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Executive Summary

Overview

The Virtual Planetary Laboratory’s interdisciplinary research effort focuses on answering a single key question in astrobiology: If we were to find a terrestrial planet orbiting a distant star, how would we go about recognizing signs of habitability and life on that planet? This question is relevant to the search for life beyond our Solar System, as outlined in NASA’s Astrobiology Roadmap Goals 1 and 7. VPL research spans many of the Roadmap objectives, but is most relevant to Objectives 1.1 (Formation and Evolution of Habitable Planets), 1.2 (Indirect and Direct Observations of Extrasolar Habitable Planets) and 7.2 (Biosignatures to be Sought in Nearby Planetary Systems).

Recent observations have brought us much closer to identifying extrasolar environments that could support life. The successful Kepler Mission has found over three thousand planetary candidates – many of them smaller than twice the diameter of the Earth – and several residing in the habitable zones of their parent stars. Kepler’s large survey and its K2 follow-on have improved our understanding of how common terrestrial planets are in the Galaxy. The future TESS mission will help find potentially habitable targets for the planned James Webb Space Telescope (JWST), which could use transit spectroscopy to probe the atmospheric composition of at least one habitable zone super-Earth. WFIRST/AFTA-C will provide the first opportunity to directly image planets in the habitable zone around nearby stars and may obtain spectra of a handful of them. In the longer term, we anticipate large spaceborne telescopes, such as NASA’s HDST, LUVOIR or AT-LAST concepts, that can directly image and obtain spectroscopy of a larger sample of potentially habitable terrestrial extrasolar planets.

The VPL provides a scientific foundation for interpretation of data from extrasolar terrestrial planet detection and characterization missions such as Kepler, TESS, JWST and HDST. To do this, the VPL uses data provided by NASA’s Earth observing and planetary exploration programs and information from Earth’s stages of evolution, to validate and develop more comprehensive models of terrestrial planets. These models allow us to simulate and explore the likely diversity of extrasolar planet environments in advance of the more challenging observations. These models are primarily used to understand the radiative and gravitational effects of stars on the planets that orbit them. Combinations of model and fieldwork are also used to understand which biologically produced gases can produce a detectable biosignature in globally-averaged planetary observations, and whether planetary processes such as photochemistry, atmospheric loss, or geology can mimic the signs of life we hope to look for in planetary environments. Finally, models and instrument simulators are used to understand how we can best extract information on a planet’s environment from astronomical observations that have no direct spatial resolution and may be quite limited in other ways.

The team required to develop and run these models is necessarily large and highly interdisciplinary. Our research encompasses single discipline efforts that produce results pertinent to our overarching habitability and biosignatures focus, all the way through to highly interdisciplinary efforts where stellar astrophysicists, planetary climate modelers, orbital dynamicists, atmospheric chemists and biologists work together to determine the effects of stellar radiation and gravitation on the habitability and biosignatures of terrestrial planets.

Our Research This Year

Our research can be divided into five main tasks: Solar System Analogs, Early Earth and Mars, The Habitable Planet, The Living Planet and The Observer. The Project Reports provided here are all subtasks of these tasks. The first four tasks explore known and simulated environments to understand the factors that affect habitability, the plausible range of terrestrial planet environments, both inhabited and uninhabited, and the global impact of life on its environment. This knowledge can be used to help prioritize newly discovered potentially habitable planets for more-detailed observational follow-up, and to generate new biosignatures to be sought in planetary spectra. The fifth task, the Observer Task, uses the models and data generated and gathered in the first four tasks to assess the remote detectability of newly postulated global signs of habitability and life.

Solar System Analogs for Extrasolar Planet Observations

In this task we use observations of Solar System planets and moons to explore the detectability of signs of habitability and life on terrestrial planets. In Schwieterman et al. (2015b), VPL team members made the first-ever detection of an absorption feature in Earth’s whole-disk spectrum due to molecular nitrogen (N2). This feature—the 4.2 μm N₂-N₂ collision-induced absorption band—was found using comparisons between observations of the distant Earth from NASA’s EPOXI mission and simulations from the VPL 3-D spectral Earth model (which is a tool for simulating the phase- and time-dependent spectrum of the Pale Blue Dot). Molecular nitrogen can be a major constituent in planetary atmospheres, but its abundance is notoriously difficult to constrain due to a lack of spectral features. Thus, this work provides a new means for determining N₂ concentrations in exoplanet atmospheres, which could be used to constrain surface pressure (thus helping to indicate liquid water stability) and could also rule out certain mechanisms for producing abiotic atmospheric oxygen.

In other Solar System work, Glein et al. (2015) used observational data from NASA’s Cassini mission and chemical models to constrain the pH of a sub-surface ocean on Saturn’s moon Enceladus, which is a key target of astrobiological interest due to its potential for originating and/or harboring life. This work suggests that Enceladus’ ocean is a Na-Cl-CO₃ solution with a very alkaline pH. Such a high pH is indicative of serpentinization of chondritic rock that, if still occurring on Enceladus at the present day, would provide an energy source for life through the production of molecular hydrogen. Thus, these results provide an example of how signatures of habitability and life from sub-surface environments could be remotely constrained.

Also, in a modeling study of another outer Solar System world—Titan—Charnay et al. (2015) explained the observed eastward propagation of dunes on this planet using a coupling between tropical methane storms and superrotation. This work provides insights into volatile cycles on dry planets, as might occur for habitable worlds that form dry or lose most of their water.

Finally, in a VPL-led study, Krissansen-Totton et al. (2016) collated spectra of planets in our Solar System with a large number of VPL planetary spectral models to determine optimum filter bands for discrimination of Earth-like spectra from other planetary types. These authors found that, for exoplanet observations in which noise is dominated by dark current, planet color can provide an efficient means of preliminary characterization.

Early to Current Earth and Mars

In this task we performed research to understand the early Earth and Mars environments, both of which serve as potential analogs for habitable environments unlike those seen on Earth today. We are expanding this line of work from past reports to span the entire histories of both planets. On Mars, we have developed explanations of modern-day measurements of the Martian atmosphere from Curiosity (SAM in particular), as well as explanations for the presence of liquid water on the surface of Mars billions of years ago. On Earth, we have done work from the origins of life all the way through the effects of anthropogenic greenhouse gas emissions on modern-day climate cycles.

Our work this year stretches all the way back to hypotheses on the origin of life on Earth and the geological environment at life’s origin. Baross and colleague proposed a concept for a ribofilm in which RNA’s origin-of-life role would have been more akin to a slowly changing platform than a spontaneous moment in time when a self-replicator arose. This paper linked the RNA world to realistic early Earth settings for the origin of life. It also presented a testable benchmark for attaining the hallmark characteristic of all Earth life: “the unity of biochemistry”. Sleep also published a review of the tectonic history of the Earth including a discussion of the conditions for the origin and evolution of early life on Earth, biological effects on global geological processes, and other concepts of high relevance to astrobiology (Sleep, 2015b).

However, most of our work this year focused on the period after the origins of life, but prior to the rise of oxygen in Earth’s atmosphere. This included a paper by Smith, Catling and colleagues (Pecoits et al., 2015) that demonstrated that photosynthesis that does not produce oxygen was likely present to account for iron formations formed 3.8 Ga (Ga = billions of years ago). Iron formation deposition tells us about major changes in the biosphere and atmosphere, and this analysis suggests the presence of photosynthetic life at the time of the very earliest sedimentary rock record on Earth. A separate study by Stüeken, Buick and colleague (Stüeken et al., 2015a) showed that the process by which biology incorporated nitrogen had evolved by 3.2 Ga. This would imply that this process is ancient and potentially not very difficult to evolve, and could plausibly exist elsewhere. This work is synergistic with separate work by Stüeken, Buick and colleague (Stüeken et al., 2015b) on the likely alkalinity of Archean lakes, which may serve as good analogs for the alkaline lake environments Curiosity is discovering on Mars.

Our work on biological nitrogen incorporation also has significant implications for the amount of nitrogen in Earth’s atmosphere. We continued VPL’s work on understanding the amount of nitrogen – and the total pressure – of Earth’s ancient atmosphere. This included the development of a proxy for the planet’s total atmospheric pressure (Som et al., submitted), some reanalysis of previously used proxies for pressure (Kavenagh and Goldblatt, 2015), research on the planet’s nitrogen budget over time (Johnson and Goldblatt, 2015), and the development of a semi-analytic treatment of Earth’s ancient nitrogen cycle (Goldschmidt abstract: http://goldschmidtabstracts.info/2015/487.pdf).

Given those constraints on pressure, we studied Earth’s ancient climate. This included studies of the maximum amount of warming that different greenhouse gases can deliver (Byrne and Goldblatt, 2015), and 1D simulations of the maximum temperatures that could have been achieved given geological constraints.

Prior to the rise of oxygen, Earth’s climate may have been driven by the thickness of an organic haze, which appears to have been intermittently thick prior to the rise of oxygen. Our work this year supported this hypothesis by expanding the global extent of the geological data sets that are consistent with such a haze (Izon, et al., 2015). In a separate study Arney, Domagal-Goldman, Meadows, Schwieterman, Charnay and colleague (Arney et al., submitted) examined the climatic effects of this haze, and studied its implications for future exoplanet observations.

A separate analysis by Krissansen-Totton, Buick and Catling of Earth’s carbon isotope record over the last 3.6 billion years shows that although this varied over time, it did not vary enough to explain the rise in atmospheric oxygen 2.4 Ga (Krissansen-Totton, et al., 2015). This was a rigorous assessment of one potential explanation for the magnitude and timing of the oxygenation of Earth’s atmosphere and oceans, one of the great unsolved problems in studies of Earth history.

Stüeken, Buick and colleague continued to look at geochemical proxies for and implications of the Earth’s rise of atmospheric oxygen. This includes work that suggests a spike in selenium weathering occurred during the brief ‘whiff’ of oxygen prior to its permanent appearance in the atmosphere (Stüeken et al., 2015c). This has implications not only for atmospheric chemistry, but also the rigorousness of weathering of the continents, and the chemistry of the oceans at the time. It also has implications for biological production of potential biosignature gases. This also led to work on the ability of one particular element – Se – both participate in novel biosignature species (Stüeken et al., 2015d) and to have revealed the redox state of the ocean during one of Earth’s past extinction events (Stüeken et al., 2015e).

All of this work is being leveraged by our team on multiple spaceflight projects. This includes a series of papers (Freissinet et al., 2015; Mahaffy and Conrad, 2015; Mahaffy et al., 2015; Stern et al., 2015; and Webster et al., 2015) from the MSL/Curiosity Sample Analysis at Mars (SAM) instrument team that included contributions from VPL team members Conrad and Domagal-Goldman. The experience of these two VPL team members, developed by studying Earth history, informed this work. This is also exemplified by papers that did not come from the MSL/SAM team, but still leveraged VPL’s expertise as developed by our Earth Through Time task. For example, this year we published a paper on the sustainability of H₂-dominated greenhouses on early Mars, which leverages both numerical and conceptual models that were previously developed by our team for simulations of the rise of oxygen on early Earth. Finally, we note that VPL contributed to both sides of a rigorous debate on the detection of methane (CH₄) by the SAM/MSL team (Webster et al., 2015 and Zahnle, 2015). NPP Postdoc Jon Toner also continued is work analyzing Phoenix Lander data, and using laboratory work to study the freezing point and water activity of salty solutions with depressed freezing points that may enable habitability in colder environments such as the Martian surface (Toner et al., 2015a,b).

The Habitable Planet

VPL’s core research is in the area of planetary habitability. This VPL task explores habitable planet formation, and the effect on planetary habitability of interactions between the potentially habitable planet, its star, other planets in the system, and the host galaxy.

VPL has theoretically explored the role of water in the atmospheres as of habitable planets. Kasting, Kopparapu and colleague (Kasting et al. 2015) found that for planets near the inner edge of the habitable zone, water vapor can still penetrate into the stratosphere and escape, implying planets do not have to develop a full-blown runaway greenhouse to lose their water over geologic time. We also showed that multiple stable states of climate could exist for a water world, including both habitable and uninhabitable states, at the same level of incident radiation, suggesting that water-rich planets in the habitable zone are not necessarily habitable (Goldblatt 2015). Zahnle and colleagues explored conditions under which a wet, Earth-like habitable planet can evolve into a dry, Dune-like habitable desert planet. Desert planets have broader habitable zones than ocean planets like Earth, and hence their origins and evolutions are pertinent to characterizing habitable zones in general (Kodoma et al. 2015).

In planet formation, Quinn and Backus ran supercomputer simulations of planet formation and migration around M dwarf stars, where planet-forming material can be relatively scarce near the star. As large planets are unlikely to form under these conditions, this work explores mechanisms for super-Earth migration and “parking” in the habitable zone. Raymond and colleagues explored the late stages of terrestrial planet formation, and showed that the migration of super-Earths through the terrestrial planet-forming region is extremely destructive (Izidoro et al. 2014), that Jupiter and Saturn can migrate outward in multiple orbital configurations (Pierens et al. 2015), and that gas giants provide a barrier to the inward migration of more distant planetary cores. Raymond and colleagues proposed that Jupiter’s presence prevented the ice giants from becoming super-Earths and identified observational tests of the model (Izidoro et al. 2015a), and showed that the ice giants’ masses and orbits can be reproduced if they formed from a population of cores whose inward migration was blocked by Jupiter. This implies a common origin for ice giants and hot super-Earths (Izidoro et al. 2015b). We show that the asteroid belt’s orbital structure is a key constraint on models of the formation of the terrestrial planets. The classical model appears to fail systematically in reproducing the inner Solar System (Izidoro et al. 2015c).

The VPL explored numerous aspects of the formation of the Moon and the subsequent coupled evolution of the Earth-Moon system. Raymond and colleagues showed that the compositional similarity between the Earth and Moon may have arisen naturally during the accretion process (Mastrobuono-Battisti et al. 2015). Zahnle, Sleep and colleagues showed that thermal blanketing by Earth’s water atmosphere limits the rate the Moon’s orbit can evolve in the first ten million years after the Moon-forming impact, provided the first good estimates of how quickly the surface of the Earth can freeze after the Moon-forming impact, and calculated the geothermal heat flow for the first hundred million years or so of the Hadean (Zahnle et al. 2015). We also extended satellite modeling to exomoons (natural satellites of exoplanets) to elucidate the role of planetary radiation on the exomoon’s habitability (Heller & Barnes, 2015).

VPL explored the role of tidal phenomena in the Solar System and in the exoplanets. Driscoll & Barnes (2015) coupled a 1-D Earth interior model to standard models of tidal processes to produce the first self-consistent geophysical model of an Earth-like planet. Their simulations showed that, contrary to prevailing opinions, planets orbiting red dwarfs are likely to maintain strong magnetic fields that may shield the planet from the host star’s activity. Raymond and colleagues created a new code for calculating tidal and spin interactions in multiple planetary systems to show how planet-planet-tide interactions affect the long-term evolution of planetary systems (Bolmont et al. 2015). Barnes (2015) showed that tidal circularization of exoplanet orbits proceeds at different rates and demonstrated how large samples of high quality transit data, e.g. from TESS, can be used to identify the boundary between rocky and gaseous exoplanets. Finally, we examined the similarities and differences between tidally-induced cracks on Enceladus and the San Andreas fault, which can be used to infer processes on tidally evolving exoplanets (Sleep, 2015b).

To explore the star’s gravitational influence on planetary habitability, VPL scientists performed N-body simulations of real and hypothetical planetary systems. Deitrick, Barnes, Quinn, Luger and colleagues used orbital stability models to reveal the full 3-dimensional orbital architecture of the Upsilon Andromedae system (Deitrick et al. 2015), the first system discovered with misaligned orbital planes. Barnes, Deitrick, Quinn, Raymond and colleagues discovered that terrestrial exoplanets in mean motion resonances with non-planar orbits can evolve chaotically for at least 10 Gyr (Barnes et al., 2015).

VPL researchers also simulated the role of stellar radiation on atmospheric escape from potentially habitable worlds, and the impacts of stellar activity on planetary atmospheres. Luger, Barnes, Meadows and Fortney demonstrated that small gaseous worlds (mini-Neptunes) in the habitable zones of red dwarfs may have their envelopes blown away by their host star, revealing a “habitable evaporated core”. Luger and Barnes (2015) also showed that the high luminosity of young M dwarfs is likely to lead to a prolonged runaway greenhouse state for planets that are discovered in the habitable zone after several billion years, potentially dessicating the planet. Hawley and collaborators examined the flaring and activity of M dwarfs stars in the Kepler field, including the long-term tracking of starspots (Davenport et al. 2015), as well as quantifying the flare rates for M dwarfs as a function of rotation period (Lurie et al. 2015). Tilley, Meadows and Hawley are currently using observed flare sequences provided by Hawley and Davenport to explore the impact of multiple flares on the photochemistry and surface UV flux of an Earth-like planet orbiting an M dwarf.

To perform a comprehensive assessment of habitability and in particular to calculate constraints on planetary orbits for newly discovered exoplanets, Barnes, Dietrick, Luger and Quinn worked on the development of the VPLanet framework, which can calculate the coupled effects of orbital, rotational, stellar, geophysical, atmospheric and climate evolution of planetary systems with potentially habitable planets. Additionally, Barnes, Meadows and Evans (2015) defined a new method to assess and compare the potential habitability of transiting exoplanets. Their approach is markedly different from classic habitable zone calculations in two important ways. It is directly tied to transit observables, like transit duration, rather than just being a function of stellar mass and orbital distance. Second, it assigns a numerical likelihood of potentially habitable conditions to each planet, allowing ranking, as opposed to the more binary habitable zone concept.

The Living Planet

In this research area, VPL team members use modeling, laboratory and field work to understand the co-evolution of the environment and biosphere, aspects of life’s global impact that could be detected remotely as biosignatures, and to explore and identify potential false positives for life.

This past year, three separate modeling investigations found cases in which O₂ and/or O₃ could build up abiotically in an exoplanet’s atmosphere and create possible false positives for life. Gao, Robinson, Yung and colleague used VPL models to show that for a planet orbiting an M dwarf with a high FUV/NUV flux ratio – if the planetary atmosphere is dry and CO₂-dominated – then recombination of the products of CO₂ photolysis is inhibited. The atmosphere can then build up CO and O₂ in 103-104 years, with abundances of abiotic O₂ and O₃ rivaling that of modern Earth (Gao et al. 2015). In this case, the abiotic source of the O₂ is made more likely due to the lack of water in the planetary spectra. However, via the same mechanism of CO₂ photolysis, Harman, Schwieterman, Kasting and colleague did a comparison of stellar types, and showed that for planets with liquid water, while abiotic O₂ should not accumulate to detectable levels around F and G stars, with K and M stars, the low near-UV flux may allow build-up of O₂ if the sinks for O₂ are low; meanwhile, O₃ could be detectable for a wider range of stars (Harman et al. 2015). Instead of CO₂, Luger and Barnes (2015) looked at extreme atmospheric H₂O loss during the early high luminosity phase of M dwarfs for planets. For planets that form at the position that will become the 5-Gyr habitable zone, atmospheric evolution is sufficiently severe that the planet can generate large amounts of atmospheric O₂ while retaining liquid water (Luger and Barnes, 2015). Thus O₂ and O₃ detection alone are not robust biosignatures but must be accompanied by knowledge of stellar parameters and a more comprehensive census of atmospheric composition and conditions on terrestrial exoplanets to rule out false positives. VPL research is now developing better ways to identify abiotically produced O₂/O₃ by determining which gases or other environmental characteristics are more likely to be present for abiotic generation. As the direct detection of nitrogen would provide a means to characterize the bulk atmosphere of potentially habitable exoplanets and constrain the likelihood of oxygen production by abiotic processes, Schwieterman, Robinson, Meadows, Misra, and Domagal-Goldman explored a novel way to detect and quantify N₂ in planetary atmospheres. Although the N₂ molecule is extremely challenging to observe in exoplanet spectra, N₂ has a collisional-induced absorption band near 4.2 µm, which is significant in Earth’s spectrum and potentially in those of Earth-like exoplanets with similarly N₂-dominated atmospheres. The VPL team quantified the potential magnitude of this spectral signature by producing synthetic transit transmission and radiance spectra using VPL radiative transfer models (Schwieterman et al. 2015b). Schwieterman, Meadows, Domagal-Goldman, Deming, Arney, Luger, Harman, Misra and Barnes (2016) published a seminal paper on using CO in transmission and O₄ in both transmission and direct imaging spectra to help discriminate between abiotic O₂/O₃ produced by atmospheric loss or photochemistry, and O₂ produced by a photosynthetic biosphere.

In other innovative work on biosignature definition, Krissansen-Totton, Catling and colleague (2015) tested the idea that atmospheric chemical disequilibrium can act as a biosignature. Performing the first, detailed quantification of thermodynamic disequilibrium in Solar System planetary atmospheres, they showed that the Earth has ~20 times the thermodynamic chemical disequilibrium of other planets because of the biosphere. They identifiy the high abundance N₂, O₂ and the presence of an ocean as the strongest disequilibrium state for our planet. Work is continuing on the topic of biosignatures and disequilibrium with an effort to identify “anti-biosignatures”, which is where gases that should be consumed by microbes are present as a result of purely abiotic processes.

We have also made significant advances this year in the area of the nature and detection of photosynthetic biosignatures, (oxygenic and anoxygenic), as well as non-photosynthetic biosignatures. Addressing the long wavelength limit of oxygenic photosynthesis, Kiang, Parenteau, Blankenship, and Siefert discovered another strain, possibly two, of a chlorophyll d-containing cyanobacterium collected from red algae at Moss Beach, California. Enrichments and isolations are currently being conducted and 16S rDNA sequences identify a new Acaryochloris strain most closely related to another found in Japan. Comparisons of its light use with other varieties of far-red photosynthesizers will help identify the mechanism that induces changes in the primary chlorophyll, and this knowledge can be used to better understand likely pigment biosignatures on planets orbiting different stellar types.

VPL research on biosignatures from anoxygenic phototrophs informs the search for life on exoplanets at a similar stage of evolution or biogeochemical state as the Archean Earth, as well as on planets orbiting M dwarfs. Adding to preliminary work on pure cultures and microbial mats of anoxygenic phototrophs, Parenteau measured more detailed reflectance spectra of field in situ samples of a variety of mat environments, uncovering striking spectral features that show that the full suite of complementary pigment niches of layered mat communities is visible at the surface. This is perhaps the first identification of an ecosystem community class of surface biosignature, as opposed to features from isolated species. Sparks and Parenteau have also measured the reflectance and transmission full Stokes polarization spectra of the same pure cultures and environmental mat samples, and found strong correlations between spectral and polarization features. To look for new gaseous biosignatures, Parenteau and Hoehler have constructed an anaerobic chamber and LED array for simulating Archean (and M-dwarf) radiation and atmospheric compositions. They are quantifying biogenic gas fluxes in anoxygenic microbial mats using a high resolution membrane inlet mass spectrometer.

Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to conduct an interdisciplinary study of the diversity and detectability of non-photosynthetic pigments as biosignatures, especially for halophiles (Schwieterman et al., 2015). This study included reflectance spectra measurements of a diverse collection of pigmented non-photosynthetic organisms and an analysis of the remote detectability of analogs of these organisms in disk-averaged spectra, with a planetary environment that includes other surfaces, an atmosphere and clouds. These spectra are now available through the existing VPL Biological Pigments Database (http://vplapps.astro.washington.edu/pigments).

At the ocean depths, Anderson and Baross, with collaborator Mitch Sogin at the Marine Biological Laboratory in Woods Hole, MA, conducted a 16S rRNA tag sequencing survey to reveal a picture of microbial colonization and dispersal both within and between hydrothermal vent systems, with results also indicating novel strains (Anderson et al. 2015). The deep, hot microbial biosphere harbors the most ancient of extant organisms and would help provide novel biosignatures and clues for the environmental distribution of life elsewhere in the universe.

Rounding together the atmosphere, surface, and subsurface interactions of life’s impacts, Hoehler, Domagal-Goldman, Som, Kasting and Meadows have modeled chemosynthetic-based biospheres by completing the coupling and benchmarking of two models: one model codifies a thermodynamic energy balance concept for habitability (Hoehler, 2009); the other model predicts a planet’s atmospheric composition and climate while balancing the redox state of the surface environment (Domagal-Goldman et al., 2014). Preliminary results on applying this to methanogenesis on early Earth were presented at AGU, and are being prepared for publication. These models will enable an ecosystem to be coupled into an interactive planetary environment including atmospheric, oceanic and subsurface components. This will allow us to predict standing biomass size, net biological productivity, and resulting gas fluxes to and from a biosphere (and therefore the nature and magnitude of any potential biosignature) with a given set of volcanic and hydrothermal inputs to the planetary surface.

The Observer

In this task we explore the detectability of signs of habitability and life for modeled observations from the previous tasks. We also observe and develop new observational, analysis and retrieval techniques to improve our understanding of the environmental properties of exoplanets for current and future observations. In exoplanet discovery and observations this year, Agol was part of the ‘Citizen Scientists’ team that discovered a new super-Neptune planet and characterized its multi-planet system (Schmitt et al., 2014). This demonstrated techniques to determine precision masses of exoplanets, which can be applied to habitable zone exoplanets in the future. Sheets and Deming (2015) measured and coadded reflected light from Kepler planets smaller than Saturn, finding that they have low (~20%) geometric albedos. This new technique probes the nature of small planet atmospheres and my eventually lead to an understanding of the atmospheres of super-Earths. In exoplanet spectroscopy, Deming participated in HST transit observations of HAT-P_11b, a Neptune-sized planet that showed clear skies with water vapor absorption clearly detected in the spectrum (Fraine et al., 2014). We hope to extend this measurement technique to transiting habitable super-Earth (to be discovered by TESS). Yung helped interpret pioneering ground-based polarimetry data on hot Jupiter HD189733b, and using a Rayleigh scattering model was able to constrain the geometric albedo of the planet to be < 0.36 (Wiktorowicz et al., 2015).

In addition to observations, we also developed several new detection techniques for planets and their moons. New, fast techniques to calculate the size of the perturbation expected to planetary orbits as planets pass by each other were developed that can potentially be used to measure the masses of Earth-sized exoplanets with JWST (Deck & Agol, 2015). Agol, Robinson and Meadows, along with undergraduates Jansen and Lacy developed new techniques to detect and characterize exomoons and their parent planets, using spectroastrometry, the measurement of the center of light in a planet/moon system at different wavelengths (Agol et al., 2015). This technique may allow for detection of potentially habitable exomoons, as well as mass measurements and disentangling of the spectra for the exoplanet/exomoon system. Misra, Krissansen-Totton and colleagues collaborated on a paper to understand whether a volcanically active planet could be identified using transmission spectra by searching for the sporadic formation of high altitude hazes due to volcanic outgassing (Misra et al., 2015) Schwieterman, Robinson, Meadows, Misra and Domagal-Goldman collaborated on a paper that uses collisional pairs of N2-N2 molecules to understand planetary atmospheric bulk composition and that may also help discriminate whether the source of oxygen in an atmosphere is biotic or abiotic. Robinson additionally co-wrote a review on techniques for 1-D thermal structure modeling for planetary and brown dwarf atmospheres (Marley & Robinson, 2015), drawing on results from many of astrobiology’s sub-fields.

To model mini-Neptunes and ultimately learn how these – likely uninhabitable – worlds can be discriminated from habitable super-Earths, NPP posdoc Benjamin Charnay, along with Meadows, Misra and Arney developed 3D models of mini-Neptune GJ1214b’s atmosphere using the Laboratoire Meteorologie Dynamique’s LMDZ. Charnay, Meadows & Leconte, (2015a), described the new LMDZ mini-Neptune model and analyzed the atmospheric circulation and the transport of tracers in GJ1214b’s atmosphere, which are important to understand the photochemistry and cloud formation on mini-Neptunes. In Charnay et al., (2015b) we performed the first 3D simulations of realistic clouds on a gaseous exoplanet, and validated the model by reproducing the observed HST transit spectrum of GJ1214b. We then predicted what information could be obtained with future telescopes and in particular showed that mini-Neptunes should show strong features from molecules longward of 3um in JWST transit spectra, even if haze precludes deeper observations at visible wavelengths. This work provides insight into the best observational techniques to decipher cloudy atmospheres, and how to distinguish mini-Neptunes from potentially habitable ocean exoplanets.

In further developments in instrument models and retrieval for potentially habitable exoplanets, Robinson and colleagues developed an instrument noise mode suitable for studying the spectral characterization potential of a coronagraph-equipped, space-based telescope and applied it to a broad set of rocky and gaseous exoplanet types (Robinson, Stapelfeldt & Marley, 2015). This is being used to explore the capability of near-future coronagraphic missions (like WFIRST-AFTA) to detect biosignatures gases in the atmospheres of nearby Earths and super-Earths. In ongoing work for VPL’s terrestrial exoplanet spectral retrieval suite, Lustig-Yaeger has completed an end-to-end retrieval suite, which is the core of the Observer task. The current version uses optimal estimation, but we are implementing MCMC and Multinest algorithms to replace OE. Luger and Lustig-Yaeger are using Gaussian processes to develop a cost function that penalizes unphysical atmospheres so that the retrieval will be constrained not just by the limitations in the spectral data, but by known characteristics of the planet and planetary system.

These tools, and the science and small exoplanet observing expertise developed by the VPL have played an integral role in the development and delivery of final reports for two NASA spacecraft concepts for exoplanet observations, Exo-Coronagraph and Exo-Starshade. Meadows is a Science and Technology Definition Team member for Exo-C (Stapelfeldt et al., 2015), Domagal-Goldman and Bill Sparks are Science and Technology Definition Team members for Exo-S (Seager et al., 2015). Both teams are continuing in an extended phase to develop these concepts beyond the original baseline. Robinson contributed modeling and predicted spectra for both final reports. Additionally, Meadows, Schwieterman, Arney, Deming and Lustig-Yaeger are working on end-to-end simulations of self-consistent Earth-like planets orbiting M dwarfs, as viewed by JWST. These simulations are being used to calculate the exposure time needed to observe diagnostic features in the exoplanet spectra, and to test the robustness of the retrieval techniques on simulated data for JWST. These results were presented by Meadows at the international conference on JWST at ESTEC in October 2015. Many of these tools and techniques also formed the basis for a WFIRST Science Investigation Team proposal that was submitted by Meadows (PI), Domagal-Goldman, Barnes, Robinson, Lincowski, Lustig-Yaeger and colleagues, to simulate forward and instrument modeling, and data analysis and retrieval, for observations of small planets taken with a WFIRST-AFTA coronagraphic mission.

Education and Public Outreach

During the past year the EPO team finalized the script and images for the first Science on Sphere show “Earth – the Pale Blue Dot” describing exoplanet detection and what Earth would look like as an exoplanet. A training manual has also been created to facilitate adoption at other institutions. The show has been put into regular rotation at the Pacific Science Center and evaluation is ongoing. The second Science On Sphere show describing the Earth through time and how life on Earth has changed during different epochs is currently under development with a scheduled completion date of Autumn 2016. This year 2 additional Astrobiology Science Communication fellows have received training, bringing the total to 4.