Brown dwarfs

Brown dwarfs are sub-stellar objects, too low in mass to sustain stable hydrogen fusion, but in which lower threshold nuclear reactions can still occur. Spanning the mass range 13–80MJ, they occupy the domain between planets and stars. Originally termed black dwarfs, the term ‘brown dwarf’ was introduced by Tarter (1976) and has been used thereafter.

This section summarises their characteristics, and places them in the context of exoplanet studies. The connection is particularly important in understanding the nature of massive orbiting planets with M ≳ 13MJ, and of the ‘free-floating planets’ found in young open clusters and star-forming regions.

The role of fusion

Some thirty years before their discovery, theories had identified a stellar/sub-stellar boundary at ∼75–80MJ, depending on chemical composition (Kumar, 1963). In the normal process of star formation, gravitational collapse releases energy which leads to increasing temperature and density, and to cores which become partially degenerate. Collapse halts when the sum of the normal gas pressure, and the free electron degeneracy pressure arising from the Pauli exclusion principle which acts as an energy sink, balances the gravitational potential.

At solar metallicity, objects above ∼78MJ reach the 3 × 107 K core temperatures necessary to initiate hydrogen fusion, and become stars. Below ∼74MJ, core temperatures never rise to the levels necessary for sustained hydrogen fusion, and these ‘failed stars’ are termed brown dwarfs.