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Moons are astronomical objects also known as natural satellites that are in orbit around larger planetary mass objects (planmos).  

Nineteen natural satellites are large enough to be round, and one, Saturn's moon, Titan, has a substantial atmosphere. image: wikipedia

Moons are a celestial bodies that orbit other celestial bodies, e.g. a planet, which is called its primary. There are 173 known natural satellites orbiting planets in the Solar System, as well as at least eight orbiting IAU-listed dwarf planets.

 As of January 2012, over 200 minor-planet moons have been discovered.There are 76 known objects in the asteroid belt with satellites (five with two satellites each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers.

There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within rings of Saturn, but only a few were tracked long enough to establish orbits. 

Planets around other stars

These are likely to have satellites as well, though numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, 

Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. 

The outer gas giants

These have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: 

the four Galilean moons, Saturn's Titan, and Neptune's Triton.

Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life, though there is currently no direct evidence of life.

Uniqueness of Earth-Moon system

The EarthMoon system is unique in that the ratio of the mass of the Moon to the mass of the Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System.

Scale of Earth moon system. image: wikipedia

 There are minor-planet systems with even greater ratios, notably the Pluto–Charon system).

Dwarf planet moons

Among the dwarf planets, Ceres and Makemake have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites. 

Haumea has two natural satellites, and Eris has one. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

Adapted from: Natural satellite. (2014, September 2). In Wikipedia, The Free Encyclopedia. Retrieved 08:12, September 23, 2014, from