B. Levrard et al 2009 ApJ 692 L9 doi:10.1088/0004-637X/692/1/L9
B. Levrard1, C. Winisdoerffer and G. Chabrier
Show affiliationsWe revisit the tidal stability of extrasolar systems harboring a transiting planet and demonstrate that, independently of any tidal model, none, but one (HAT-P-2b) of these planets has a tidal equilibrium state, which implies ultimately a collision of these objects with their host star. Consequently, conventional circularization and synchronization timescales cannot be defined because the corresponding states do not represent the endpoint of the tidal evolution. Using numerical simulations of the coupled tidal equations for the spin and orbital parameters of each transiting planetary system, we confirm these predictions and show that the orbital eccentricity and the stellar obliquity do not follow the usually assumed exponential relaxation but instead decrease significantly, eventually reaching a zero value only during the final runaway merging of the planet with the star. The only characteristic evolution timescale of all rotational and orbital parameters is the lifetime of the system, which crucially depends on the magnitude of tidal dissipation within the star. These results imply that the nearly circular orbits of transiting planets and the alignment between the stellar spin axis and the planetary orbit are unlikely to be due to tidal dissipation. Other dissipative mechanisms, for instance interactions with the protoplanetary disk, must be invoked to explain these properties.
celestial mechanics; planetary systems: formation; planetary systems: protoplanetary disks
97.82.Fs Substellar companions; planets
97.10.Cv Stellar structure, interiors, evolution, nucleosynthesis, ages
97.10.Fy Circumstellar shells, clouds, and expanding envelopes; circumstellar masers
Issue 1 (2009 February 10)
Received 2008 August 18, accepted for publication 2008 December 11
Published 2009 January 21
B. Levrard et al 2009 ApJ 692 L9
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