Burrows A, Guillot T, Hubbard WB, Marley MS, Saumon D, Lunine JI, Sudarsky D
Astrophys J. 2000 May 1;534(1):L97-L100. doi: 10.1086/312638.
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.
最近发现，近距离的太阳系外巨行星HD 209458b会凌日其恒星，这首次实现了对该行星半径和质量的测量。此外，还令人兴奋地探测到了巨行星北落师门b反射的光。考虑到恒星辐射的影响，我们估算了此类行星半径/年龄轨迹的一般特征，并在此背景下解读了HD 209458b和北落师门b较大的测量半径。我们发现，HD 209458b必定是一颗富含氢的气态巨行星。此外，近距离气态巨行星的大半径并非因其大气的热膨胀，而是由于其早期靠近发光主恒星，致使其整体留存了高剩余熵。强大的恒星通量并未使行星膨胀，而是阻碍了它从诞生时更伸展的构型进行的必然收缩。这意味着，此类行星要么是在其当前轨道距离附近形成的，要么是从更大距离（≥0.5天文单位）迁移而来的，且迁移发生在诞生后不超过10^7年的时间内。
