On the radii of close-in giant planets

A. Burrows, T. Guillot, W. B. Hubbard, M. S. Marley, D. Saumon, J. I. Lunine, D. Sudarsky

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162 Scopus citations


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 τ 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 τ 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.

Original languageEnglish (US)
Pages (from-to)L97-L100
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - May 1 2000
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planetary systems
  • Planets and satellites: general
  • Stars: individual (HD 209458, τ Bootis)


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