Effects of mass loss for highly-irradiated giant planets

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  • 15 September 2005
Abstract
We present calculations for the evolution and surviving mass of highly-irradiated extrasolar giant planets (EGPs) at orbital semimajor axes ranging from 0.023 to 0.057 AU using a generalized scaled theory for mass loss, together with new surface-condition grids for hot EGPs and a consistent treatment of tidal truncation. Available theoretical estimates for the rate of energy-limited hydrogen escape from giant-planet atmospheres range over four orders of magnitude, when one holds planetary mass, composition, and irradiation constant. Yelle (2004, Icarus 170, 167-179) predicts the lowest escape rate. Baraffe et al. (2004, A&A 419, L13-L16) predict the highest rate, based on the theory of Lammer et al. (2003, Astrophys. J. 598, L121-L124). Scaling the theory of Watson et al. (1981, Icarus 48, 150-166) to parameters for a highly-irradiated exoplanet, we find an intermediate escape rate, ~ 100 times higher than Yelle's but ~ 100 times lower than Baraffe's. With the scaled Watson theory and the scaled Yelle theory we find modest mass loss, occurring early in the history of a hot EGP. Particularly for the Yelle theory, the effect of tidal truncation sets the minimum mass limit, well below a Saturn mass for semimajor axes greater than or equal to 0.023 AU. This contrasts with the Baraffe model, where hot EGPs are claimed to be remnants of much more massive bodies, originally several times Jupiter and still losing substantial mass fractions at present.

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