What is a Moon?
Unlike the term "planet," there is no official definition of a moon. However, the common definition for a moon is almost universal. A moon is a celestial object within a solar system that orbits a body other than a star. The term "natural satellite" is synonymous with moon.
Origin of the Moons
Current theories suggest there are two types of moons based on origin. The first type are those moons that formed by accretion; that is, during the formation of the Solar System, material orbiting a young planet joined together to form a moon. An example of this type of moon is the Earth's moon. The second type of moon is an object that formed, not while orbiting a young planet, but elswhere in the Solar System and was later captured by a planet's gravitational force. An example of this type of moon is Mars' moon Phobos, which is believed to be a captured asteroid.
Number of Moons in the Solar System
At present, there are approximately 340 known moons in the Solar System, with 169 of them orbiting planets, 7 orbiting dwarf planets, 104 orbiting asteroids, and 58 orbiting trans-Neptunian objects. Listed below are the numbers of moons orbiting each planet:
- Mercury: 0
- Venus: 0
- Earth: 1
- Mars: 2
- Jupiter: 64
- Saturn: 62
- Uranus: 27
- Neptune: 13
It is quite possible that there are planetary moons that have yet to be discovered.
The sizes of the moons in our solar system vary greatly. The smallest known planetary moon in the Solar System is believed to be Mars' non-spherical moon Deimos, which has dimensions of 15 x 12.2 x 10.4 km. The largest moon in the Solar System is Jupiter's moon Ganymede, whose diameter is 3,280 km, making it larger than the planet Mercury.
It is interesting to note that Earth's moon is, proportionately speaking, the biggest natural satellite compared to the size of the planet it orbits.