Reply #3: If it is above 17 Jupiter masses it is a Brown Dwarf star, below 17 and it's a Gas Giant planet. [View All]

[Odin2005]

IIRC 17 Jupiter masses is the minimum amount of mass needed for an object to start fusing deuterium ("heavy hydrogen"), but deuterium is not very abundant so the fusion only lasts for a short time and "true" proton-proton fusion never starts. Objects that are massive enough for deuterium fusion but not proton-proton fusion are called Brown Dwarfs.

Brown Dwarfs and Gas Giants are also thought to tend to have different origins. It is thought that most brown dwarfs are formed like other starts, but just happen to not get enough mass for proton-proton fusion. Gas Giants start out like most planets do, from the disk of debris surround a young star, in the outer solar system you have lots of ice that allow proto-planets of 10 earth-masses to form, when that point is reached these giant ice-balls start sucking up the surrounding gas and become Gas Giants