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A moon is an object in space that orbits around a planet. The most famous example is the Earth’s moon, of course, but there are moons going around all the planets in the Solar System except for Mercury and Venus. Even Pluto and many asteroids have moons.
As of September 2008, there are 335 objects in the Solar System classified as moons. 167 of them are orbiting the planets, 6 are orbiting dwarf planets, 104 are orbiting asteroids, and 58 are going around Trans-Neptunian objects.
The largest moon in the Solar System is Jupiter’s Ganymede, measuring 5,268 km across. This is actually bigger than Mercury. The second largest moon is Saturn’s Titan at 5,152 km across. The largest moon orbiting a dwarf planet is Pluto’s Charon, measuring 1,186 km across.
Astronomers believe that moons were formed out of the same protoplanetary disk that formed the planet. Excess material didn’t join the planet, but spun out into a moon orbiting the planet. But there are more exotic ways that moons might have formed around some planets. The Earth’s moon was probably formed billions of years ago when a Mars-sized protoplanet smashed into the Earth. The collision created a debris cloud that condensed back into the Moon. And Martian moons Phobos and Deimos resemble asteroids. It’s thought that maybe Mars captured them millions of years ago.
Most moons orbit in the same direction as the rotation of their planet. This is called a regular or prograde orbit. Most moons have a regular orbit, but there are some exceptions, like Neptune’s Triton, which orbits in a retrograde direction.
Some moons are tidally locked to their planet. This is a situation where the moon’s rotation time matches its orbital period. The Moon is a perfect example of this. It takes the same amount of time to complete one orbit around the Earth as it takes to complete one rotation on its axis. Because of this, the Moon always shows the same face to the Earth.
Humans have only set foot on one moon in the Solar System… the Moon. Additionally, NASA’s Cassini spacecraft sent the Huygens probe down to explore the atmosphere and surface of Saturn’s moon Titan.
If you’d like more information on moons, here’s a link to NASA’s World Book on Jupiter, which has the most moons in the Solar System. And here’s a link to the Saturn page, which has the second highest number of moons.
We’ve also recorded several episodes of Astronomy Cast just about planets’ moons. Try this, Episode 57: Jupiter’s Moons.