Some planets have moons, others don't. It looks like that, in general, large planets have more and larger moons.

It may be assumed that Jupiter, Saturn and Uranus are typical cases concerning their moons. They all have satellite systems which look like miniature planetary systems. And that is, basically, what they are. The moons formed in their orbits around their planets just like the planets orbiting the sun. (Note that their orbits, except for the tiny outer moonlets of Jupiter and Saturn's Phoebe, which are most likely captured asteroids or cometary cores, lie within the planet's equatorial plane, even in case of grossly-tilted Uranus.) Ring systems complete with `shepherd' moonlets might also be standard case for gas giants (all jovian planets in our solar system have them!).

Neptune is different. The system looks as if it has been through a great deal. The ring system with its accompanying moonlets is intact, but the rest looks as if something had wreaked havoc upon it some time ago. One moon, Triton, orbits on a retrograde orbit, the other, Nereid, on a highly eccentric one. This system seems to have been disturbed by something. The most popular theory is that something bad happened which not only messed up Triton's and Nereid's orbits, but also threw out another large moon entirely -- the one we now know as Pluto. However, this assumption is now considered obsolete. The state-of-the-art theory, however, claims that Triton originally was an independent body, a giant comet just like Pluto, which was captured by Neptune -- and thereby messed up Nereid's orbit. If this was true, it would mean that there might be quite many more Pluto-sized objects around at the outer fringe of the solar system.

Now consider the terrestrial planets. Mercury and Venus have no moons (it is now pretty certain that Neith, that `ghost moon' of Venus which used to fool astronomers for over 200 years, is not really there), Earth has one, Mars has two. Mars's two moons are tiny, while Earth's is a really large one -- it would make a dignous companion of a large jovian planet, or a small but respectable planet (definitely much more than an asteroid!) in itself. It is therefore sometimes said that Earth and Moon are a binary planet.

It is almost certain that Earth's moon is a rare, exceptional case. Terrestrial planets normally just don't have moons that big. The currently best accepted theory is that Earth once collided with another big terrestrial body. Much material was thrown out of Earth's body during that event, and from this formed the Moon. Note that such a collision is a fairly unlikely event, hence Earth/Moon might be a real freak of nature. The normal case seems to be that a terrestrial planet has a few tiny moons, or none at all.

Regarding Mars's moons, it is not certain where they have come from. They might have formed from debris in orbit around Mars which was left over from the formation of the planet, or (considered more likely) they might be captured asteroids.

Finally, let's have a look at the Pluto/Charon system. Pluto is accompanied by a moon which has half of Pluto's diameter and perhaps 1/10 or 1/8 of its mass. This means that Pluto and Charon are much more clearly a binary planet (or, for that matter, a huge binary comet) than Earth and Moon (imagine Earth being orbited by Mars instead of the Moon!), especially if one considers two properties of this system that Earth and Moon do not share:

Pluto and Charon are tidally locked to each other. Most moons always turnthe same side towards their planet. Our moon does, for example, does this.This is also the case with Charon, but also with Pluto: its rotationperiod is equal to the revolution period of Charon. Thus, on Pluto themoon never rises, nor sets. It just stays put in the sky as seen from onehemisphere of Pluto; from the other, it can never be seen.

The mass centre lies in open space between the two bodies, while in thecase of Earth and Moon it lies, though noticeably displaced from Earth'scentre, well within Earth.

So the rule seems to be that terrestrial planets usually have few tiny moonlets or no moons at all and jovian planets have miniature planetary systems and rings. But there are always ones which break the rule.