Theoretical radii of extrasolar giant planets: The cases of TrES-4, XO-3b, and HAT-P-1b

Xin Liu, Adam S. Burrows, Laurent Ibgui

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To explain their observed radii, we present theoretical radius-age trajectories for the extrasolar giant planets ( EGPs) TrES-4, XO-3b, and HAT-P-1b. We factor in variations in atmospheric opacity, the presence of an inner heavy-element core, and possible heating due to orbital tidal dissipation. A small, yet nonzero, degree of core heating is needed to explain the observed radius of TrES-4, unless its atmospheric opacity is significantly larger than a value equivalent to that at 10 × solar metallicity with equilibrium molecular abundances. This heating rate is reasonable, and corresponds for an energy dissipation parameter (Qp) of ∼103.8 to an eccentricity of ∼0.01, assuming 3 × solar atmospheric opacity and a heavy-element core of Mc = 30 M. For XO-3b, which has an observed orbital eccentricity of 0.26, we show that tidal heating needs to be taken into account to explain its observed radius. Furthermore, we reexamine the core mass needed for HAT-P-1b in light of new measurements and find that it now generally follows the correlation between stellar metallicity and core mass suggested recently. Given various core heating rates, theoretical grids and fitting formulae for a giant planet's equilibrium radius and equilibration timescale are provided for planet masses Mp = 0.5, 1.0, and 1.5 MJ with a = 0.02-0.06 AU, orbiting a G2V star. When the equilibration timescale is much shorter than that of tidal heating variation, the "effective age" of the planet is shortened, resulting in evolutionary trajectories more like those of younger EGPs. Motivated by the work of B. Jackson et al., we suggest that this effect could indeed be important in better explaining some observed transit radii.

Original languageEnglish (US)
Pages (from-to)1191-1200
Number of pages10
JournalAstrophysical Journal
Issue number2
StatePublished - Nov 10 2008

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planetary systems
  • Planets and satellites: general
  • Stars: individual (GSC 02620-00648, HAT-P-1, XO-3)


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