23May2015

Why Does Earth Only Have One Moon?

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.

That is, 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. Source:  TestTube

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