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Characterization of Exoplanet Atmospheres with the Optical Coronagraph on WFIRST

Lacy, B. and Shlivko, D. and Burrows, A. (2019) Characterization of Exoplanet Atmospheres with the Optical Coronagraph on WFIRST. Astronomical Journal, 157 (3). Art. No. 132. ISSN 1538-3881. doi:10.3847/1538-3881/ab0415.

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Wide-Field Infrared Survey Telescope (WFIRST)-CGI is a NASA technology demonstration mission that is charged with demonstrating key technologies for future exo-Earth imaging missions in space. In the process, it will obtain images and low-resolution spectra of a handful to a dozen extrasolar planets and possibly protoplanetary disks. Its unprecedented contrast levels in the optical will provide astronomers' with their first direct look at mature, Jupiter-sized planets at moderate separations. This paper addresses the question: what science can be done with such data? An analytic noise model, which is informed by the ongoing engineering developments, is used to compute maximum achievable signal-to-noise ratios and scientifically viable integration times for hypothetical star–planet systems, as well as to investigate the constraining power of various combinations of WFIRST-CGI photometric and spectral observations. This work introduces two simple models for planetary geometric albedos, which are inspired largely by the solar system's gas giants. The first planet model is a hybrid Jupiter–Neptune model, which separately treats the short and long wavelengths where chromophores and methane dominate absorption, respectively. The second planet model fixes cloud and haze properties in CoolTLusty to match Jupiter's albedo spectrum, it then perturbs only the metallicity. MCMC retrievals performed on simulated observations are used to assess the precision with which planet model parameters can be measured subject to different exposure times and observing cases. Planet radius is recovered within ±15% for all observing cases with both the hybrid model and the CoolTLusty metallicity grid. Fit results for both models' parameterizations of geometric albedo spectra demonstrate that a rough indication of the metallicity or methane content should be possible for some WFIRST-CGI targets. We conclude that real observations will likely be able to differentiate between extreme cases using these models, but will lack the precision necessary to uncover subtle trends.

Item Type:Article
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URLURL TypeDescription Paper
Lacy, B.0000-0002-9420-4455
Burrows, A.0000-0002-3099-5024
Additional Information:© 2019. The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2018 January 22; revised 2018 November 20; accepted 2019 January 31; published 2019 March 5. The authors would like to acknowledge support for this research under their NASA WFIRST-SIT award # NNG16PJ24C, NASA grant NNX15AE19G, and NASA JPL subcontracts no. 1538907, 1529729, 1513640 and 1534432-B-4.25. We are grateful to Bijan Nemati and Jeremy Kasdin for insightful conversations, and we thank Sergei Yurchenko and Alex Howe for help with the ExoMol methane database.
Funding AgencyGrant Number
Subject Keywords:planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: detection; planets and satellites: gaseous planets
Issue or Number:3
Record Number:CaltechAUTHORS:20190305-082645109
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Official Citation:B. Lacy et al 2019 AJ 157 132
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:93518
Deposited By: George Porter
Deposited On:05 Mar 2019 16:39
Last Modified:16 Nov 2021 16:58

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