Thursday, October 19, 2017

Phase Offsets and the Energy Budgets of Hot Jupiters

Phase Offsets and the Energy Budgets of Hot Jupiters


Schwartz et al

Thermal phase curves of short-period planets on circular orbits provide joint constraints on the fraction of incoming energy that is reflected (Bond albedo) and the fraction of absorbed energy radiated by the night hemisphere (heat recirculation efficiency). Many empirical studies of hot Jupiters have implicitly assumed that the dayside is the hottest hemisphere and the nightside is the coolest hemisphere. For a given eclipse depth and phase amplitude, an orbital lag between a planet's peak brightness and its eclipse (a phase offset) implies that planet's nightside emits greater flux. To quantify how phase offsets impact the energy budgets of short-period planets, we compile all infrared observations of the nine planets with multi-band eclipse depths and phase curves. Accounting for phase offsets shifts planets to lower Bond albedo and greater day--night heat transport, usually by ≲1σ. We find a somewhat higher nightside temperature for WASP-43b, but the planet still exhibits an unusually high day-night temperature contrast compared to other planets of the same irradiation temperature. For WASP-12b, our more accurate analysis suggests that the planet has a slightly lower Bond albedo, and much greater day-night recirculation efficiency than previously reported. The planet no longer fits the trend of increasing day-night temperature contrast with greater instellation.

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