Astrophysical calculations show that any star that is smaller than about 1/10th of the mass of the sun cannot sustain hydrogen fusion reactions at its core. These failed stars never light up. Instead they wander the galaxy as warm, dark balls of hydrogen known as brown dwarfs.
Brown dwarfs probably form through the same process that lead to ordinary stars but merely on a smaller scale. If that’s correct, planets should also form in the protoplanetary disks of gas and dust around brown dwarfs. Indeed, astronomers have seen a number of protoplanetary disks of this type.