<P> Moons are classed in two separate categories according to their orbits: regular moons, which have prograde orbits (they orbit in the direction of their planets' rotation) and lie close to the plane of their equators, and irregular moons, whose orbits can be pro - or retrograde (against the direction of their planets' rotation) and often lie at extreme angles to their planets' equators . Irregular moons are probably minor planets that have been captured from surrounding space . Most irregular moons are less than 10 kilometres (6.2 mi) in diameter . </P> <P> The earliest published discovery of a moon other than the Earth's was by Galileo Galilei, who discovered the four Galilean moons orbiting Jupiter in 1610 . Over the following three centuries only a few more moons were discovered . Missions to other planets in the 1970s, most notably the Voyager 1 and 2 missions, saw a surge in the number of moons detected, and observations since the year 2000, using mostly large, ground - based optical telescopes, have discovered many more, all of which are irregular . </P> <P> Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of 1.6 km (1.0 mi). For a very short time in 1974, Mercury was thought to have a moon . </P> <P> Venus also has no moons, though reports of a moon around Venus have circulated since the 17th century . </P>

Which planet is 5.5 of earth's mass and has no moon