Why Does Earth Only Have One Moon?

Sorry for the advertisement Trace Dominguez
by Jack J. Shamama

Pluto is 1/6th the size of Earth, but it has 5 moons. Jupiter has 67. Why does the Earth have just one moon?

One way of categorizing the planets of our solar system is to divide them between terrestrial and jovian. The terrestrial planets, Mars, Earth, Venus, and Mercury only have three moons between them (Mars has two, Phobos and Deimos, and the Earth has one). The Jovian planets, by contrast, are teeming with moons: At latest count, Jupiter has 67, Saturn has over 60, Uranus has 27, and Neptune has 14. As NASA's New Horizons probe approaches Pluto, it's counting five moons in orbit around the dwarf planet. 

Current astronomical theory suggests that a planet can acquire a moon by one of three possible ways:

1. They collide with another body that causes a chunk to begin to orbit.
2. They capture it using their gravity to snap a passing celestial body.
3. They form alongside a planet from an accretion disk.

It's believed that Earth got our moon through the first theory. Many scientists believe that a Mars-sized body known as Theia collided with Earth around 4.5 billion years ago. The second theory explains how Mars got its moons, since Mars sits close to the asteroid belt and it could have possibly grabbed Phobos and Deimos from there. Theory two may also explain Pluto's five tiny moons. Pluto's smallest satellites, Nix and Hydra, are little icy bodies floating around the Kuiper belt which probably got too close and trapped by Pluto's gravity. The accretion disks in the third theory is probably how Jupiter and Saturn got so many moons of their own.

Of the hundreds of moons orbiting planets in our solar system, some may have geological features so similar to Earth, many think they could be good candidates for life. Jupiter's moons Ganymede may have a salty ocean and Europa may have more water on it than Earth! What moon do you hope we visit next? Let us know in the comments.

Learn More:

How Many Moons Does Mercury Have? (Universe Today)
"Since the Earth has a moon, and Saturn has more than 60, here's a good question: does Mercury have moons? Unfortunately, the answer is no, Mercury has no moons."

How Many Moons Does Each Planet Have? (eNotes.com)
"All planets except Mercury and Venus have at least one moon. Saturn has the greatest number of moons-18. In 1995, the Hubble Space Telescope detected what appeared to be four additional moons of Saturn; however, that discovery has yet to be confirmed."

Why does the Earth have only one moon? (Cornell University)
"There are two kinds of planets in the solar system: the Terrestrial planets (Mercury, Venus, Earth, Mars), and the Jovian planets (Jupiter, Saturn, Uranus, Neptune). While the Jovian planets have a total observed number of moons of close to 90, the Terrestrial planets have only 3 (the Moon, and two small moons around Mars). This huge difference is linked to the formation of the solar system."