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[31.05] Mass Loss for Highly-Irradiated Giant Planets

W. B. Hubbard, A. Burrows, I. Hubeny, D. Sudarsky, M. F. Hattori (U. Arizona)

We present calculations for the 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 (Icarus 170, 167-179, 2004) predicts the lowest escape rate. Baraffe et al. (A&A 419, L13-L16, 2004) predict the highest rate, based on the theory of Lammer et al. (ApJ 598, L121-L124, 2003). Scaling the theory of Watson et al. (Icarus 48, 150-166, 1981) to parameters for a highly-irradiated exoplanet, we find an intermediate escape rate, ~10² higher than Yelle's but ~10² 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 the distances investigated. 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.

Supported by NASA Grant NAG5-13775 (PGG) and NASA Grant NNG04GL22G (ATP).

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.