An outline of the paper: HST Spectral Mapping of L/T Transition Brown Dwarfs Reveals Cloud Thickness Variations. by Apai et al.
2M2139 and SIMP0136 are two brown dwarfs at the transition between L and T spectral types, that fluctuate in brightness on timescales of a few hours. Since the L-T transition is characterised by the disappearance of silicate/iron clouds in brown dwarfs, the variability is thought to be caused by patches of clouds rotating in and out of view as the brown dwarfs spin. Their rotation periods are 8 hours and 2 hours, compared to 10 hours for Jupiter.
The authors have measured the changes in the spectrum of the two objects in the near infrared (1-2 microns) with the WFC3 camera on the Hubble Space Telescope. Measuring the variations in the spectrum over several hours can show whether the brightness fluctuations are due to holes in the clouds, temperature changes in the cloud deck, or the presence of several distinct populations of clouds.
This is similar to the study of Buenzli et al. earlier this year (see our post), except that the two objects have variations of much higher amplitude (about 5% for SIMP0136 and up to 25% for 2M2139), so that they afford more constraining scenarios.
Apai et al. find that the best way to reproduce the Hubble observations is with a mixture of two cloud layers, high clouds and low clouds.
In the best fit models, one of the cloud layers is distributed in at least three large patches, on a uniform background provided by the other layer.Strangely enough, the observed changes in the spectra can be equally well explained by the rotation of dark spots on a bright background, or by bright spots on a dark background – like the proverbial puzzle of whether zebras have black stripes on white or the opposite (the truth of course is that zebras are green with black and white stripes, and this holds the key to solving the cloud problem in brown dwarfs).
A look at Jupiter shows how simplistic a cloud distribution in terms of two layers and circular patches might be. Indeed, the Hubble Space Telescope has recently acquired a long sequence of Jupiter images that will allow us to consider it “as an exoplanet” and test our capacity to recover cloud features from integrated lightcurves.
The Apai et al. study is sobering in the context of exoplanet atmospheres: the interpretation remains difficult and ambiguous even with vastly more precise and abundant data that we could dream of for exoplanets.