Here this from Alex Rudolph of the physics department at Harvey Mudd College,
"Jupiter is called a failed star because it is made of the same elements (hydrogen and helium) as is the Sun, but it is not massive enough to have the internal pressure and temperature necessary to cause hydrogen to fuse to helium, the energy source that powers the sun and most other stars.
"However, Jupiter has only about 0.1 percent the mass of the sun, and as it is definitely not a star, we can't really call the solar system a double star. It is interesting to note, however, that more than half of all stars in the sky are part of a binary, triple, or higher multiple star system (binaries being the most common). So the Sun is unusual in being a loner.
"As for why Jupiter failed to become a star--it probably had to do with the accident of the sun grabbing most of the mass early in the formation of the solar system, while in other systems the mass was more equitably distributed; in binary star systems, for example, the masses of the stars are commonly roughly equal. Stellar formation is a hot topic of current research, as astronomers are trying to fathom the still-mysterious details of the birth process.
