The upper atmosphere of HD17156b

HD17156b is a newly found transiting extrasolar giant planet that orbits its G-type host star in a highly eccentric orbit (e 0.67) with an orbital semimajor 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 modeling. 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 of its upper atmosphere. Contrary to the well-known transiting planet HD209458b, we find that the atmosphere of HD17156b is unlikely to undergo fast hydrodynamic escape at any point along the orbit, even if the upper atmosphere is almost entirely composed of atomic hydrogen and H⁺, and infrared cooling by H⁺ ₃ ions is negligible. The nature of the upper atmosphere is sensitive to the composition of the thermosphere, and in particular to the mixing ratio of H₂, as the availability of H ₂ 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.