An outline of the paper: The Brown Dwarf-Exoplanet Connection by Adam J. Burgasser
Measuring the atmospheres of exoplanets can be a tricky business (trust me, I’ve been trying). It not only requires the use of the biggest and best telescopes to collect the scarce photons, but it also requires a cautious approach to understand if what we are studying are real atmospheric features or contamination from the Earth’s atmosphere or the instruments.
Brown dwarfs have a lot of things in common with exoplanets in terms of atmospheric properties. They are a lot easier to observe than exoplanets and thus can act as good templates for understanding exoplanets atmospheres.
In the next couple of posts I will compare exoplanets and brown dwarfs.
Brown dwarfs fall into the temperature regime of hot Jupiters. However, in most cases they are much easier to study in detail due to their isolation. With temperatures varying from about 600 to 2000 K, they are interesting analogs to hot Jupiter atmospheres. Unlike stars, brown dwarfs never reach the pressures and temperatures necessary to sustain stable hydrogen fusion in their core. Unlike planets, during the earlier parts of their lifetime, some burning of deuterium does take place.
Similar, but not the same!
Comparing the atmospheres of exoplanets and brown dwarfs requires a cautious approach though. Brown dwarfs are heated from their interiors whilst hot Jupiters are heated from above the atmosphere by the star they orbit. For hot Jupiters, eccentricity effects as well as large day/night temperature contrasts make the equilibrium temperature a poor proxy for the photospheric temperature, and the irradiation from above can lead to the formation of a stratosphere much hotter than the equilibrium temperature. Not knowing the photospheric temperature leads to a plethora of possibilities for atmospheric structure. This is because several elements can condensate to form clouds (see post on Clouds in T-Y dwarf atmospheres).
Large and young (< 300 Myr) exoplanets on wide orbits (~10-70 AU) (i.e HR 8799 bcd, β Pictoris) are more comparable to brown dwarfs, because their photospheres are predominantly dominated by internal heating.
Stay tuned for the next post on Exoplanets and Brown Dwarfs.
Featured image credit: Image credit: NASA/JPL-Caltech