Jupiter would have become a star if it had about 75 times as much matter, or mass, as it has. Mass is the principal factor that determines whether gas and dust can contract to form a star. If the mass is less than about 8% that of our Sun, the core can not heat up to the 10 million degrees Kelvin needed to initiate a nuclear reaction. But if the mass were more than 100 times that of the Sun’s, the sheer pressure of out-flowing radiation would tear the newly formed star apart.
Although Jupiter is the most massive planet in our system, with 318 times more mass than the Earth (see size comparison in the picture), it is still far too small to become even a faint red dwarf. Its core temperature is calculated to be only about 36,000 degrees Kelvin, the amount of heat remaining from the time of its formation. Even if Jupiter had acquired enough mass during the embryonic stages of the solar system to become a star, things would be much the same for us as they are today. The only difference: Jupiter would shine as brightly as the full moon, though concentrated to a smaller size, and would be easily visible in the daytime. Yet such a cool red star would have an insignificant effect on our climate. Furthermore, even a stellar Jupiter, 75 times as heavy as the planet, would not destroy the stability of Earth’s orbit around the Sun; the Sun would still be gravitational master of the solar system.