Asked by: Louise Dryden, Cardiff
Jupiter is often called a ‘failed star’ because, although it is mostly hydrogen like most normal stars, it is not massive enough to commence thermonuclear reactions in its core and thus become a ‘real star’. But the term ‘failed star’ is a bit of a misnomer. Theoretically, any object at all could be made into a star, simply by adding enough matter to it. With enough mass, the internal pressure and temperature of the object will reach the threshold needed to start thermonuclear reactions. That threshold is the least for the simplest element, hydrogen. In order to turn Jupiter into a star like the Sun, for example, you would have to add about 1,000 times the mass of Jupiter. But, to make a cooler ‘red dwarf’ you would only need to add about 80 Jupiter masses. Although the exact numbers are still a bit uncertain, it is possible that a ‘brown dwarf’ could still form (in which deuterium, rather than hydrogen, fuses in the star’s core) with only about 13 Jupiter masses. So, Jupiter cannot and will not spontaneously become a star, but if a minimum of 13 extra Jupiter-mass objects happen to collide with it, there is a chance it will.