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Session 70 - Searching for Other Planetary Systems.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[70.03] On the Nature of the Companion of 51 Peg

T. Guillot, A. Burrows, W. B. Hubbard, J. I. Lunine, D. Saumon (U. of Arizona)

The recent suggestion that a planet of about 1 Jupiter mass (M_J) orbits at 0.05 A.U. from the star 51 Peg rises three questions: Can a giant planet form so close to a star? Can it be transported there? Can it survive? This work focuses on the latter question: We calculate evolution tracks of hydrogen-helium planets between 0.3 and 3\,M_J for different star-planet separations (0.01 to 5.2 A.U.), and determine their radii, luminosity and structure.

We show that, after a relatively short time scale (less than 10^8 years), a hydrogen-helium giant planet orbiting within 0.1\,A.U. of a G2 star becomes partially radiative. Its radius is then very close to the equilibrium radius (obtained when the planet is in complete equilibrium with the star), and significantly different from the radius it would have if it was fully convective. This is independent of its evolution history. Furthermore, the equilibrium radius is close to the radius of Jupiter (R_J=70000\,km), and then well under the Roche limit, except for tiny star-planet separations (<0.01\,A.U.). We also show that at these distances, evaporation due to Jeans escape or UV radiation remains small. Hence, these objects can survive very close to their parent star. This is also true for objects formed with heavier elements, as they have even smaller radii: an icy Jupiter would have a radius of about 1/2 R_J, and it would be of about 1/3 R_J if of chondritic composition.

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