Asked by: Adam Dooley, Manchester
Stars burn as a result of thermonuclear reactions deep in their cores. An object with a mass less than about eight percent of the Sun’s cannot ignite significant nuclear reactions in its core. These objects are ‘failed’ stars called brown dwarfs. In their youth, brown dwarfs do generate some energy from fusing deuterium into helium nuclei, but older brown dwarfs only radiate a small amount of heat due to their slow contraction.
The dividing line between these brown dwarfs and gas planets occurs at about 1 per cent of the mass of the Sun. Objects less massive than that can never achieve the core temperatures required for thermonuclear reactions. This corresponds to about 13 times the mass of Jupiter, meaning that Jupiter itself is incapable of ever ‘igniting’. Jupiter lies pretty close to the limit of what we’d call a gas giant. But by definition, if a gas giant is massive enough to ‘ignite’ deuterium fusion it is not a gas giant at all, but a brown dwarf.