Satellites:

Natural satellites are natural secondary objects that revolve in a closed orbit about a planet or star, referred to as the primary of the satellite. The moon is the Earth's satellite, just as the Earth itself is a satellite of the sun, although the moon and Earth are close enough in size to be considered sometimes as a double-planet system. The moon orbits the Earth once in about 27 days. Of the nine planets in the solar system, only two- Mercury and Venus- lack natural satellites. Several planets have multiple satellites; Saturn, with more than 20 satellites, has the most. The motion of most of the solar system's known satellites about their planets is direct- that is, from west to east- and in the same direction as the rotation of their planets. Only a few satellites of the large outer planets revolve n the retrograde direction- that is, from east to west- and opposite to the direction of rotation of their planets; they were probably captured by the planets' gravitational fields some time after the formation of the solar system. Many astronomers believe that Pluto, which moves in an independent orbit about the sun, is an escaped satellite of Neptune; in 1978 Pluto itself was discovered to have a satellite.