Thursday, May 5, 2016

A Transition in the Composition of Hot Jupiter Clouds

A transition in the composition of clouds in hot Jupiters


Parmentier et al


Over a large range of equilibrium temperatures, clouds shape the transmission spectrum of hot Jupiter atmospheres, yet their composition remains unknown. Recent observations show that the \emph{Kepler} lightcurves of some hot Jupiters are asymmetric: for the hottest planets, the lightcurve peaks before secondary eclipse, whereas for planets cooler than ∼1900K, it peaks after secondary eclipse. In this paper we use the thermal structure from 3D global circulation models to determine the expected cloud distribution and \emph{Kepler} lightcurves of hot Jupiters. We demonstrate that the change from a visible lightcurve dominated by thermal emission to one dominated by scattering (reflection) naturally explains the observed trend from negative to positive offset. For the cool planets the presence of an asymmetry in the \emph{Kepler} lightcurve is a telltale sign of the cloud composition, because each cloud species can produce an offset only over a narrow range of effective temperatures. Silicate clouds reproduce the observations in the 1600−1900K equilibrium temperature but cannot explain offsets for cooler planets. In contrast, manganese sulfide clouds match the offsets for equilibrium temperatures between 1300 and 1700K. We suggest that a transition occurs between silicate and manganese sulfide clouds at a temperature near 1600K, analogous to the L/T transition on brown dwarfs. The cold trapping of cloud species below the photosphere naturally produces such a transition and predicts similar transitions for other condensates. We predict that most hot Jupiters should have cloudy nightsides, consistent with the lack of thermal emission observed in some of them. Partial cloudiness should be common at the limb and on the dayside, affecting the retrieved molecular abundances, whereas the dayside emission may often be dominated by a cloud-free hot spot.

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