How are moons formed / where do moons come from?

Moons, or more accurately 'natural satellites,' are natural objects that can be said to be in orbit around another body (called the 'primary')*. This is a rather woolly definition which leads to some ambiguities: when two bodies are so similar in size that they orbit around a point in free space (as opposed to a point beneath the surface of one of the bodies) they are usually referred to as a double body. Pluto and its moon, Charon, are an example of this, although they are not generally referred to as a double body (testament to the woolliness of the definitions!) Small bodies that fit the (rather loose) definition of a moon are often referred to as 'moonlets,' although again there is no hard and fast definition of this.


Types of moon
Moons are split into two categories:

Regular satellites
Regular satellites are natural satellites which:

• Orbit relatively close to their primary (parent) body;
• Have a prograde orbit (i.e. they orbit in the same direction as their parent body spins);
• Orbit in the same plane (or near enough) as their primary;
• Have a near-circular orbit.
  

Irregular satellites
Irregular satellites are natural satellites which exhibit one or more of the following

• Orbit distantly from their parent body:
• Have a retrograde orbit (i.e. they orbit in the opposite direction to their primary's spin);
• Orbit in a plane which is significantly inclined compared to that of their parent body's;
• Have a highly eccentric (noticeably elliptical) orbit
  

So where do they come from?
Different types of moon, perhaps unsurprisingly, are thought to come from different origins. I'll discuss some of the theories here.

Accretion
This is the theory that some moons were created in much the same way as the planets themselves. Whereas the planets formed from clumps of matter joining together from the disc of gas and dust that orbited the sun, this theory states that some moons formed from clumps of matter that formed in discs of gas and dust that orbited the parent planet. As certain clumps of matter became larger, they 'hoovered' up the rest of the disc. Most of the regular satellites would have been formed this way.

Capture
In the early solar system many bodies formed all over the place. Some were small; some grew large and became planets; some collided and broke into smaller pieces. Whilst the larger lumps (planets, and some other bits) settled into wide, almost circular orbits around the sun, all sorts of debris was hurtling around the solar system. Many of these bits and pieces collided with the larger bodies and became a part of them. Others missed the planets, but passed close enough to one to be 'captured' by its gravity. Many of these orbits are wide and eccentric, and on an entirely different plane to that of the planets, and hence is how most of the irregular moons came to be.

Other theories
There are a number of other theories that have been put forward for moon formation, but the two mentioned above are the most common for explaining the majority of the moons in our solar system. These other theories include:

• Fission: Matter is thrown off a body that is spinning very fast. This matter then enters orbit around the parent planet
• Impact: A big enough body hits the parent hard enough for matter to be ejected into space. This matter enters orbit around the parent planet and cools and condenses, forming a moon.

Why bother talking about moons? They're just boring lumps of rock.
Not so! There are 170 known moons (at the time of writing) in our solar system, and many of them are sources of great interest to anybody who's interested in space, and continually challenge the way scientists and astronomers think. For example:

• Ganymede: The largest known moon, and 9th largest body in the solar system (it's bigger than the planet Mercury!)
• Europa: Has a crust made largely of ice, and there is speculation that there may be a liquid ocean underneath it. It is possible that Europa is the largest store of liquid water that we know of (yes, even above the Earth), and is arguably the best bet for finding life on another body in our solar system.
• Io: The most volcanically active body in the solar system (including the planets!)
• Titan: Has an atmosphere thicker than Earth's.
• Triton: The only large moon in the solar system to have a retrograde orbit around its parent.
• Our moon (Luna): Is the largest moon in the solar system relative to its parent planet (by a long way).
• And many more...
  

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* The term 'natural satellites' can apply to anything that fits this description. For example, it could apply to a planet when you're studying a star, or it could equally be used to refer to a dwarf galaxy that is in orbit around a larger galaxy.