Alien planet and asteroid. Computer artwork of an asteroid (upper left), an alien (extrasolar) planet (upper right) and the star the planet is orbiting (background). The planet is a massive gas giant, also known as a brown dwarf. They are too small to have the mass required to start the nuclear reactions associated with stars, but are far larger than most planets. Numerous large gas giant planets have been discovered around other stars. Most of them are larger than Jupiter and orbit much closer to the star than any of the planets in our Solar System. They are known as hot Jupiters.