Definition of Exoplanets and Brown Dwarfs

This chapter reviews the definition of exoplanets and of brown dwarfs. Emphasis is given to the separation of these two populations. A traditional view is to declare {\guillemotleft} planet {\guillemotright} objects with a mass < 13 M Jup and {\guillemotleft} brown dwarf {\guillemotright} objects with a mass > 13 M Jup . By analogy with Solar System planets, a better definition is to call {\guillemotleft} planets {\guillemotright} objects formed by accretion of dust and planetesimals in a disk. An by extension of the primitive introduction of the word {\guillemotleft} brown dwarf {\guillemotright} for failed stars by Jill Tarter, this term must be reserved to objects formed by gravitational collapse in a molecular gas cloud. The two definitions do not coincide since a {\guillemotleft} brown dwarf {\guillemotright} can have a mass down to about 6 Jupiter mass. And there is no physical reason to assert that a 20 Jupiter mass object has not been formed by accretion. From there, the difficulty is to decide if an object of say 20 Jupiter mass is formed by dust accretion or by gravitational collapse. A future observational test to solve this difficulty is presented.