Like the sun, Jupiter is composed predominantly of hydrogen & helium. But unlike the sun, it lacks the necessary amount to begin fusion, the process that fuels a star. Jupiter would need to be 75-80 times more massive than it is at present to be considered a star. If all of the planets in the solar system had formed as part of the gas giant, it still would not have sufficient mass. Still, by itself, Jupiter is 2 & 1/2 times larger than all of the other planets in the solar system combined.