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2015 Annual Science Report

VPL at University of Washington Reporting  |  JAN 2015 – DEC 2015

Exoplanet Detection and Characterization: Observations, Techniques and Retrieval

Project Summary

In this task, VPL team members use observations and theory to better understand how to detect and characterize extrasolar planets. Techniques to improve the detection of extrasolar planets, and in particular smaller, potentially Earth-like planets are developed, along with techniques to probe the physical and chemical properties of exoplanet atmospheres. These latter techniques require analysis of spectra to best understand how it might be possible to identify whether an extrasolar planet is able to support life, or already has life on it.

4 Institutions
3 Teams
8 Publications
0 Field Sites
Field Sites

Project Progress

Exoplanet and Exomoon Detection: We 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 & Deck 2016; Jontoff-Hutter et al. 2016, Deck & Agol 2016). 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.

Constraining Planetary Albedos: This year, Sheets and Deming (2015) improved and extended their study of the albedos of short-period Kepler planet candidates using the statistical technique of averaging secondary eclipse photometry for groups of planets with similar radii. They 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. They are in the process of extending this study to Kepler long-cadence data, that enables better resolution in radius due to the much larger number of eclipses. They study planets in three radius ranges: 1-2 Earths, 2-4 Earths, and 4-6 Earths. All three groups are dark, with albedos similar to their results from the short cadence data. However, they are also studying the effect of non-zero orbital eccentricities, that may cause their inferred albedos to be adjusted upward. 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).

Modeling Mini-Neptunes and Super-Earths: To model mini-Neptunes and ultimately learn how these – likely uninhabitable – worlds can be discriminated from habitable super-Earths, Charnay, 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. For more information on this project, see the NPP Report by Benjamin Charnay in this Annual Report.

Instrument Simulation Models: Drake Deming has also extended and improved his JWST transiting exoplanet simulation codes. The goal is to model the degree to which the atmospheres of nearby transiting super-Earths will be amenable to characterization of their major molecular constituents using transit and eclipse spectroscopy by JWST. He is applying the simulations to planets anticipated to be discovered by the TESS mission, as projected by the TESS science team. Drake is also addressing the issue of how to maximize the atmospheric science return from JWST observations, given that more than a dozen different spectroscopic modes will be available.

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. 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.

Terrestrial Exoplanet Spectral Retrieval Code: 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.

VPL Expertise Helps in Mission Concept Development: 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, and at the STScI Workshop on Transiting Exoplanet Science with JWST. 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.