Introduction

This chapter is devoted to planet formation and to the early stages of evolution of low-mass objects, including low-mass stars, brown dwarfs and exoplanets. We first summarize the general properties of current exoplanet observations (Section 15.2) and describe the two main planet formation models based on disk instability and on the core-accretion scenario, respectively (Section 15.3). Recent progress of the latter formation model allows sophisticated population synthesis analyses which provide fully quantitative predictions that can be compared to the observed statistical properties of exoplanets (Section 15.3.5). The last part of this chapter is devoted to the distinction between brown dwarfs and planets, in terms of structure and evolutionary properties. The existence of a mass overlap between these two distinct populations of low-mass objects is highlighted by the increasing discoveries of very massive exoplanets (M ≳ 5MJ) and by the identification of planetary mass brown dwarfs in young clusters (M ≲ 10MJ) These discoveries stress the importance to define signatures which could allow to disentangle a brown dwarf from a planet. We first analyse the effect of accretion on the evolution of young brown dwarfs and the resulting uncertainties of evolutionary models at ages of a few million years. We also analyse different specific signatures of brown dwarfs and planets such as their luminosity at young ages, their radii and their atmospheric properties.

Important progress has been made within the past few years regarding the theory of low mass stars (m < 1M⊙) and brown dwarfs. The main improvements concern the equation of state of dense plasmas and the modelling of cool and dense atmospheres, necessary for a correct description of such objects. These theoretical efforts now yield a better understanding of these objects and good agreement with observations regarding color-magnitude diagrams of globular clusters, mass-magnitude relationships and near-IR color-magnitude diagrams for young open clusters. However uncertainties still remain regarding synthetic optical colors and the complex problem of dust formation in the coolest atmosphere models.

Improvement of the theory

Very low mass (VLM) stars and brown dwarfs (BD) are dense and cool objects, with typical central densities of the order of 100−1000 gr.cm−3 and central temperatures lower than 107 K. Under such conditions, a correct equation of state (EOS) for the description of their inner structure must take into account strong correlations between particles, resulting in important departures from a perfect gas EOS (cf. Chabrier & Baraffe (1997)). Important progress has been made in this field, in particular by Saumon, Chabrier & Van Horn (1995) who developed an EOS specially devoted to VLM stars, BD and giant planets. Since the EOS determines essentially the mechanical structure of these objects, and thus the mass-radius relationship, it can be tested against observations of eclipsing binary systems.