Tidal Evolution of Close-in Extrasolar Planets

The distribution of eccentricitieseofextrasolarplanetswithsemimajor axesa > 0:2 AU isveryuniform,andvalues for e are relatively large, averaging 0.3 and broadly distributed up to near 1. For a < 0:2 AU, eccentricities are much smaller (most e < 0:2), a characteristic widely attributed to damping by tides after the planets formed and the protoplanetarygasdiskdissipated.Mostpreviousestimatesofthetidaldampingconsideredthetidesraisedontheplanets,but ignored the tides raised on the stars. Most also assumed specific values for the planets’ poorly constrained tidal dissipation parameter Qp. Perhaps most important, in many studies the strongly coupled evolution between e and a was ignored. We have now integrated the coupled tidal evolution equations for e and a over the estimated age of each planet, and confirmed that the distribution of initialevalues of close-in planets matches that of the general population for reasonable Q values, with the best fits for stellar and planetary Q being � 10 5.5 and � 10 6.5 , respectively. The accompanying evolutionofavaluesshowsmost close-in planetshadsignificantly largeraatthestart oftidalmigration. The earlier gas disk migration did not bring all planets to their current orbits. The current small values of a were only reachedgraduallyduetotidesoverthelifetimesoftheplanets.Theseresultsmayhaveimportantimplicationsforplanet formation models, atmospheric models of ‘‘hot Jupiters,’’ and the success of transit surveys. Subject headingg celestial mechanics — planetary systems: formation — planetary systems: protoplanetary disks