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Project Progress

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 observations this year, Raymond was part of the team that discovered Kepler 186f, the first Earth-sized planet found in the habitable zone of an M dwarf star (Quintana et al., 2014). Agol was involved in detecting and characterizing new multi-planet systems in the Kepler data, including the first 7-transiting planet system, Kepler-90, (Lissauer et al., 2014; Rowe et al., 2014). He also contributed to the planetary system found by 'Citizen Scientists’, Planet Hunters 3 (Schmitt et al., 2014) for which Agol helped to develop an efficient and accurate code for transit timing inversion (Deck et al., 2014). Agol and Deming helped analyze and interpret Hubble and Spitzer Space Telescope observations of short-period giant planets, including detection of water absorption features and potential planet variability (Deming et al., 2013), which are being interpreted with the help of 3D RHD simulations (Dobbs-Dixon & Agol, 2013). Several of these techniques are now being extended to smaller planets.

Misra, Meadows and Crisp completed modification of the VPL’s line-by-line radiative transfer model (SMART) to generate a state-of-the-art transit transmission model that includes the effects of gas absorption, cloud and aerosol extinction, refraction, and the effects of stellar limb darkening. The model has been validated against ATMOS limb spectra of the Earth and lunar eclipse spectra. We have used the model to show that simultaneous measurements of the absorption features from the O₂-O₂ dimer molecule and molecular oxygen (O₂) can be used as a new technique to probe planetary atmospheric pressure for oxygenated terrestrial atmospheres and biosignatures (Misra et al., 2014a). We have shown that inclusion of refraction decreases the detectability of spectral absorption features in transit transmission, and that this effect is dependent on atmospheric composition, the size of the star, and the planet-star distance (Misra et al., 2014b), and we have postulated that refraction effects in transit transmission observations could be used to discriminate between planets with and without clouds (Misra et al., 2014c). Work is currently underway on a publication that describes the effects of scattering on terrestrial exoplanet spectra.

Deming and Sheets used the predictions from Misra’s models to begin searching for refracted light near transit, to estimate the degree of cloud coverage and scale height of super-Earth atmospheres as an important precursor to JWST spectroscopy during transit. Deming and Sheets also measured the albedos of small transiting exoplanets by coadding Kepler data for each planet class, to search for secondary eclipses. Observations for planets of similar radius were grouped and transformed to a common orbital phases scale so that data from multiple planets could be coadded (Sheets and Deming, 2014). The resultant stacked eclipses have signal-to-noise ratios exceeding one million, and they can detect reflected light from super-Earth planets between one and two earth radii in size. They initially used this technique on short-cadence Kepler data, and planets smaller than Saturn (Sheets and Deming 2014). They have now extended their analysis to Kepler’s long cadence data of 56 super-Earths. They were able to detect reflected light from close-in super-Earth planets between one and two Earth radii and conclude that these objects have a relatively low albedo, indicating significant absorption in their atmospheres or surfaces. Kopparla and Yung have undertaken a modeling study using the vector radiative transfer model VLIDORT to study the phase space of expected atmospheric composition and the observable polarization signal for a range of potential exoplanets (Kopparla et al., 2014). Line, Crisp and Yung developed, tested, and published the relative performance of three commonly used remote sensing retrieval algorithms (optimal estimation, Markov-Chain Monte Carlo, and Bootstrap Monte Carlo) for interpreting realistic, synthetic spectra of exoplanets. (Line et al., 2013) and used the retrieval algorithms to search for chemical disequilibria in observations of exoplanetary atmospheres (Line &Yung, 2013) and to undertake a systematic retrieval of secondary eclipse spectra of nine planets to determine their C/O ratios. Lustig-Yaeger analyzed existing exoplanet spectra to determine that broadband observations were often inadequate for detection of molecular species, but that their accuracy could be enhanced with even a small section of spectroscopic data (Lustig-Yaeger et al., 2015). Lustig-Yaeger, Meadows and Crisp are currently developing VPL’s retrieval model, which is at the core of the proposed fifth task. This model will combine VPL’s existing forward models of planetary environments developed by Crisp, Robinson, Misra and Meadows, with instrument models for direct imaging and transit transmission missions, obtained or developed by Meadows and Deming, and the sophisticated retrieval models developed by Line. The resulting model will also use all known information about the planet and planetary system to more robustly determine the family of environmental conditions that best fit the observed planetary spectrum.