title: On the Radii of Close-in Giant Planets.
abstract: The recent discovery that the close-in extrasolar giant planet HD 209458b transits its star has provided a first-of-its-kind measurement of the planet's radius and mass. In addition, there is a provocative detection of the light reflected off of the giant planet tau Bootis b. Including the effects of stellar irradiation, we estimate the general behavior of radius/age trajectories for such planets and interpret the large measured radii of HD 209458b and tau Boo b in that context. We find that HD 209458b must be a hydrogen-rich gas giant. Furthermore, the large radius of a close-in gas giant is not due to the thermal expansion of its atmosphere but to the high residual entropy that remains throughout its bulk by dint of its early proximity to a luminous primary. The large stellar flux does not inflate the planet but retards its otherwise inexorable contraction from a more extended configuration at birth. This implies either that such a planet was formed near its current orbital distance or that it migrated in from larger distances (>/=0.5 AU), no later than a few times 107 yr of birth.
authors.initials: A
authors.lastname: Burrows
authors.initials: T
authors.lastname: Guillot
authors.initials: WB
authors.lastname: Hubbard
authors.initials: MS
authors.lastname: Marley
authors.initials: D
authors.lastname: Saumon
authors.initials: JI
authors.lastname: Lunine
authors.initials: D
authors.lastname: Sudarsky