Gaseous Planets, Protostars, and Young Brown Dwarfs: Birth and Fate

We review recent theoretical progress aimed at understanding the formation and the early stages of evolution of giant planets, low-mass stars, and brown dwarfs. Calculations coupling giant planet formation, within a modern version of the core accretion model that includes planet migration and disk evolution, and subsequent evolution yield consistent determinations of the planet structure and evolution. Uncertainties in the initial conditions, however, translate into large uncertainties in the luminosity at early stages. It is thus not possible to say whether young planets are faint or bright compared with low-mass young brown dwarfs. We review the effects of irradiation and evaporation on the evolution of short-period planets and argue that substantial mass loss may have occurred for these objects. Concerning star formation, geometrical effects in protostar core collapse are examined by comparing one-dimensional and three-dimensional calculations. Spherical collapse is shown to significantly overestimate the core inner density and temperature and thus to yield incorrect initial conditions for pre-main-sequence or young brown dwarf evolution. Accretion is also shown to occur nonspherically over a very limited fraction of the protostar surface. Accretion affects the evolution of young brown dwarfs and yields more compact structures for a given mass and age, thus fainter luminosities, confirming previous studies for pre-main-sequence stars. This can lead to severe misinterpretations of the mass and/or age of young accreting objects from their location in the Hertzsprung-Russell (HR) diagram. Since accretion covers only a limited fraction of the protostar surface, we argue that newborn stars and brown dwarfs should appear rapidly over an extended area in the HR diagram, depending on their accretion history, rather than on a well-defined birth line. Finally, we suggest that the distinction between planets and brown dwarfs be based on an observational diagnostic, reflecting the different formation mechanisms between these two distinct populations, rather than on an arbitrary, confusing definition.