The theory of brown dwarfs and extrasolar giant planets

Abstract

Straddling the traditional realms of the planets and the stars, objects below the edge of the main sequence have such unique properties, and are being discovered in such quantities, that one can rightly claim that a new field at the interface of planetary science and astronomy is being born. This article extends the previous review of Burrows and Liebert (1993) and describes the essential elements of the theory of brown dwarfs and giant planets. It discusses their evolution, atmospheric composition, and spectra, including the new spectroscopic classes L and T. Particular topics which are important for an understanding of the spectral properties include the effects of condensates, clouds, molecular abundances, and atomic opacities. Moreover, it discusses the distinctive features of these extrasolar giant planets that are irradiated by a central primary, in particular, their reflection spectra, albedos, and transits. Overall, the theory explains the basic systematics of substellar-mass objects over three orders of magnitude in mass and age, and a factor of 30 in temperature. CONTENTS

An extreme planetary system around HD 219828

“…The distinction between the two types of objects has been argued to be related to the deuteriumburning mass limit near 13 M Jup (Burrows et al 2001). The lack of a discontinuity in the mass distribution of companions to solar-type stars at the brown-dwarf regime (Udry & Santos 2007;Sahlmann et al 2011) suggests, however, that a distinction based on the formation mechanisms may be more representative (Chabrier et al 2014).…”

Background

“…Thus, brown dwarfs are generally a little smaller in size than planets like Jupiter, and the most massive ones are the smallest (and thereby most dense). Since objects that degenerate are no longer generating energy, they are on an endless slide to cooler, fainter states (Chabrier and Baraffe 2000;Burrows et al 2001). It is worth noting that stars like the Sun also end in a similar phase after fusion ceases; they are then known as "white dwarfs" (which are much more massive and so can be as small as the Earth and initially much hotter -hence "white").…”

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

“…Kumar referred to the objects below this limit as 'black' dwarfs, but the name 'brown dwarf' proposed by Tarter (1975) is the one that has been adopted. More recent models set the hydrogen burning mass limit at 0.072-0.075 M for a solar composition and somewhat higher for lower metallicities (Chabrier & Baraffe 1997;Burrows et al 2001).…”