Constraining the atmosphere of GJ 1214b using an optimal estimation technique

We explore cloudy, extended H2-He atmosphere scenarios for the warm super-Earth GJ 1214b using an optimal estimation retrieval technique. This planet, orbiting an M4.5 star only 13 pc from the Earth, is of particular interest because it lies between the Earth and Neptune in size and may be a member of a new class of planet that is neither terrestrial nor gas giant. Its relatively flat transmission spectrum has so far made atmospheric characterization difficult. The Non-linear optimal Estimator for MultivariateE spectral analySIS (NEMESIS) algorithm is used to explore the degenerate model parameter space for a cloudy, H2-He-dominated atmosphere scenario. Optimal estimation is a data-led approach that allows solutions beyond the range permitted by ab initio equilibrium model atmosphere calculations, and as such prevents restriction from prior expectations. We show that optimal estimation retrieval is a powerful tool for this kind of study, and present an exploration of the degenerate atmospheric scenarios for GJ 1214b. Whilst we find a family of solutions that provide a very good fit to the data, the quality and coverage of these data are insufficient for us to more precisely determine the abundances of cloud and trace gases given an H2-He atmosphere, and we also cannot rule out the possibility of a high molecular weight atmosphere. Future groundand space-based observations will provide the opportunity to confirm or rule out an extended H2-He atmosphere, but more precise constraints will be limited by intrinsic degeneracies in the retrieval problem, such as variations in cloud top pressure and temperature. DOI: https://doi.org/10.1093/mnras/stt1204 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-90729 Published Version Originally published at: Barstow, J K; Aigrain, S; Irwin, P G J; Fletcher, L N; Lee, J M (2013). Constraining the atmosphere of GJ 1214b using an optimal estimation technique. Monthly Notices of the Royal Astronomical Society, 434(3):2616-2628. DOI: https://doi.org/10.1093/mnras/stt1204 MNRAS 434, 2616–2628 (2013) doi:10.1093/mnras/stt1204 Advance Access publication 2013 July 29 Constraining the atmosphere of GJ 1214b using an optimal estimation technique J. K. Barstow,1,2‹ S. Aigrain,1 P. G. J. Irwin,2 L. N. Fletcher2 and J.-M. Lee2,3 1Department of Astrophysics, Denys Wilkinson Building, University of Oxford, Oxford OX1 3RH, UK 2Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, UK 3Institute for Theoretical Physics, University of Zürich, CH-8057 Zürich, Switzerland Accepted 2013 June 27. Received 2013 June 20; in original form 2013 April 26