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Inflating and Deflating Hot Jupiters: Coupled Tidal and Thermal Evolution of Known Transiting Planets
We examine the radius evolution of close in giant planets with a planetevolution model that couples the orbital-tidal and thermal evolution.For 45 transiting systems, we compute a large grid ofcooling/contraction paths forward in time, starting from a large phasespace of initial semimajor axes and eccentricities. Given observationalconstraints at the current time for a given planet (semimajor axis,eccentricity, and system age), we find possible evolutionary paths thatmatch these constraints, and compare the calculated radii toobservations. We find that tidal evolution has two effects. First,planets start their evolution at larger semimajor axis, allowing them tocontract more efficiently at earlier times. Second, tidal heating cansignificantly inflate the radius when the orbit is being circularized,but this effect on the radius is short-lived thereafter. Oftencircularization of the orbit is proceeded by a long period while thesemimajor axis slowly decreases. Some systems with previouslyunexplained large radii that we can reproduce with our coupled model areHAT-P-7, HAT-P-9, WASP-10, and XO-4. This increases the number ofplanets for which we can match the radius from 24 (of 45) to as many as35 for our standard case, but for some of these systems we are requiredto be viewing them at a special time around the era of current radiusinflation. This is a concern for the viability of tidal inflation as ageneral mechanism to explain most inflated radii. Also, large initialeccentricities would have to be common. We also investigate theevolution of models that have a floor on the eccentricity, as may be dueto a perturber. In this scenario, we match the extremely large radius ofWASP-12b. This work may cast some doubt on our ability to accuratelydetermine the interior heavy element enrichment of normal, noninflatedclose in planets, because of our dearth of knowledge about theseplanets' previous orbital-tidal histories. Finally, we find that the endstate of most close in planetary systems is disruption of the planet asit moves ever closer to its parent star.
| Detection of Thermal Emission of XO-2b: Evidence for a Weak Temperature Inversion
We estimate flux ratios of the extrasolar planet XO-2b to its host starXO-2 at 3.6, 4.5, 5.8, and 8.0 μm with Infrared Array Camera on theSpitzer Space Telescope to be 0.00081 ± 0.00017, 0.00098 ±0.00020, 0.00167 ± 0.00036, and 0.00133 ± 0.00049,respectively. The fluxes provide tentative evidence for a weaktemperature inversion in the upper atmosphere, the precise nature ofwhich would need to be confirmed by longer wavelength observations.XO-2b substellar flux of 0.76 × 109 ergcm-2 s-1 lies in the predictedtransition region between atmospheres with and without upper atmospherictemperature inversion.
| Evidence for a lost population of close-in exoplanets
We investigate the evaporation history of known transiting exoplanets inorder to consider the origin of observed correlations between mass,surface gravity and orbital period. We show that the survival of theknown planets at their current separations is consistent with a simplemodel of evaporation, but that many of the same planets would not havesurvived closer to their host stars. These putative closer-in systemsrepresent a lost population that could account for the observedcorrelations. We conclude that the relation underlying the correlationsnoted by Mazeh et al. and Southworth et al. is most likely a linearcut-off in the M2/R3 versus a-2 plane,and we show that the distribution of exoplanets in this plane is inclose agreement with the evaporation model.
| Wasp-7: A Bright Transiting-Exoplanet System in the Southern Hemisphere
We report that a Jupiter-mass planet, WASP-7b, transits the V = 9.5 starHD 197286 every 4.95 d. This is the brightest discovery from theWASP-South transit survey so far and is currently the brightesttransiting-exoplanet system in the southern hemisphere. WASP-7b is amongthe densest of the known Jupiter-mass planets, suggesting that it has amassive core. The planet mass is 0.96+0.12-0.18 M Jup, the radius is0.915+0.046 -0.040 R Jup, and thedensity is 1.26+0.25 -0.21ρJup (1.67+0.33 -0.28 gcm-3).
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