We discussed an impressive paper today in journal club, Sing et al.’s (2014) study of the transiting, hot Jupiter WASP-31 b.WASP-31 b weighs about half as much as Jupiter but has a radius more than 50% larger, giving it a very low density for a Jupiter-like planet.
Using the Hubble Space Telescope, Sing and colleagues observed several, spectrally-resolved transits of WASP-31 b (i.e., in several different colors), resulting in several of the black data points in the figure at left.
The colors of the planet’s transit tells us a lot about the composition and nature of WASP-31 b’s atmosphere. For example, the blue and green curves in the figure at left include spectral features from water, but the black data points don’t show the same features.
However, the data points DO show a clear detection of potassium (K) in the planet’s atmosphere (as indicated in the figure). Potassium, while interesting, is not completely unexpected in WASP-31 b’s atmosphere since it is very hot, > 1,500 K.
But the fact that only the very highest point in the potassium spectral feature is seen and not the broad wings on either side probably indicates the planet has a thick cloud deck. Since the peak of the potassium feature results from gas high in the planet’s atmosphere and the wings from lower in the atmosphere, Sing and colleagues can estimate the pressure level of the cloud deck, about 10 mbar.
For comparison, clouds on the Earth typically form at about 500 mbar. In the Earth’s atmosphere, 10 mbar is about the level where meteors burn up, so WASP-31 b’s clouds are VERY high in its atmosphere.