The Brown Dwarf - Exoplanet Connection

Brown dwarfs are commonly regarded as easily-observed templates for exoplanet studies, with comparable masses, physical sizes and atmospheric properties. There is indeed considerable overlap in the photospheric temperatures of the coldest brown dwarfs (spectral classes L and T) and the hottest exoplanets. However, the properties and processes associated with brown dwarf and exoplanet atmospheres can differ significantly in detail; photospheric gas pressures, elemental abundance variations, processes associated with external driving sources, and evolutionary effects are all pertinent examples. In this contribution, I review some of the basic theoretical and empirical properties of the currently known population of brown dwarfs, and detail the similarities and differences between their visible atmospheres and those of extrasolar planets. I conclude with some specific results from brown dwarf studies that may prove relevant in future exoplanet observations. 1. A Brown Dwarf Primer Brown dwarfs are stellar objects with insufficient mass to sustain core hydrogen fusion reactions, resulting in a steady decline in both luminosity and effective temperature (Teff ) with time. The mass limit for sustained hydrogen fusion is roughly 0.072 M⊙ (75 Jupiter masses) for a Solar metallicity gas mixture, increasing to 0.090 M⊙ for a pure hydrogen gas (e.g., Chabrier & Baraffe 2000). This mass limit establishes a formal division between “stars” and “brown dwarfs”, although such a division is not necessarily relevant to how these objects form. While there is ongoing debate over the details of brown dwarf formation (the roles of gas turbulence, fragmentation and dynamical interactions; see recent reviews by Luhman et al. 2007 and Whitworth et al. 2007), observational evidence indicates brown dwarfs are created in a manner similar to, or at least coincident with, stars, via gravitational collapse of dense cores within giant molecular clouds. As a brown dwarf’s energy reservoir arises primarily from the gravitational potential energy released in their initial contraction, the luminosity, Teff and emergent spectral energy distribution of a brown dwarf depend primarily on mass and age, and secondarily on elemental abundances, bulk properties (e.g., rotation) and external drivers (e.g., the presence of close companion). The interdependence of these factors on brown dwarf observables Small contributions also arise from brief periods of lithiumand deuterium fusion for objects more massive than ∼0.065 M⊙ and ∼0.012 M⊙, respectively. The latter limit is considered a possible dividing line between “brown dwarfs” and “planets” (see Basri & Brown 2006), an issue that will not be touched upon here.