On the radii of close-in giant planets

The recent discovery that the close-in extrasolar giant planet HD 209458b t ransits its star has provided a first-of-its-kind measurement of the planet 's radius and mass. In addition, there is a provocative defection of the li ght reflected off of the giant planet tau Bootis b. Including the effects o f stellar irradiation, we estimate the general behavior of radius/age traje ctories for such planets and interpret the large measured radii of HD 20945 8b and tau Boo b in that context. We find that HD 209458b must be a hydroge n-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 configur ation at birth. This implies either that such a planet was formed near its current orbital distance or that it migrated in from larger distances (grea ter than or equal to 0.5 AU), no later than a few times 10(7) yr of birth.