THE STATISTICS OF ALBEDO AND HEAT RECIRCULATION ON HOT EXOPLANETS

If both the day-side and night-side effective temperatures of a planet can be measured, it is possible to estimate its Bond albedo, 0 < A(B) < 1, as well as its day-night heat redistribution efficiency, 0 < epsilon < 1. We attempt a statistical analysis of the albedo and redistribution efficiency for 24 transiting exoplanets that have at least one published secondary eclipse. For each planet, we show how to calculate a sub-stellar equilibrium temperature, T-0, and associated uncertainty. We then use a simple model-independent technique to estimate a planet's effective temperature from planet/star flux ratios. We use thermal secondary eclipse measurements-those obtained at lambda > 0.8 mu m-to estimate day-side effective temperatures, T-d, and thermal phase variations-when available-to estimate night-side effective temperature. We strongly rule out the null hypothesis of a single A(B) and epsilon for all 24 planets. If we allow each planet to have different parameters, we find that low Bond albedos are favored (A(B) < 0.35 at 1 sigma confidence), which is an independent confirmation of the low albedos inferred from nondetections of reflected light. Our sample exhibits a wide variety of redistribution efficiencies. When normalized by T-0, the day-side effective temperatures of the 24 planets describe a uni-modal distribution. The two biggest outliers are GJ 436b (abnormally hot) and HD 80606b (abnormally cool), and these are the only eccentric planets in our sample. The dimensionless quantity T-d/T-0 exhibits no trend with the presence or absence of stratospheric inversions. There is also no clear trend between T-d/T-0 and T-0. That said, the six planets with the greatest sub-stellar equilibrium temperatures (T > 2400 K) have low epsilon, as opposed to the 18 cooler planets, which show a variety of recirculation efficiencies. This hints that the very hottest transiting giant planets are qualitatively different from the merely hot Jupiters. We propose an explanation of this trend based on how a planet's radiative and advective times scale with temperature: both timescales are expected to be shorter for hotter planets, but the temperature dependence of the radiative timescale is stronger, leading to decreased heat recirculation efficiency.