Hazes and clouds on brown dwarfs

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Brown dwarfs (objects 12 to 80 times heavier than Jupiter, between planets and stars) are similar to hot Jupiter in several respects. Their atmospheres have similar temperatures (1000 to 3000 K) and composition (hydrogen and helium gas with a few percents of heavier elements). The same kind of aerosols can form in both atmospheres: clouds metallic oxides, silicates, and iron.

Of course there are also some differences. The gravity is higher on brown dwarfs, which compresses their atmosphere into a thin layer (a few dozen kilometres instead of hundreds for hot Jupiters). Another important difference is that brown dwarfs are isolated in space and receive heat only from their interior, while hot Jupiters are heated by their star.

Some brown dwarfs show strong variations in brightness, and the standard explanation for this is that we are seeing gaps in the silicate/iron clouds, the so-called “patchy cloud” scenario.

More detailed observations of one variable brown dwarf with the Hubble Space Telescope in 2012 have shown that the picture was not so simple. The brightness variations of that brown dwarf are due to temperature fluctuations in the clouds rather than holes in the cloud cover. (see this post)

This year, two large sets of observations, one with the Spitzer infrared space telescope and the other with the Hubble Space Telescope, offer us a much more vivid view of brown dwarf atmosphere.

The results are spectacular, and they bury the “patchy cloud” scenario.

1- The Spitzer observations show that all brown dwarfs have spotted surfaces to a certain degree, and that there is no marked increase of variability around the temperature were silicate clouds are expected to break (around 1400 K). [see this paper]

2- The Hubble observations show that the observed variations of brightness are not due to holes in the cloud cover, but rather a combination of cloud thinning and temperature changes in the atmosphere. They suggest a different pattern for brown dwarfs on either side of the 1400 K transition. For the hotter type, the variations are mainly caused by a patchy layer of high hazes far above the visible clouds. For the cooler type, the variations are caused by thickness and temperature variations in the main clouds. [see this paper]

illustr_post_browndwarfs

Can we draw a parallel with hot Jupiters? The observation techniques are so different for the two classes of objects that direct comparison is difficult, but we certainly do see the signature of both high-altitude hazes and lower clouds in hot Jupiters, at comparable temperatures. The temperature in the atmosphere of HD 209458b, for instance, straddles both sides of the 1400K boundary.

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.