From Brown Dwarfs to Planets

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Abstract
The burst of discoveries of planets and brown dwarfs during the past year has revealed an unexpected diversity of planetary systems. 51 Pegasi, the first extrasolar planet detected in orbit around a solar-type star appears to be the prototype of a new family of Jovian planets, the «Hot Jupiters». Though the physical mechanisms underlying the orbital decay of protoplanets are already understood, we are far from having a general view of planetary formation and of the diversity of planetary systems. If the orbital decay of protoplanets is sometimes so efficient, what is the frequency of planetary systems like ours, what is the frequency of telluric planets, what is the limit between planets and brown dwarfs?