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The Sellers Exoplanet Environments Collaboration

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The Sellers Exoplanet Environments Collaboration (SEEC) is a multi-disciplinary, cross-divisional effort to study the broad diversity of exoplanet atmospheres and climate, using the wide range of scientific and technical resources available at GSFC. We foster new collaborations and leverage existing expertise across Planetary, Earth, Astrophysics and Heliophysics Divisions to apply knowledge of Solar System bodies to improve our models of exoplanets and better prepare for current and future exoplanet observations.Our team includes researchers working on generalizing atmosphere and solar wind models and radiative transfer schemes to simulate a wide range of planetary conditions, as well as work on improved simulations and analysis of future observations of exoplanets to better understand what we can learn about the diversity of planetary properties across space and time.

Director: Avi Mandell (Code 693)
Deputy Director: Bill Danchi (Code 667)
Leadership Team: Vladimir Airapetian (671), Luke Oman (Code 614), Alex Glocer (Code 673), Giada Arney (693), Marc Kuchner (667)
Complete Team List


Key Projects

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The Exoplanet Modeling and Analysis Center (EMAC)
The Exoplanet Modeling and Analysis Center (EMAC) leverages interdisciplinary expertise from astrophysicists, Solar System scientists, Earth systems scientists, and heliophysicists for the development of community modeling and analysis tools, and for the application of these tools to the prediction and interpretation of spaceflight measurements of habitable environments in support of the search for life in and beyond the Solar System.
ROCKE-3D Global Circulation Model for Exoplanets
The ROCKE-3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics) team, a multi-institution collaborative project, will perform 3D GCM simulations of past climates of Earth and other rocky Solar System planets to broaden our understanding of planetary habitability, to use similar simulations to assess the habitability of rocky exoplanets, and to produce synthetic disk-integrated spectra and phase curves of these planets.

Research Highlights:

High-temperature condensate clouds in super-hot Jupiter atmospheres
Deciphering the role of clouds is central to our understanding of exoplanet atmospheres, as they have a direct impact on the temperature and pressure structure, and observational properties of the planet. Super-hot Jupiters occupy a temperature regime similar to low mass M-dwarfs, where minimal cloud condensation is expected. However, observations of exoplanets such as WASP-12b (Teq ~ 2500 K) result in a transmission spectrum indicative of a cloudy atmosphere. We re-examine the temperature and pressure space occupied by these super-hot Jupiter atmospheres, to explore the role of the initial Al- and Ti-bearing condensates as the main source of cloud material.

Citation: Wakeford, H., et al. "High-temperature condensate clouds in super-hot Jupiter atmospheres". 2017, MNRAS.
A Cloudy Atmosphere for the Promising JWST Target WASP-101b
We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury (PanCET) program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 (WFC3) observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H2O absorption features and we rule out a clear atmosphere at 13σ. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features.

Citation: Wakeford, H., et al. "HST PanCET program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b". 2017, ApJ.
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST
We present here an open-source Python package and online interface for creating observation simulations of all observatory-supported time-series spectroscopy modes. This noise simulator, called PandExo, relies on some aspects of Space Telescope Science Institute's Exposure Time Calculator, Pandeia. We describe PandExo and the formalism for computing noise sources for JWST. Then, we benchmark PandExo's performance against each instrument team's independently written noise simulator for JWST, and previous observations for HST. We find that \texttt{PandExo} is within 10% agreement for HST/WFC3 and for all JWST instruments.

Citation: Batalha, N. E., Mandell, A., et al. "PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST". 2017, PASP
A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c
Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. Here, we report a space-based measurement of the combined transmission spectrum of the two inner planets made possible by a favorable alignment resulting in their simultaneous transits on 04 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at 10-σ levels; TRAPPIST-1 b and c are hence unlikely to harbor an extended gas envelope as they lie in a region of parameter space where high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres. Many denser atmospheres remain consistent with the featureless transmission spectrum---from a cloud-free water vapour atmosphere to a Venus-like atmosphere.

Citation: de Wit, Julien; Wakeford, H, et al. "A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c". 2016, Nature.
Challenges to Constraining Exoplanet Masses via Transmission Spectroscopy
To investigate the effects of planetary mass on transmission spectra, we present simulated observations of super-Earths with atmospheres made up of mixtures of H2O and H2, both with and without clouds. We model their transmission spectra and run simulations of each planet as it would be observed with JWST using the NIRISS, NIRSpec, and MIRI instruments. We find that significant degeneracies exist between transmission spectra of planets with different masses and compositions, making it impossible to unambiguously determine the planet's mass in many cases.

Citation: Batalha, N. E., et al. "Challenges to Constraining Exoplanet Masses via Transmission Spectroscopy" 2017, ApJ
Was Venus the first habitable world of our solar system?
Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present day Venus orbital parameters, an ocean volume consistent with current theory and measurements, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite an incident solar flux 46-70% higher than modern Earth receives.

Citation: Way, M.J., Del Genio, A.D., Kiang, N.Y., Sohl, L.E., Grinspoon, D.H., Aleinov, I., Kelley, M., Clune, T. "Was Venus the first habitable world of our solar system?" 2016, GRL
Effects of variable eccentricity on the climate of an Earth-like world
We investigate two scenarios that involve evolution of the Earth-like planet's orbital eccentricity from 0--0.283 over 6500 years, and from 0--0.066 on a time scale of 4500 years. In both cases we discover that they would maintain relatively temperate climates over the time-scales simulated. More Earth-like planets in multi-planet systems will be discovered as we continue to survey the skies and the results herein show that the proximity of large gas giant planets may play an important role in the habitability of these worlds. These are the first such 3-D GCM simulations using a fully-coupled ocean with a planetary orbit that evolves over time due to the presence of a giant planet.

Citation: Way, M.J., Georgakarakos, N. "Effects of variable eccentricity on the climate of an Earth-like world". 2017, ApJ Lett
NIR-driven moist upper atmospheres of synchronously rotating temperate terrestrial exoplanets
H2O is a key molecule in characterizing atmospheres of temperate terrestrial planets, and observations of transmission spectra are expected to play a primary role in detecting its signatures in the near future. Detectability of H2O absorption features in transmission spectra depends on the abundance of water vapor in the upper part of the atmosphere. While the stratospheric water vapor mixing ratio of the Earth is less than 10-5 due to the cold trap, the efficiency of the cold trap depends on atmospheric properties. Here we study the 3D distribution of atmospheric H2O for synchronously rotating Earth-sized aquaplanets using the GCM ROCKE-3D, and examine the effects of total incident flux and stellar spectral type. We observe a more gentle increase of the water vapor mixing ratio in response to increased incident flux than 1D models suggest, in qualitative agreement with the climate-stabilizing effect of clouds around the substellar point previously observed in GCMs applied to synchronously rotating planets. However, the water vapor mixing ratio in the upper atmosphere starts to increase while the surface temperature is still moderate. This is explained by the circulation in the upper atmosphere driven by the radiative heating due to absorption by water vapor and cloud particles, causing efficient vertical transport of water vapor. Consistently, the water vapor mixing ratio is found to be well correlated with the near-infrared portion of the incident flux. Our results imply that various levels of water vapor mixing ratio in the upper atmosphere may be expected for synchronously rotating temperate terrestrial planets, and that for the more highly irradiated ones the H2O absorption features in the transmission spectra are strengthened by a factor of a few, loosening the observational demands for a direct H2O detection.

Citation: Fujii, Y., Del Genio, A.D., Amundsen, D.S. "NIR-driven moist upper atmospheres of synchronously rotating temperate terrestrial exoplanets". Submitted to ApJ, 2017
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D)
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM) developed at the NASA Goddard Institute for Space Studies for the modeling of atmospheres of Solar System and exoplanetary terrestrial planets. Its parent model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and 21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing effort to expand the capabilities of ModelE2 to handle a broader range of atmospheric conditions including higher and lower atmospheric pressures, more diverse chemistries and compositions, larger and smaller planet radii and gravity, different rotation rates (slowly rotating to more rapidly rotating than modern Earth, including synchronous rotation), diverse ocean and land distributions and topographies, and potential basic biosphere functions.

Citation: Way, M., Aleinov, I., Amundsen, D.S., Chandler, M., Clune, T., Del Genio, A.D., Fujii, Y., Kelley, M., Kiang, N.Y., Sohl, L., Tsigaridis, K. "Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) 1.0: A General Circulation Model for simulating the climates of rocky planets". 2017, submitted to ApJ Supp. Ser., 2017
Born Dry: Kepler's Ultra-Short-Period Planets Formed Water-Poor
Recent surveys have uncovered an exciting new population of ultra-short-period (USP) planets with orbital periods less than a day. These planets typically have radii <1.5 Earth radii, indicating that they likely have rocky compositions. This stands in contrast to the overall distribution of planets out to ~100 days, which is dominated by low-density sub-Neptunes above 2 Earth radii, which must have gaseous envelopes to explain their size. However, on ultra-short-period orbits, planets are bombarded by intense levels of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely vulnerable to losing their envelopes to atmospheric photo-evaporation. Using models of planet evolution, I show that the rocky USP planets can easily be produced as the evaporated remnants of sub-Neptunes with H/He envelopes and that we can therefore understand the observed dearth of USP sub-Neptunes as a natural consequence of photo-evaporation. Critically however, planets on USP orbits could often retain their envelopes if they formed with very high-metallicity water dominated envelopes. Such water-rich planets would commonly be >2 Earth radii today, which is inconsistent with the observed evaporation desert, indicating that most USP planets likely formed from water-poor material within the snow-line. Finally, I examine the special case of 55 Cancri e and its possible composition in the light of recent observations, and discuss the prospects for further characterizing this population with future observations.

Citation: Lopez, E. D. "Born Dry in the Photo-Evaporation Desert: Kepler's Ultra-Short-Period Planets Formed Water-Poor". 2016, in revision with MNRAS
Predictions for the Transition Between Rocky Super-Earths and Gaseous Sub-Neptunes
One of the most significant advances by NASA's Kepler Mission was the discovery of an abundant new population of highly irradiated planets with sizes between that of the Earth and Neptune, unlike anything found in the Solar System. Subsequent analysis showed that at ~1.5 R there is a transition from a population of predominantly rocky super-Earths to non-rocky sub-Neptunes, which must have substantial volatile envelopes to explain their low densities. Determining the origin of these highly irradiated rocky planets will be critical to our understanding of low-mass planet formation and the frequency of potentially habitable Earth-like planets. These short-period rocky super-Earths could simply be the stripped cores of sub-Neptunes, which have lost their envelopes due to atmospheric photo-evaporation or other processes, or they might instead be a separate population of inherently rocky planets, which never had significant envelopes. We suggest an observational path forward to distinguish between these scenarios. Using models of atmospheric photo-evaporation we show that if most bare rocky planets are the evaporated cores of sub-Neptunes then the transition radius should decrease as surveys push to longer orbital periods. On the other hand, if most rocky planets formed after their disks dissipate then these planets will have formed without initial gaseous envelopes. In this case, we use N-body simulations of planet formation to show that the transition radius should increase with orbital period. Moreover, we show that distinguishing between these two scenarios should be possible in coming years with radial velocity follow-up of planets found by TESS. Finally, we discuss the broader implications of this work for current efforts to measure η, which may yield significant overestimates if most rocky planets form as evaporated cores.

Citation: Lopez, E. D. & Rice, K. "Predictions for the Period Dependence of the Transition Between Rocky Super-Earths and Gaseous Sub-Neptunes and Implications for η". 2016, in revision with MNRAS.
Pale Orange Dots: The Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets
One of the most significant advances by NASA's Kepler Mission was the discovery of an abundant new population of highly irradiated planets with sizes between that of the Earth and Neptune, unlike anything found in the Solar System. Subsequent analysis showed that at ~1.5 R there is a transition from a population of predominantly rocky super-Earths to non-rocky sub-Neptunes, which must have substantial volatile envelopes to explain their low densities. Determining the origin of these highly irradiated rocky planets will be critical to our understanding of low-mass planet formation and the frequency of potentially habitable Earth-like planets. These short-period rocky super-Earths could simply be the stripped cores of sub-Neptunes, which have lost their envelopes due to atmospheric photo-evaporation or other processes, or they might instead be a separate population of inherently rocky planets, which never had significant envelopes. We suggest an observational path forward to distinguish between these scenarios. Using models of atmospheric photo-evaporation we show that if most bare rocky planets are the evaporated cores of sub-Neptunes then the transition radius should decrease as surveys push to longer orbital periods. On the other hand, if most rocky planets formed after their disks dissipate then these planets will have formed without initial gaseous envelopes. In this case, we use N-body simulations of planet formation to show that the transition radius should increase with orbital period. Moreover, we show that distinguishing between these two scenarios should be possible in coming years with radial velocity follow-up of planets found by TESS. Finally, we discuss the broader implications of this work for current efforts to measure η, which may yield significant overestimates if most rocky planets form as evaporated cores.

Citation: Arney, G. N., V. S. Meadows, S. D. Domagal-Goldman, D. Deming, T. Robinson, G. Tovar, E. T. Wolf, E. Schwieterman. "Pale Orange Dots: The Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets." 2017, Astrobiology.
Inner Edge of the Habitable Zone Around M-dwarfs
Terrestrial planets at the inner edge of the habitable zone of late-K and M-dwarf stars are expected to be in synchronous rotation, as a consequence of strong tidal interactions with their host stars. Previous global climate model (GCM) studies have shown that, for slowly-rotating planets, strong convection at the substellar point can create optically thick water clouds, increasing the planetary albedo, and thus stabilizing the climate against a thermal runaway. However these studies did not use self-consistent orbital/rotational periods for synchronously rotating planets placed at different distances from the host star. Here we provide new estimates of the inner edge of the habitable zone for synchronously rotating terrestrial planets around late-K and M-dwarf stars using a 3-D Earth-analog GCM with self-consistent relationships between stellar metallicity, stellar effective temperature, and the planetary orbital/rotational period. We find that both atmospheric dynamics and the efficacy of the substellar cloud deck are sensitive to the precise rotation rate of the planet. Around mid-to-late M-dwarf stars with low metallicity, planetary rotation rates at the inner edge of the HZ become faster, and the inner edge of the habitable zone is farther away from the host stars than in previous GCM studies. For an Earth-sized planet, the dynamical regime of the substellar clouds begins to transition as the rotation rate approaches ~10 days. These faster rotation rates produce stronger zonal winds that encircle the planet and smear the substellar clouds around it, lowering the planetary albedo, and causing the onset of the water-vapor greenhouse climatic instability to occur at up to ~25% lower incident stellar fluxes than found in previous GCM studies. For mid-to-late M-dwarf stars with high metallicity and for mid-K to early-M stars, we agree with previous studies.

Citation: Kopparapu, R. K., Wolf, E. T., Haqq-Misra, J., Yang, J., Kasting, J. F., Meadows, V. S., Terrien, R., Mahadevan, S. "The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models". 2016. Astrophysical Journal.
Climate cycles on planets can reduce the width of the habitable zone
The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. Recent calculations have suggested that planets in the outer regions of the HZ cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to "Snowball Earth" episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we build on previous studies with an updated energy balance climate model to calculate this "limit cycle" region of the HZ where such cycling would occur. We argue that an abiotic Earth would have a greater CO2 partial pressure than today because plants and other biota help to enhance the storage of CO2 in soil. When we tune our abiotic model accordingly, we find that limit cycles can occur but that previous calculations have overestimated their importance. For G stars like the Sun, limit cycles occur only for planets with CO2 outgassing rates less than that on modern Earth. For K- and M-star planets, limit cycles should not occur; however, M-star planets may be inhospitable to life for other reasons. Planets orbiting late G-type and early K-type stars retain the greatest potential for maintaining warm, stable conditions. Our results suggest that host star type, planetary volcanic activity, and seafloor weathering are all important factors in determining whether planets will be prone to limit cycling.

Citation: Haqq-Misra, Jacob; Kopparapu, Ravi Kumar; Batalha, Natasha E.; Harman, Chester E.; Kasting, James F. "Limit Cycles Can Reduce the Width of the Habitable Zone". 2016. Astrophysical Journal.
Climate cycles on early Mars could have kept it warm when Sun was less bright
Climate cycles on early Mars could have kept it warm when Sun was less bright. Highlight text: For decades, scientists have tried to explain the evidence for fluvial activity on early Mars, but a consensus has yet to emerge regarding the mechanism for producing it. One hypothesis suggests early Mars was warmed by a thick greenhouse atmosphere. Another suggests that early Mars was generally cold but was warmed occasionally by impacts or by episodes of enhanced volcanism. These latter hypotheses struggle to produce the amounts of rainfall needed to form the martian valleys, but are consistent with inferred low rates of weathering compared to Earth. Here, we provide a geophysical mechanism that could have induced cycles of glaciation and deglaciation on early Mars. Our model produces dramatic climate cycles with extended periods of glaciation punctuated by warm periods lasting up to 10 Myr—much longer than those generated in other episodic warming models. The cycles occur because stellar insolation was low, and because CO2 outgassing is not able to keep pace with CO2 consumption by silicate weathering followed by deposition of carbonates. While CO2 by itself is not able to deglaciate early Mars in our model, we assume that the greenhouse effect is enhanced by substantial amounts of H2 outgassed from Mars' reduced crust and mantle. Our hypothesis can be tested by future Mars exploration that better establishes the time scale for valley formation.

Citation: Batalha, Natasha E.; Kopparapu, Ravi Kumar; Haqq-Misra, Jacob; E.; Kasting, James F. "Climate cycling on early Mars caused by the carbonate-silicate cycle. 2016. Earth and Planetary Science Letters.
A list of Kepler candidate planets in the habitable zone from Q1-Q17 DR24 data
The NASA Kepler mission has discovered thousands of new planetary candidates, many of which have been confirmed through follow-up observations. A primary goal of the mission is to determine the occurrence rate of terrestrial-size planets within the Habitable Zone (HZ) of their host stars. Here we provide a list of HZ exoplanet candidates from the Kepler Q1-Q17 Data Release 24 data-vetting process. This work was undertaken as part of the Kepler HZ Working Group. We use a variety of criteria regarding HZ boundaries and planetary sizes to produce complete lists of HZ candidates, including a catalog of 104 candidates within the optimistic HZ and 20 candidates with radii less than two Earth radii within the conservative HZ. We cross-match our HZ candidates with the stellar properties and confirmed planet properties from Data Release 25 to provide robust stellar parameters and candidate dispositions. We also include false-positive probabilities recently calculated by Morton et al. for each of the candidates within our catalogs to aid in their validation. Finally, we performed dynamical analysis simulations for multi-planet systems that contain candidates with radii less than two Earth radii as a step toward validation of those systems.

Citation: Kane, Stephen R.; Hill, Michelle L.; Kasting, James F.; Kopparapu, Ravi Kumar; Quintana, Elisa V.; Barclay, Thomas; Batalha, Natalie M.; Borucki, William J.; Ciardi, David R.; Haghighipour, Nader; Hinkel, Natalie R.; Kaltenegger, Lisa; Selsis, Franck; Torres, Guillermo. "A Catalog of Kepler Habitable Zone Exoplanet Candidates". Astrophysical Journal, 2016
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