The gravitational collapse of a nebula (containing gas and dust) leads to the birth of a star. Newborn stars are often found with circumstellar disks in which planets may form.

We find that about half the stars are members of binary or multiple star systems. In cases where the stars are close, there is probably not enough space for planets to form. However, wide binaries may accomodate planets (e.g., 16 Cyg B).

Stellar companions may be as small as 0.08 solar masses. Below this limit the central temperatures are insufficient to start nuclear burning. Collapsing clouds probably do create such small objects. These slowly cooling objects are called BROWN DWARFS.

Only a few brown dwarfs have been found in the sensitive radial velocity surveys (so they are rare compared to planets). One of the first discovered is a companion to the nearby star Gliese 229.

Planets, on the other hand, form by the accumulation of disk material. The dividing line in mass between a planet and a brown dwarf is about 10 Jupiter masses (= 0.01 solar masses).