Category Archives: Earths in Other Systems

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.

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)

Building Planetary Systems

Mikayla Mace is a UA journalism graduate student who is writing a nice series of article on our Earths in Other Solar Systems work. She has just posted a piece on our Genesis Database, the mother of all planet formation simulations, featuring Gijs Mulders and Fred Ciesla. The Genesis Database will help us understand how habitable earth-like planets can form and around which stars are they more likely to exist:

Building Planetary Systems: The Genesis Database

Not the ocean planet you were told about: By weight Earth has only very little water, below a percent.

Not the ocean planet you were told about: By weight Earth has only very little water, below a percent.

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