Exoplanets and Brown Dwarfs


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.

Exoplanets (filled squares) and brown dwarfs in a gravity vs effective temperature plot. On the left, some transitions for the atmospheric structure and spectrum are shown: condensation of titanium, vanadium, lithium and potassium, transition from carbon monoxide to methane, from nitrogen to ammonia, condensation of water. The right axis shows the M-L-T-Y spectroscopic sequence. Typical hot Jupiters have equilibrium temperatures around the L-T transition in brown dwarfs. The gravity on brown dwarfs is 10 to 100 times higher. Fig. 2 from the paper.

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.

An artist’s impression of what brown dwarfs of different spectral type might look like to our eyes. To the left is an L Dwarf, with a typical temperature ranging from ~1300 K to ~2500 K, showing a cross-hatched structure due to large convective cells. A T-dwarf, temperature range of ~750 K to ~1500 K, is starting to show belts and zones, characteristic of Jovian planets. The Y-dwarfs are thought to have temperatures ranging from ~300K to ~750K. In the image above a purple hue has been given. This is a purely artistic contribution as we don’t yet know what Y-dwarfs look like at visible wavelengths.

Featured image credit: Image credit: NASA/JPL-Caltech


About Author

Editor of Exoclimes.com. Observational exoplanet and brown dwarf astronomer studying the atmospheres of exoplanets. Interested in public outreach and conveying my interest in astronomy to others. Follow me on Twitter or Google+. (More)