I’ve been asked this question a lot as our exoplanet faculty search ran its course this winter and spring. God forbid a potential exoplanet find may actually turn out to be a brown dwarf! But why the distinction? There is a case to be made that any object that cannot initiate internal hydrogen fusion is a planet, period. BDs would then just be planets on the more massive end of the spectrum. On the other hand, some prefer to think that BDs should be thought of as more like stars since they have a similar formation mechanism. But do they? Increasingly the evidence is that BDs can form both in a star-like manner as well as through physical processes in a circumstellar disk, where planets are also formed. Should BDs be preserved as a separate third category of object? Perhaps, since they seem able to form both through disk processes and direct collapse of interstellar cloud fragments. Planets may be defined as objects that form only through an initial accumulation of solids in circumstellar disks. Stars form in all the same ways as BDs but can be distinguished from them by having a high enough internal temperature to initiate hydrogen fusion.
I was asked to write an article in Science magazine about some of these matters, and it just appeared in the July 6 issue: