Astronomers have discovered planetary systems with multiple planets. The orbits of Solar System planets are nearly circular. Among the other dwarf planets, has no known moons. Habitability for hypothetical planets orbiting brown dwarfs has been studied. The outer Solar System is beyond the asteroids, including the four giant planets. Aries contains several stars with extrasolar planets. These were the first extrasolar planets discovered. The inner Solar System includes the four terrestrial planets and the asteroid belt. The concept is applicable to other planets and celestial bodies. Two satellites of the outer planets possess significant atmospheres. The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. This is useful for calculating the behaviour of planets and comets and such. It is the only spacecraft to have visited either of these two ice giant planets. There is no official definition of extrasolar planets. This was the first evidence that anything other than the planets orbited the Sun.