The Upper Atmosphere of HD17156b

TL;DR

This study models the upper atmosphere of the highly eccentric exoplanet HD17156b, revealing its atmospheric stability, circulation, and composition variations, and predicting observable features distinct from other transiting exoplanets.

Contribution

First 3D atmospheric model applied to an eccentric exoplanet, providing insights into its stability, composition, and potential observational signatures.

Findings

HD17156b's atmosphere is unlikely to escape hydrodynamically.

Atmospheric properties are highly sensitive to H2 mixing ratio.

Predictions made for thermosphere-ionosphere system observable by future observations.

Abstract

HD17156b is a newly-found transiting extrasolar giant planet (EGP) that orbits its G-type host star in a highly eccentric orbit (e~0.67) with an orbital semi-major axis of 0.16 AU. Its period, 21.2 Earth days, is the longest among the known transiting planets. The atmosphere of the planet undergoes a 27-fold variation in stellar irradiation during each orbit, making it an interesting subject for atmospheric modelling. We have used a three-dimensional model of the upper atmosphere and ionosphere for extrasolar gas giants in order to simulate the progress of HD17156b along its eccentric orbit. Here we present the results of these simulations and discuss the stability, circulation, and composition in its upper atmosphere. Contrary to the well-known transiting planet HD209458b, we find that the atmosphere of HD17156b is unlikely to escape hydrodynamically at any point along the orbit, even if the upper atmosphere is almost entirely composed of atomic hydrogen and H+, and infrared cooling by H3+ ions is negligible. The nature of the upper atmosphere is sensitive to to the composition of the thermosphere, and in particular to the mixing ratio of H2, as the availability of H2 regulates radiative cooling. In light of different simulations we make specific predictions about the thermosphere-ionosphere system of HD17156b that can potentially be verified by observations.