reality, no patch of the stellar photosphere has the same exact spectrum as the stellar chord. The difference between the assumed lightsource (stellar disk) and the actual lightsource of unknown spectrum (photosphere under the chord) imprints itself onto the transmission spectrum observed.

The effect itself is not new: Some of the best published studies tried to develop a correction for the suspected stellar contamination. In doing so, almost all groups assumed a linear relation between the photometric variability of the stars and the covering fraction (basically: more variability means more spots).

In our new paper we worked with Mark Giampapa — a solar/stellar astrophysicist —  on the first comprehensive study of this effect and its impact on transit spectra and exoplanet density measurements.

This project brought about important, surprising, and concerning results.

Our team — part of the larger Earths in Other Solar Systems project — has created toy models of a star with starspots and faculae to assess the connection between stellar variability and stellar spot covering fraction; we then used state-of-the-art stellar atmospheric models to predict the stellar contamination in the transit spectra of the planets, which we then compared with atmospheric absorbers (including biosignatures) that could be detected in transmission spectra now and in the near future. Finally we also assessed the impact of the apparent size (and therefore density) of the exoplanets: could starspots lead to an apparently lower planet density (more volatiles/gaseous envelope)?

Photometric variability amplitudes are very poor tracers of stellar spot/facula coverage.

Our findings are detailed here, but the key points are:

  • The amplitude of stellar variability (photometrically determined brightness variations) that is sometimes used as a basis to argue for low spot covering fractions is an extremely poor measure of the stellar heterogeneity: the linear correlation many published papers assume is wrong in most cases.
  • Considering the actual spot covering fraction range that typical stellar variability amplitude really translates to, a much broader range of stellar contamination is possible than previously considered.
  • The contamination is not only limited to changing the slope of the spectrum (which is the effect most are aware of), but it will also introduce spectral features – especially for red dwarf host stars, whose spectra are rich in molecular features. The results are concerning: without additional information, it can be extremely difficult or downright impossible to distinguish water absorption in the star from water absorption in the planet. We find that even molecules not present in the stellar photosphere (O2 and O3) are difficult to identify given the large contamination from the star.
  • For TRAPPIST-1, currently the most exciting planetary system for transit spectroscopy studies, we predict that stellar contamination could be 4-7 times greater than the intrinsic planetary features.
  • Finally, we also show that for host stars with a larger number of spots, the planet density calculated will be too low – errors as larger as 15-25% are to be expected. The error is huge if one’s goal is to understand the possible composition of a small, mostly rocky planet; but less of a problem for hot jupiters.

Expected range of stellar contamination for M-type stars. The contamination can suppress or mimic several of the key absorbers expected from planetary atmospheres, including water and oxygen.

The stellar contamination in transit spectra can be very significant: in fact, our predictions are that the contamination levels are high enough the be present in numerous Hubble Space Telescope studies already published.

How can we recognize and distinguish stellar contamination from genuine planetary atmospheric features?

We are working on this and testing multiple ideas; multi-epoch data, high-resolution spectra, and better understanding of the spot and facula properties are likely to be part of the solution. Of course, the larger fraction of the exoplanet community thinks about this challenge, the more likely it is that we can solve the problem to inform upcoming Hubble and James Webb Space Telescope observations, which may then lead us to sampling habitable zone exo-earth atmospheres within the next five years.

Further reading: Rackham, Apai, Giampapa 2017 Astrophysical Journal, in press (arXiv)

Extrasolar Storms: Belts, Spots, and Waves in Brown Dwarfs

Our new paper came out today in Science, presenting evidence for bands, zones, spots, and waves in brown dwarfs and a model that explains well several until-now mysterious changes in the brightnesses of brown dwarfs.

Podcast: Learn more about our project from the Science Magazine’s podcast!


I am excited about our results because they open a new window on very fundamental processes in brown dwarfs (atmospheric circulation, heat exchanges, and cloud formation) and, at the same time, they also explain a number of past observations that puzzled brown dwarf experts. As always with brown dwarfs, the results are much more far-reaching than people often realize: brown dwarfs are excellent proxies for giant exoplanets: often what we cannot learn from giant exoplanets  we learn from brown dwarfs.

Brown dwarfs and Exoplanets: This is the decade of exoplanets, so one may wonder why are brown dwarfs important. News often describes brown dwarfs as “failed stars”, a label I find misleading: in fact, most brown dwarfs are much more similar to giant planets than to stars. What’s more, it is almost certain that the brown dwarf population contains a large number of ejected giant planets — bona fide exoplanets that were booted from their natal systems by more massive siblings. Known brown dwarfs have temperatures between 250 K to about 2,500K — completely overlapping with the temperatures of giant exoplanets; the compositions of many brown dwarfs are likely very similar or identical to many of the giant exoplanets. But most excitingly, the physical and chemical processes in brown dwarf and exoplanet atmospheres are the same; the identical processes, combined with the fact that brown dwarfs are much easier to study is the reason why we learn so much about exoplanets from brown dwarfs.

A great video summary of our results by JPL:

So, what’s new? Our study shows atmospheric circulation in brown dwarfs for the first time: it shows that brown dwarfs have bands and zones, spots, and that cloud thickness in the zones is continuously changed by atmospheric waves. We found that brown dwarfs are similar to the gas giants in the Solar System (in that they have zonal circulation) , but that they are more like Neptune and less like Jupiter (their brightness variations are driven by large-scale waves in zones rather than Great Red Spot-like storms as in Jupiter). The waves are an interesting piece of the puzzle: we see large-scale waves in the solar system planets (including Earth), but we have not yet seen waves with wavelengths similar to the entire planet — like the ones we now found in brown dwarfs.

Why is atmospheric circulation important? Atmospheric circulation — large-scale flows of air in atmospheres — is very important as it sets how heat and particles/droplets/gas are distributed in a planet. For example, in Earth atmospheric circulation (such as Hadley cells) transport heat between the warmer equatorial regions to the cool polar regions and this circulation pattern not only determines the temperature distribution, but also sets which regions on Earth are dry or rainy and how clouds form over the planet.

Morning Glory clouds in Australia are atmospheric waves rendered visible by cloud formation.

What are these waves? On a fundamental level waves are changes that propagate through a medium. For example, dropping a pebble in a lake will force the water to move away from its equilibrium  — and that change will propagate across the surface of the lake. Atmospheres have many different types of waves: for example, (gravity) waves are common and they often propagate on the interface of warm air sitting on top of cold air — these waves are invisible to us (as air is mostly transparent), but they can lead to spectacular sights when clouds highlight them. (This Berkeley meteorology class’s page gives a couple of cool examples). The two examples shown here are small waves — atmospheric circulation is driven by large-scale waves, with wavelengths that are hundreds or thousands of kilometers.

Atmospheric waves at the interface of rapidly flowing air (above) and near-stationary air (below), leading to mixing and heat transport. Photo by Benjamin Foster.

What does this tell us about exoplanets? Whatever we find in brown dwarfs should be pretty much be the same for most giant exoplanets in the galaxy — only the rare hot jupiters (very heavily irradiated by their host stars) should look different (but even for those, the underlying processes that shape their atmospheres should be the same). So, based on our results we would expect that most giant exoplanets will have zonal circulation; we should expect that their atmospheres are not homogeneous, structureless, but in fact should display large brightness variations in the infrared. We should also expect that giant waves will propagate in their atmospheres (parallel with their equators) and that these waves will change the thickness of the clouds. Our next steps will be to figure out what processes drive these waves (probably some combination of heat transport, winds, and rotation) and to improve the cloud models — the same cloud models that are used to interpret exoplanet atmospheres, too. Importantly, we also learned that the atmospheres of gaseous exoplanets should have regions with very different appearance: where the clouds are thinner or lower, we can see into the deeper, hotter atmosphere. This should be an important warning for most current studies that use one-dimensional atmospheric models: in other worlds they assume that every bit of the planet is like the rest.

I am also excited about our results because they demonstrate how much we can learn from unresolved data — basically, from a single pixel. This is crucial as this is all we are going to get for exo-earths: we will not be able to build large enough telescopes to take detailed images of the surfaces of exoplanets — but we will still be able to learn about their atmospheres (and surfaces) from time-resolved observations!

Near-infrared images show bands on Jupiter. Brighter regions have thinner cloud cover, allowing radiation to escape more easily from its hotter interior.

How did we do it? We used the Spitzer Space Telescope and watched the brown dwarfs rotate. As they rotate their brightness changes: when a brighter spot rotates in the visible hemisphere, Spitzer will see the brightening. The brown dwarfs we observe take between 1.5 and 13 hours to turn around fully: we used Spitzer to observe 32 complete rotations of each brown dwarf. This allowed us not only to map the cloud distribution, but also how it changes from rotation to rotation and also over longer timescales: our observations were following the brown dwarfs for more than a year. We then used a novel computer algorithm developed by my colleague Theodora Karalidi to figure out how the brown dwarfs look and how the clouds change. Another team member, Mark Marley from NASA Ames, used a different set of models (cloud structure and light propagating through the clouds) to help figure out how high in the atmosphere are the clouds we see. Initially, we expected that the changes we see are driven by Great Red Spot-like stable features (the GRS has been seen in Jupiter for more than 300 years) — but the brightnesses of the brown dwarfs changed way too much to be explained by spots, Waves, however, worked extremely well. We then realized that the waves and bands not only explain our own data, but a humber of other puzzling findings reported by other teams. We had an excellent team of experts who all contributed different pieces to solving this puzzle.

Very large and very rapid changes in the lightcurve of 2M1324. From Apai et al. (2017, Science).

What’s next? One of our next steps is to expand this study to directly imaged giant exoplanets, which will allow us to explore how cloud properties and dynamics change with the mass of the objects — this cannot be done well with the sensitive, but low-resolution Spitzer Space Telescope. We are using the Hubble Space Telescope in our program Cloud Atlas, to prepare coarse cloud maps for about a dozen or so cool brown dwarfs and exoplanets.



Exoplanets: Headlines from the Future

The field of exoplanet is exploding: on a typical day about a dozen new peer-reviewed exoplanet studies are published and most weeks see announcements of multiple discoveries: new results range from the compositions and structures of exoplanet atmospheres through new findings on exoplanet formation and exoplanet population to exciting discoveries of the smallest, coolest, or lowest-mass planets. Exoplanets all over the headlines. But what discoveries will be in the headlines ten and twenty years from now?

Surprisingly, this question is very important now. It is important because most discoveries today are made by telescopes that were designed and built ten to twenty years ago – and what discoveries we may make in the future depends on what telescopes, instruments, and space missions we are building now. Different telescopes and observational techniques are great for answering different questions: no telescope can do it all.

Karl Stapelfeldt, NASA Exoplanet Exploration Program Chief Scientist, with a possible cover from a future New York Times.

So, what questions we will be able to answer in the future depends on what telescopes we build now, which in turn depends on the questions we think are going to be important.

I posted yesterday on the preprint server arXiv a report we worked on with a dedicated group of exoplanet experts and which we recently delivered it to the NASA EXOPAG Executive Committee and to the NASA Astrophysics Advisory Committee (more on the abbreviations at the bottom of the page)*. The study builds on input from the exoplanet community to identify the most interesting science questions that we may be able to study in the future with direct imaging missions – that is, space telescopes that can directly image exoplanets (separating their light from that of their host stars).

To be clear, our report does not determine or advise NASA on which missions we should build – that will be done by multiple other committees – but reports on what science questions the community thinks are the most important and potentially solvable questions. Our study informs and guides the community and NASA (and various observatories and organizations) when deciding on future exoplanet strategies.

So, what do astronomers think about the future of exoplanet research?

Even though we have learned a lot about exoplanets in the past decade, it is clear that we are just scratching the surface of the universe of amazing, exotic, and surprising worlds. Reflecting this our group started with a huge list of questions – close to a hundred of them, everything we wanted to know about exoplanets. Too many questions to be useful, but through discussions and analysis we weeded out questions that seemed to be intractable even in the best foreseeable future. This cut down our list, but still left us with too many questions. After lengthy discussions we were able to combine many of the questions into more general or fundamental questions, which again led to a shorter list.

Then the work really started: we needed to understand which of our questions will be answered in the next decade or so by telescopes already being built (such as NASA’s JWST, TESS, or the 30m-class ground-based telescopes) — none of those questions were interesting for our report. With the truly amazing work being done on exoplanets now, many of the obvious questions on our list will, in fact, be answered by 2030.

This process left us with high-level, important, but often very tough questions that will not be answered with any of the telescopes currently existing or being built. They will be the big questions in   a few decades. These are the questions that require truly powerful new telescope(s).

Many of the questions have to do with habitable worlds, which is not surprising. Still, some focus on gas exoplanets and some on ice giants (think cold or hot exo-neptunes) or super-earths. (In our report we did not focus on directly searching for and characterizing extrasolar life, because it was being addressed in a parallel report, SAG16 – but we covered how habitable worlds can be characterized).

The nine questions we identified naturally fell in three categories: Questions in Category A  aimed at exploring planetary systems: what are their structures, components, how do they form and evolve, what combinations of planets and planetesimal belts are common, etc. Although much progress will be made on these questions over the next two decades by telescopes being built now, we found that no telescope will be able to give us the complete picture: some will detect only close-in planets, others only dust disks, yet others only planets far out.

Questions in Category B are questions about individual planets: what are their atmospheres made of, do they have clouds and hazes and if so, where do those come from? Which of the (small) planets are truly habitable, i.e., that they have liquid water on their surfaces?

Finally, Questions in Category C aim to understand how planets work. These were some of the toughest questions, especially those about rocky planets. These worlds are the most difficult to detect, yet they can be so diverse (we think): just consider how different Mercury, Mars, Venus, and Earth are! In the future we will want to know not only how these planets look now, but why — how did they evolve to be the worlds they are. Unlike massive gas giant planets, whose strong gravity will hold on to pretty much all the stuff they formed with, puny rocky planets often lose their atmospheres (Earth and Mars definitely did).

This study has been fun: over one and a half year we held virtual and physical meetings to explore ideas and methods for exoplanet characterization; I found the list of questions we converged to to be really exciting.

Perhaps the most important questions are, however, those that directly aid our search for life on other planets. It is clear that the search for life around other stars is going to be with one of the most fundamentally important experiments ever conducted; but it is also clear that it is going to be extremely difficult. Not only is it technically difficult to detect the gases in the atmospheres of earth-like planets that could reveal life, but it is similarly difficult to interpret them. In fact, none of the atmospheric signatures we think we could detect in exoplanets would allow us to conclude that we found life unless we have a pretty good understanding of the planet. This is because all biosignature gases we could possibly detect could also be produced by some odd geological or atmospheric processes — all without life. To exclude those “false positives”, we must know the worlds in detail.

Questions in Category C aim exactly at this: Is there a geological activity on a planet? How did it evolve? What processes set is atmospheric circulation?

Many of the tough, but also very exciting questions go beyond astrophysics and connect to planetary sciences, geophysics, geochemistry, and atmospheric sciences: fortunately, we could draw on multi-disciplinary expertise from the NASA NExSS group to explore these questions.

Our report was a community effort – we received input from a large number of exoplanet scientists who volunteered their time and expertise to explore what the future should bring. For me, it has been a thrilling experience to work with such a great team and to try to figure out if and how we could in explore oceans, volcanism, climate, and other exotic properties of exoplanets in the future – for all the exciting discoveries we are making today, I am sure that the future will be even cooler.

Of course, we can be sure of one thing: With all the exciting questions we can identify, there will be many surprises and unexpected discoveries.

So, even though our report helps us to guess some of the topics in which future exoplanet discoveries will be made – I, for one, will surely follow closely the exoplanet news even twenty years from now.

The Coolest Exoplanet Imaged – The Discovery of GJ 504b

Exciting news for planet hunters: Working with the 8m Subaru telescope at Mauna Kea the international SEEDS team announced the exciting discovery of a new directly imaged planet – this new planet is exciting not only because very few planets have been directly imaged yet, but also because this one is different from all others seen until now.

Japan's 8.2m Subaru telescope at the summit of Mauna Kea, Hawaii.

Japan’s 8.2m Subaru telescope at the summit of Mauna Kea, Hawaii.

In short, this planet is cool because it is cool; and it is also exciting because it orbits a sun-like star. Let me explain why do these properties make this an exceptional discovery.

The Coolest Planet Yet Imaged
Exoplanets are very hard to image: they are faint and always close to very bright stars. In fact, this task is so difficult that right now we can only see the easiest planets: the ones that are the largest, furthest away from their host stars, and brightest. As you would expect, the hotter a planet is the brighter it shines, especially in the infrared, where direct imaging searches are conducted. Not surprisingly, planets imaged until now are all hot super-jupiters, as these are the brightest possible planets.
Unfortunately, these planets are rare and the vast majority of stars imaged by the few competing teams (including ours) do not seem to have such planets.

Most planets directly imaged until now had temperatures 800-900 K or higher (some as high as 1,800 K!). This is much hotter than Jupiter (170 K), Earth (288 K), and even warmer than Venus (737 K). Once a planet is imaged we can start exploring its atmosphere in detail – as we have done for over a dozen directly imaged hot planets.
The new planet GJ 504b is a record-holder: its temperature is only 510 K! Although still hotter than Earth, this planet is much cooler than the previous directly imaged planets and now offers an opportunity to explore how the atmosphere of such a warm giant planet look like!

Super-Jupiters around Sun-like Stars

Like all other directly imaged exoplanets, GJ 504b is a super-jupiter, but it differs from all the others in an important aspect: GJ 504b is the first such planet to orbit around a sun-like star. Most previously imaged exoplanets, curiously, orbit around much more massive stars (A-type stars). Although over 300 sun-like stars have been searched for large-separation super-jupiters, until now none has been found, suggesting an important difference in the planetary systems between sun-like and more massive stars (more on this interesting topic later). GJ 504b also turns out to have lower mass than all but one directly imaged exoplanet.

The newly discovered planet around the nearby star GJ 504. The image was taken by the international SEEDS team using the Subaru telescope.

The newly discovered planet around the nearby star GJ 504. The image was taken by the international SEEDS team using the Subaru telescope.

Although lighter than its counterparts around more massive stars, GJ 504b is still between 3-9 times more massive than our own Jupiter! Not only is it massive, the radius of this planet’s orbit is at least 44 AU. This is again surprising if you consider that Jupiter, the most massive planet in the Solar System, is only 5.4 AU from the Sun and at orbits this long the Solar System has virtually no mass in planets – which probably has to do with how much mass was available to form planets that far from the Sun.
So, the puzzle is: How can a star seemingly similar to the Sun be able to form such a massive planet so far out?

Interestingly, GJ 504 also joins the small set of known planet host stars that are visible to the naked eye. At a dark site you will be able to see GJ 504 (it is 5.2 magnitude) without a telescope in the Virgo constellation, although of course you will not be able to see the planet.

Fortunately, this exciting planet is equatorial and thus visible from telescopes at both the northern and southern hemispheres. The planet will be again visible from February next year and surely will be among the hottest targets for all adaptive optics systems.

The detection of GJ 504b is a very exciting next step toward cooler and lower-mass planets and very soon we will learn how the atmosphere of such a warm planet works! Congratulations to the SEEDS team!

Paper by Kuzuhara on arXiv

The Substellar Zoo: From Brown Dwarfs to Super-Earths

In this illustration by Frost all stars have planetary systems (which is about right) with regularly placed, circular orbits and planets like those in the solar system (mostly wrong).

In this illustration by Isaac Frost all stars have planetary systems (which is about right) with regularly placed, circular orbits and planets like those in the solar system (mostly wrong).

I particularly like Frost’s illustration from 1846 which shows how planetary systems were thought to look like in a post-Newtonian universe: in essence, Frost’s universe is filled with copies of the solar system – planets orbit each star. Interestingly, 130 years later the Star Wars universe was not that different: the desert planet Tatooine, the snow planet Hoth, or the forest moon Endor all strongly resemble Earth (not surprising for a movie shot mainly in California). From the vintage print of Frost to the last century’s most visionary sci-fi movie, exoplanetary systems remained just like the solar system and all planets remained similar to Earth (perhaps apart from the exotic collection of tentacled man-eating monsters).

Last week, when traveling from Budapest to Italy by train, I stopped briefly in Venice. Curiously, this charming town known for its canals, palaces, gondolas, and art exhibitions is also arguably the birthplace of cutting-edge exoplanet science: in 1995 in a single conference three important discoveries were announced. The discovery of the first brown dwarfs and the first extrasolar planet orbiting another star, 51 Pegasi b. These exciting discoveries marked the end of universe filled with Solas System ‘clones’ and brought about a reality that is much more interesting and often surprising. Now, there are enough different exoplanet types that they may appear to be a small zoo – but the main distinctions are simply mass, temperature, and composition.

Venice, the birthplace of the substellar zoo: The first exoplanet orbiting a star and the first brown dwarfs were announced here in 1995.

Venice, the birthplace of the substellar zoo: The first exoplanet orbiting a star and the first brown dwarfs were announced here in 1995.


So, what type of sub-stellar objects and exoplanets exist?

Brown Dwarfs: Brown dwarfs are gaseous objects that have too low mass too low (and therefore too low central pressure) to drive fusion reactions, from which stars get their energy supply. The stellar/substellar boundary is defined by the ability of the object to fuse hydrogen into helium and thus produce large amount of energy. The precise mass limit depends on the exact composition of the object, but it is about 70 Jupiter masses (or 0.08 solar masses or about 22,000 earth masses). Objects less massive than this but more massive than planets are called brown dwarfs. The lower boundary is often quoted as 13 Jupiter mass, which has been proposed as a natural break: objects more massive than this will very briefly fuse deuterium and generate some energy temporarily, while the less massive will never be able to drive fusion reactions.

Exoplanets: Objects less massive than brown dwarfs but larger than asteroid sizes that orbit other stars are called exoplanets (or extrasolar planets). Exoplanets are a very diverse group and come in many flavors.

Planetary-mass Objects: If Jupiter would be somehow ejected from the Solar System and become unbound, would it still be a planet? Not according to the current definition. We can now find in great number unbound gaseous objects that are lighter than brown dwarfs but as they do not orbit stars they are not planets. Lacking a better name the term planetary-mass objects has been coined for these.

It is interesting to note that many astronomers adopts a stricter definition for extrasolar planets: only objects that formed from material orbiting a star are called included. This definition addresses the status of the strange objects like 2MASS1207b – a planetary mass object that orbits a brown dwarf at such a large distance that it is not possible for it to have formed like a planet would.

The faint red companion 2M1207b is about 5-7 times as massive as jupiter. Although it has a mass low enough to be a planet, it is far enough from its host that it could not have formed from a disk, like planets do. Thus, it is an example for a planetary-mass object.

The faint red companion 2M1207b is about 5-7 times as massive as jupiter. Although it has a mass low enough to be a planet, it is far enough from its host that it could not have formed from a disk, like planets do. Thus, it is an example for a planetary-mass object.

Among exoplanets we often speak about very different objects:

Super-Jupiters: Exoplanets more massive than Jupiter but less massive than 13 jupiter masses, the planet/brown dwarf boundary. Examples include the four known planets in the HR 8799 system, Beta Pictoris b or the recently discovered GJ 504b.

Hot Jupiters: Gas giants planets with masses similar to Jupiter (which is 320 earth masses) that orbit very close to their host stars and thus have extremely high temperatures, usually well over 1,000 K (about 1,300 F). Examples include 51 Peg b, HD 209456b, and Corot-1b.

Hot Neptunes: Giant planets with masses around 20 earth masses that orbit their hosts stars on very short orbits (days) and due to the vicinity to their host stars they have very high temperatures, similarly to hot jupiters.

Super-earths: Planets with masses between 2 and 10 earth masses. Objects in this category may be fully rocky (i.e. jumbo versions of earth), may be ocean planets (with hundreds of times more water than earth has), but they have also enough gravitational pull to hold on to very massive gaseous envelopes (think of a mix between earth and neptune). Although the Solar System has no such planet, super-earths are now found in a rapidly increasing number and as they are easier to characterize than smaller planets, are set to be very important targets for astronomers in the coming years.

Earth-sized planets: Planets with radius similar to Earth. Note, that the density of earth-sized planets may cover a relatively large range and some of these planets may be more massive or  less massive than Earth. Even more importantly, an Earth-sized planet may have a similar size to Earth, and may be a much hotter dry rock or a deep-frozen icy body, depending on how close it is to its host star. Most such small planets known currently have been found by the Kepler space telescope; this transiting planet search mission can only measured the sizes of the planets and not their masses. 

Earth-like planets: An earth-like planet (or exo-earth) has very similar same size, mass, and temperature to Earth. If these key parameters are similar, there is a good chance that the conditions on the surface of the Earth-like planets is similar to those on Earth – but remember, that during most of Earth history the atmosphere and temperatures were very different from those on modern Earth. Nevertheless, finding and characterizing Earth-like planets is a key goal of astronomy and astrobiology.

Sub-Earths: In the Solar System two out of the four rocky planets are much smaller than Earth: Mars is 11%, while Mercury is only 5.5% of Earth’s mass! The Kepler space telescope‘s amazing accuracy has allowed the detection of planets smaller than Earth in a few exceptional cases, such as the three planets in the Kepler-42 system. Planet formation models predict that sub-earth-mass planets should be very common, even if we can currently only detect a few.

An amazing diversity, isn’t it?