Solar System, the Sun and the celestial bodies orbiting the Sun, including the nine planets and their satellites; the asteroids and comets; and interplanetary dust and gas. The term can also refer to a group of celestial bodies orbiting another star, typically the phrase "solar system" refers to the system that includes Earth and the Sun. The dimensions of the solar system are measured in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, orbits about 39 AU from the Sun. The boundary between the solar system and interstellar space(called the heliopause)is estimated to occur near 100 AU. Comets, however, achieve the greatest distance from the Sun; they have highly eccentric (oval) orbits ranging out to 50,000 AU or more.