The hottest planet

Mirror image The newly discovered 'hot Jupiter' HD 149026b is the hottest planet known. At around 2,300 K, its temperature is higher than that of many low-mass stars. Its atmosphere is unusual too, since it seems to instantly re-radiate any light that falls upon it. The smallest known transiting planet, HD 149026b, has an 8-µm brightness temperature of 2300 ± 200 K. The planet's predicted temperature for uniform, spherical, blackbody emission and zero albedo (unprecedented for planets) is 1,741 K. The measurement matches the prediction for a planet on which each patch of surface area instantaneously re-emits all absorbed light as a black body. Of the over 200 known extrasolar planets, just 14 pass in front of and behind their parent stars as seen from Earth. This fortuitous geometry allows direct determination of many planetary properties 1 . Previous reports of planetary thermal emission 2 , 3 , 4 , 5 give fluxes that are roughly consistent with predictions based on thermal equilibrium with the planets’ received radiation, assuming a Bond albedo of ∼0.3. Here we report direct detection of thermal emission from the smallest known transiting planet, HD 149026b, that indicates a brightness temperature (an expression of flux) of 2,300 ± 200 K at 8 µm. The planet’s predicted temperature for uniform, spherical, blackbody emission and zero albedo (unprecedented for planets) is 1,741 K. As models with non-zero albedo are cooler, this essentially eliminates uniform blackbody models, and may also require an albedo lower than any measured for a planet, very strong 8 µm emission, strong temporal variability, or a heat source other than stellar radiation. On the other hand, an instantaneous re-emission blackbody model, in which each patch of surface area instantly re-emits all received light, matches the data. This planet is known 6 , 7 , 8 , 9 to be enriched in heavy elements, which may give rise to novel atmospheric properties yet to be investigated.