Category Archives: Exoplanet Exploration

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: https://jobregister.aas.org/ad/770e6378

Contact me with any questions (apai@arizona.edu)!

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.

Mont Blanc, Slopes, Skiers, and the HST/WFC3 Ramp Effect

Hanging out at the Servoz Train Station in the French Alps

Hanging out at the Servoz Train Station in the French Alps

It is a beautiful, sunny, but cool day in the little village of Servoz in the French Alps: surrounded by breathtaking snow-capped mountains – among them the legendary Mont Blanc – I am sitting on a tiny railway station waiting for the little red mountain train that will carry me out of the valley. With still an hour to go before the train I am hoping that getting out will be easier than getting in: this was the first time a lecture I was about to give had to be postponed due to an avalanche.
I had spent five amazing days here, three of which at the Les Houches School for Physics, where Nicolas Iro, Francois Forget, and Valerio Lucarini organized an outstanding school on planetary circulation. This was definitely a school to remember – great lectures, lots of discussions, great food and skiing opportunities allowed by the long mid-day breaks, and fun evening discussions, all set in a picturesque Alpine setting. This was also an important meeting for me personally because we announced here the publication of our new study which, I believe, is an important step in exoplanet characterization with the Hubble Space Telescope. It may even be important for Hubble’s successor, the James Webb Space Telescope. Our study offers a solution to the infamous “ramp effect”. This vaguely understood effect has been plaguing all Hubble Space Telescope observations — among them some of the most beautiful data on exoplanet atmospheres — ever since the instrument was installed in the memorable 2009 Hubble Servicing Mission 4.
Hubble was never designed to do exoplanets: it really was built to be a cosmology machine, mankind’s most advanced telescope (for science) peering into the depths of the universe and — due to the fact that the speed of light is finite — also to the depth of time. Its sharp vision has revealed galaxies back to about one billion years after the Big Bang (or about 12.5 billion years ago). Yet, somewhat paradoxically, that sharp vision is not enough to explore planets even around the closest stars. Planets orbit their bright host stars so closely that even Hubble’s resolution and image quality are not enough to distinguish the light of the planets from the light of their host stars. But that is not the end of the story for Hubble: as a testament to human ingenuity, different teams of astronomers realized that for some systems observing brightness changes in time can help separate the light of the planet from that of the star. Soon after different techniques popped up that used this idea: from changes in the star’s brightness we are now able to deduce when a planet passes in front of its host star (planetary transit) and from precise measurements of the level of dimming in different colors we can start figuring out what the atmosphere is made of (if it contains water vapor, for example, the planet will appear larger at the wavelengths where light can be absorbed by water than at wavelengths where it can’t). For other systems, observations showed that very close in and very hot Jupiter-like planets – as they orbit their host stars – will add varying levels of extra light to the pixel where the star’s light is collected. With a very hot dayside, these planets are very bright and depending on how much of their bright dayside we see (in different parts of their orbits) the extra light will vary. This allowed astronomers to create the first crude maps of hot jupiters, a spectacular result from the last decade. This is now an unexpected but booming field of discovery and results from planets received visibility and generated excitement on par with the exciting cosmology results. And this was just a beginning – exoplanet programs more ambitious than ever are being executed on Hubble.
Even with constant illumination HST's WFC3 detector does not show a flat line but a hook-like pattern known as the "ramp effect". A major challenge for precise time-resolved observations required by exoplanet observations.

Even with constant illumination HST’s WFC3 detector does not show a flat line but a hook-like pattern known as the “ramp effect”. A major challenge for precise time-resolved observations required by exoplanet observations. From Zhou et al. 2017

But there was one problem that continued to annoy astronomers, continued to limit HST’s accuracy for these measurements. It led to both some unreliable results and to forcing astronomers to discard large amounts of precious Hubble time. The problem became known as “the hook” or the “ramp effect”. Although HST exoplanet programs rely on extracting tiny changes in time, Hubble – simply put – will see all stars change regardless of whether there are planets around them. Measuring a typical star’s brightness with Hubble – which should yield a precisely flat line – will instead result in a funny hook-shaped light curve: a shape that will be different for each star and even for each Hubble orbit observations of the same star! This effect is typically 1.5% – it is small for most other studies with Hubble, but huge for exoplanets where we are after much tinier effects. How can we then use Hubble to map clouds on other planets if even stars appear to change?
The Les Houches school had many participants who have been developing impressive models for how exotic exoplanet atmospheres should behave – and compared their model predictions to mostly data from Hubble, most of which was affected by – to different levels – by the infamous ramps. Therefore, in my lecture I decided to highlight both the problem and the solution we found for it.

To explain how this works, let me tell you more about tourists in the Alps. After the school ended on Friday I stayed for the weekend in Les Houches and, following Alain Leger’s advice, I used Saturday to go up to one of the highest peaks – Aiguille du Midi (3,842m), which offered incredible views of the Chamonix valley. I took a thrilling cable car ride up to the top – a whopping 2,807-meter ascent in just about forty minutes!

Moonrise and Aiguille du Midi with its astonishing viewtower

Moonrise and Aiguille du Midi with its astonishing viewtower

On top of the peak is a crazy tower – it seems small from below (see the photo with the moonrise that I took from the village of Les Houches) but standing on top of it is a majestic and humbling experience. At the top of the tower is a viewing terrace with one of the most beautiful and panoramic views I have ever seen. Standing on the terrace on top of a ten-story-high needle-like tower carefully balancing on a 100 meter-high cliff, buffeted by strong, icy winds, and blinded by the bright, untamed sun of the high altitudes, I can all but wonder about the raw power of nature. Gigantic mountains and majestic peaks all around  – among them Mont Blanc (4,810m), Dome du Gouter (4,304m), Mont Maudit, Aiguille de Verte (4,121m). I went up on a good day but the low-level of oxygen (only 45% of that at sea level), the high wind, the sun, and the brightness of the snow offered a glimpse of what it may be like to try to climb one of these stairs (although the only climb I did was the stairs to cafeteria, one of the highest in Europe).
The peaks of Les Drus (3754m)

The peaks of Les Drus (3754m) and Aigulle Verte (4121 m)

Dozens of skiers traveled with me on the cable car from Chamonix to the top of the mountain: most equipped not only with skis but with ice axes, ropes, and climbing harness. I suspected that their goal was not the cafeteria but to ski down from the top, which must be an incredible (and seemingly dangerous) experience.
From the peak I could watch these skiers; holding on to chains and inching on top of an impossibly steep cliff to reach the slightly gentler slope that does not end in a thousand meter free fall. Then they started, one by one, their descent – following a gradient in gravitational potential energy.
Skiers in the Alps - starting from almost 4,000 meters they will ski down to Chamonix.

Skiers in the Alps – starting from almost 4,000 meters they will ski down to Chamonix.

Chamonix valley - surrounded by majestic peaks.

Chamonix valley – surrounded by majestic peaks.

Interestingly and unknown to the skiers, they follow a similar pattern – for a very similar reason — to what we have seen in the HST data.
Imagine now that you want to figure out how many people are brought up to the mountain top by the cable cars by counting the skiers that arrive back to Chamonix. You could determine that, say 50%, of the people on average will ski (while the rest of us enjoy the view and a glass of French wine); so you would know that for every person you count at the bottom there was – on average – another one that went up. That is easy. However, if you were to count the people arriving to the bottom via ski – right after the arrival of the first cable car – you would see first only a few arriving, then more and more, until the number of skiers arriving each minute reaches an approximately constant value. If you plot the number of skiers arriving in 5-minute chunks of time, you may get a curve that is similar to what HST measures when it observes a star of constant brightness. You may wonder why do you see fewer skiers first, then more skier a bit later, even though the cable cars run precisely on schedule and they are always packed to full capacity.
View from Aiguille du Midi
The solution for both skiers and for HST has to do with what happens to them after departure and before arrival. Experienced skiers can ski all the way down; they start one by one and arrive roughly the same amount of time later than they started. But some skiers – perhaps the less experienced or less lucky – will fall, often hitting obstacles covered in snow. Recovering from a fall could be easy or – if the skier hit a bad obstacle – could take a longer time. With many skiers going down, some of those hidden obstacles will be visible as skiers will be trying to stand up and recover from the fall: as long as a skier is stuck at an obstacle, other skiers will easily see and avoid those.
If you start observing the arrival of the first skiers in the morning, those that arrive first are the ones who did not fall – then you start seeing skiers arrived who started early, but fell once. Just by counting the skiers’ arrival rate you may think that cable car is not bringing up skier at a constant rate – but if you look carefully and you understand the pattern, you can figure out how many obstacles are there and what is the chance that a skier hits an obstacle.
In our paper – led by University of Arizona graduate student Yifan Zhou – we proposed a model in which HST’s detector is like a slope with hidden obstacles (traps). When the detector is illuminated — say, by a bright star — electric charges will be freed that will travel in the detector (helped by an electrical potential difference) until they are detected. However, if a charge hits an obstacle, which are most likely imperfections in the detector, they can get trapped and it will take time until they can find their way out of the traps, leading to their delayed arrival. Yifan has done a great job in translating this idea into a set of equations and then went on to show that this works perfectly for many different HST datasets! It is an exciting result that was accepted to the Astronomical Journal (http://adsabs.harvard.edu/abs/2017arXiv170301301Z) and which we are already using to revisit some of the most interesting HST exoplanet observations.
It seems to work so well that, who knows, one day we may even use it to figure out how many people ski down from the Aiguille du Midi peak.
Eventually, the little red train did come and is taking me to the next adventure – the UK Exoplanet Community meeting. Scotland, here I come!
 Daniel Apai at Aiguille du Midi

The Future of Exoplanet Research

By Daniel Apai
Includes interview with Nick Siegler and Shawn Domagal-Goldman

Over the weekend, at the Hilton on the San Diego Bay, a small group met to speak about the present and future of NASA’s Exoplanet Exploration program. To someone not in the field of exoplanets the talks and debates may have resembled science fiction: giant space telescopes, rockets and spacewalks, hyper-precise measurements of stellar motion, search for alien life, exploration of volcanism on exoplanets, laser-combs, starshades, and other Earths across the Galaxy were just a few of the topics that were debated. The memorable images included cows illuminated by lasers in a Nevada desert. It was a fun meeting and a timely one, too.

EXOPAG Meeting San Diego

The field of exoplanets is hotter than ever: we learned that planets are literally everywhere and that planets with sizes similar to Earth are the most common among the known planets. Many of the stars (probably 1 in 4) harbor about-earth-sized planets with stellar heating similar to Earth. Not only did we learn about the frequency of the planets, but also about their properties. New missions and instruments are being built and planned, conferences and school galore, and amazing discoveries are made almost weakly. The enthusiasm is palpable in the field; yet. we know that reaching our grand goal of finding extraterrestrial life is going to be anything but easy.

We can only find life if it produces a signature that is detectable from vast – literally astronomical – distances. Seen from space humans, trees, elephants, or even whales are undetectable and unremarkable, yet Earth would reveal its secret to an outside observer through the surprising abundance of a highly reactive gas, molecular oxygen. Oxygen is and has been produced by  advanced photosynthetic organisms, first in the ocean and then on land. About 2.3 billion years ago oxygen has saturated the planet’s surface and rapidly accumulated in vast amounts in our atmosphere, From that point on Earth’s atmosphere became a glowing indicator of life for the entire Galaxy – at least, for civilizations that are slightly better in building telescopes than we are.

So, starting from the only example we have, NASA’s Exoplanet Exploration program is aiming to build a telescope that will look for oxygen or other similarly odd gases in other earth-like planets atmospheres as possible signatures of life.

In a perhaps unusual consensus, the exoplanet community is united behind the most important goal, surveying nearby exo-earths for biosignatures. Few other approaches to detecting extraterrestrial life seem feasible. Although the goal is clear, possible approaches and ideas are plenty: the abundance of proposed approaches stems from the fact that no telescope that exists today (or at least, accessible to astronomers) is capable enough to directly search for biosignatures in known exo-earths. Building one that will be up for the job is not going to be easy: in fact, right now, we do not know how good exactly that telescope would need to be, what capabilities it would have to have — and we don’t know how we would build it.

Guided by the vision of finding extraterrestrial life, astronomers, astrobiologists, technologists, engineers, project managers are all working together to come up with concrete plans for such telescopes. Our goal is to create at least two different designs for life-finding telescopes by 2019. The year is important, because in 2020 the astronomical community will issue a major report, the Decadal Survey. This study will set the strategy for NASA for 2020s and beyond and will determine whether planning and construction of such a telescope can begin in a few years or we need to wait another decade.

What the best telescope design is will depend on what questions we want to answer and on the properties of planets, too: our meeting in San Diego explored these issues as well as the technology development needed to build a telescope more ambitious than anything very built. For example, one possible telescope design would use a “starshade” – a giant (think fifty meters or hundred and fifty feet) flower-petal-shaped mask. The strange mask would fly tens of thousands of miles in front of the telescope and could, if positioned precisely, cancel out the light of the host star completely, revealing the faint planets. However, nothing like this has ever been flown in space or used in ground – so a Northrop-Grumman team of engineers is testing this idea in the night in a dark Nevada desert, shining bright a light to a telescope from miles away and covering the light with a small starshade mask in between. One night however, a cow, perhaps intrigued by the strange glowing flower in the desert, wandered into the light beam and photo-bombed the experiment, thus becoming part of the history of space exploration.

The San Diego meeting was exciting and fun: a lot of progress has been made recently, but much more needs to be done in the next three years to finalize plans for a space telescope that can look for life on other Earths

At the meeting I also grabbed the opportunity to interview two experts who approach this question from different angles: Dr. Nick Siegler, who is the Chief Technology of NASA’s Exoplanet Exploration Program; and Dr. Shawn Domagal-Goldman, astrobiologist and biosignature-expert at the NASA Goddard Space Flight Center.

Several important studies of space telescope design and science questions will be carried out over the next year or two, pushing our technology and understanding toward the long-term goal. It will be exciting to see how this group of smart people figures out solutions to problems that were thought to be impossible to solve, and how it will overcome unexpected barriers, such as curious cows.

Launching Toward Other Earths – EOS Updates from the PI

News and updates on NASA’s Earths in Other Systems Project from PI Daniel Apai. May 10, 2015.

NASA's new Nexus for Exoplanet System Science program offers an ambitious, novel approach to study and understand habitable exoplanetary systems.

NASA’s new Nexus for Exoplanet System Science program offers an ambitious, novel approach to study and understand habitable exoplanetary systems.

Sunday early morning with a coffee in my hand, sitting next to giant blooming Saguaro cacti and citrus trees in Tucson with the spectacular Catalina mountains in the background. Two tiny hummingbirds angrily hover around each other in the air, in a surreal, high-speed aerial fight over the nectar drops in our bottlebrush flowers. A rare, quiet moment to reflect on the launch of our Earths in Other Systems project and the five years ahead of us in this exciting endeavor.

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Morning Coffee with Saguaros and Catalina Mnts

After almost two years in planning and preparation, our Project EOS has finally began: an exciting meeting at NASA HQ has launched NASA’s new Nexus for Exoplanet System Science program (which is funding EOS), we published the first paper with EOS results and investigators, the first postdoctoral researchers and a program coordinator are joining our project in May, our website is also online, and we began preparations for transforming a group of offices at the Steward Observatory of The University of Arizona into the EOS “Headquarters”.

EOS Overview

Our EOS team studies the formation of planets capable for sustaining life through three closely connected questions.

Project EOS is an ambitious, exceptionally large-scale research project that combines different disciplines and research techniques to understand how Earth-like planets form. While we now know that Earth-sized planets that receive similar amount of energy from their host stars as Earth does are common in the Galaxy, we do not know how similar these worlds are to Earths: do they only have the same size, but very different compositions, or are many of these worlds truly Earth-like, each carrying in it a potential for rich and complex living systems to emerge? Consider Venus, Earth’s “evil twin”:  81% as massive as Earth and orbiting at 72% of the Earth-Sun distance, it is a world that — seen from hundreds of lightyears — could appear misleading similar to Earth. Yet, through differences in its formation and evolution Venus has become a world with a surface and atmosphere astonishingly different from Earth: entirely devoid of water, lacking plate tectonics and its ability to bury CO2 and stabilize its, Venus’s thick CO2 atmosphere traps the incoming solar radiation and heats up to about 740 K (464 C). Or consider the opposite extreme: NASA’s Kepler mission has found a new type of planets, super-Earths, to be very common in the Galaxy. Many of these super-earths may have very low densities, an evidence that they must have lot of water and light, extended atmospheres. And a “lot of water” here means hundreds or thousands of Earth oceans’s worth of water, completely covering the silicate mantle of the planets, most likely in hundreds of km-thick high-pressure water ice layers, below thick liquid oceans or high-pressure steam atmospheres. These “water worlds” may be just inhospitable to life as the hot, acidic, bone-dry desert Venus has evolved into.

View from the hellish surface of Venus from the Soviet Venera probes.

View from the hellish surface of Venus from the Soviet Venera probes.

How many of the planets in the solar neighborhood are truly Earth-like — moderately rich in volatiles and organics — is an essential question to answer if we want to carry out a meaningful search for extraterrestrial life: for surveying nearby Earths for signatures of life is going to be one of the most complex and challenging endeavors in science yet.

In Project EOS twenty-five of the best experts from five disciplines will work together over the next five years to understand how the composition and volatile and organics budget of newly formed Earth-sized planets is set. In a fascinating set of projects we will look at the smallest scales and back in time, probing the mineralogy and composition of micron-sized grains in ancient meteorites using the most sophisticated microscopic techniques, to explore the history of volatiles and organics in planetary building blocks at the time when the Solar System was young. We will also use optical, radio, and infrared telescopes to study young stars and, around them, planetary systems in formation to piece together the incredible story of a dusty disk rapidly transforming itself into a planetary system that may support life. In search of new knowledge our team will travel to most continents on Earth and will use telescopes in the Sonoran Desert, the Chilean Atacama Desert and on Hawaii’s Mauna Kea; the Hubble and Spitzer Space Telescopes. We will also build powerful computer models for the planet formation process and use these to inspect the details and fill out the gaps; we will  compare the predictions of these models to the properties of exoplanetary systems: planetary orbits, masses, densities, atmospheric compositions. If we succeed, what we learn here will guide our and NASA’s search for life beyond Earth.

I am fortunate enough to work with a team of truly outstanding scientists from the diverse fields, all working toward a shared goal. Over the next five years, our team will also be joined by a dynamic group of young students and postdoctoral researchers: the team at its largest will include over forty researchers. But we will reach an and involve much larger groups: Our results will find their way to the courses we teach and we will also build up a team of Other Earths Ambassadors – citizen scientists excited by the search for life on other planets and eager to contribute.

We will share the excitement and news from the EOS project through blog updates, public talks, Twitter and Facebook posts; join us and follow the blog and twitter feeds and you will learn about our science results, discoveries, travels, and about exploring other worlds, directly from the front line.

Twitter: @EOSNExSS, @danielapai

NExSS Kick-off Meeting at NASA HQ

Two weeks ago NASA has announced its new Nexus for Exoplanet System Science, which may prove to be a major change in the way NASA will fund exoplanet science in the future. Our UA-led team was part of the first selection and I, the principal investigator of our project, joined the program’s two-day kick-off meeting at NASA HQ. The meeting was exciting, inspiring, and challenging at the same time. There have been several press releases and articles about the program in various online and printed media; what follows is my own personal perspective on the meeting.
Introduction to VPL by Victoria MeadowsView from 17th Floor in Crystal City

Questions, Problems, & Puzzles

NASA has invited the principal investigators and key members of 16 NASA-funded teams working on topics related to exoplanet habitability, as well as the directors of the new initiative to discuss and debate the best format and goals for the new program. The teams were selected from regular proposal  submissions to different NASA programs through the usual peer-review process, but invited to NExSS in addition to their selection to carry out the research they proposed.

The motivation for launching NExSS, as I understand, comes from the rapidly growing importance of extrasolar planet habitability research within many different NASA programs. The recent restructuring of NASA research grant programs (XRP, Habitable Worlds, etc.) further emphasized planetary habitability studies across many programs, which led to different aspects of habitability funded through different channels, without a good way to coordinate research between the programs. In addition, planetary  habitability-related proposals accounted for a very large fraction of the major proposals that responded to the latest opportunity to join the NASA Astrobiology Institute.

NExSS is a new approach to study extrasolar planets: the program’s idea is to combine various studies of planetary habitability funded through existing NASA programs into a new framework – one in which the teams collaborate and have influence over the broader, longer-term research directions.

Although many people at NASA have been involved in and contributed to launching NExSS, Mary Voytek, senior scientist for astrobiology, is the chief architect of the new program and program officers Christina Richey and Doug Hudgins, among others, also played important roles. Shawn Domagal-Goldman has also provided important input and advice for the new program.

Our meeting began with short talks at a NASA HQ auditorium, which included welcomes by Jim Green and Paul Hertz, the directors of the NASA Planetary Sciences and Astrophysics divisions. They expressed excitement about exoplanet research, emphasized the need for studying planets as “systems” and they strongly endorsed connecting research projects in different disciplines that address exoplanet habitability. Their enthusiastic support of NExSS was a clear demonstration of how strongly NASA is supporting the new interdisciplinary research coordination network.

We were also welcome by the three new co-directors of NExSS, Dawn Gelino, Natalie Batalha, and Anthony Del Genio, who work on various aspects of exoplanet research.

Next, lightning talks by the leads of each of the 16 teams introduced the scope of the teams; the single-slide presentations gave the first insights into the surprising breadth of NExSS. The NExSS teams have been selected from a set of projects submitted and selected for regular NASA programs (e.g., XRP, Habitable Worlds, Astrobiology, Heliophysics), so the sixteen teams brought very different expertise and perspectives to the table.

The projects also covered a broad spectrum in size, ranging from a few 1-2 investigator grants through a number of medium-sized teams to a few really large teams with multi-million dollar grants. These latter programs are our University of Arizona-led Earths in Other Solar Systems team (PI: Apai), the Arizona State University-led team (PI: Desch), a team led by NASA Goddard Institute for Space Sciences (PI: Del Genio), a team led by Berkeley (PI: Graham), and the one at Hammond University (PI: Moore). NASA’s press release and the team websites provide more information about the teams; I will instead focus on the kick-off meeting.

DistantEarths-1027

Working hard on putting the puzzle pieces together at the NASA NExSS Kick-Off meeting.

In contrast to the more usual top-down approach, our group’s first task is to brainstorm on its own purpose and definition. This has been an unusual responsibility; most committees are tasked to chart a course to reach a specific goal on a well-defined timescale. Defining our own goals and purpose is much more challenging; however, it also gave us the valuable opportunity to brainstorm and debate on the importance and achievability of different science goals over various timescales.

NASA has contracted a small company, KnowInnovate, to facilitate the creative process; this small team — two brothers — helped us move forward in the complex debate. Indeed, it has proven challenging for our team to converge on a set of well-defined goals in its first meeting; but by the end of the meeting we did identify our next steps and, I believe, made progress forward in surveying the questions, problems, and goals for the field.

Questions, Questions, Questions

Questions, Questions, Questions

The 2-day discussion resulted in covering most vertical surfaces of the meeting room with neon-colored sticky post-it notes, each with a question, problem, goal, or idea relevant for exoplanet studies. Arranged thematically, by importance, or by timescale, these stickies captured well the complexity and the heavily connected nature of next decade’s exoplanet research.

There discussion was productive and interesting; the number of questions and problems identified, and their complexity, is daunting, to say the least. Questions ranged from the impact of stellar hosts on the habitable planets through the importance of the formation and evolution of planetary systems to the unknowns of planetary interiors and life’s impact on the planet.DistantEarths-1029

Nevertheless, in a process that built on large quantities of coffee, snacks, and post-it notes, we identified some short-term steps and topics of immediate interests. These included establishing working groups on topics relevant for many questions (missing experimental data, cloud physics and chemistry), plans for workshops/conferences to connect to the community, blog-type snippets on new exoplanet research papers, just to name a few.DistantEarths-1028

It has been exciting to see a launch of a new program and one the exoplanet community can so actively shape. From my perspective, the NExSS group’s most important goal is interfacing and connecting: both within the group – in which we had a great start – and also with the broader community. The NExSS Executive Council will gradually change as PIs rotate in and out of the group over the next years, but I am very hopeful that the group will maintain its collaborative spirit as we put together the pieces of this exciting, but complex extrasolar puzzle.

You can follow our team’s work and results on Twitter (@EOSNExSS) or by subscribing to email announcements on our website ( http://otherearths.org ).