There's actually still scientific dispute about the exact answer to this question. The solar system formed from a giant cloud of gas and particles (literally stardust--the remains of a star that had exploded) billions of years ago. The "dust" clumped together to form the planets. Some of the leftover dust formed the moons of the planets (although the origin of Earth's moon is still up for debate). Some of this dust was too close to the planet to form stable moons, and instead formed rings. It is likely that large rings couldn't form on inner planets (Mercury, Venus, Earth, Mars) because the rings are made of frozen icy dust, and the sun is too hot this close to it for the rings to form. Additional fact: Mars has two moons, one of which is very close to the planet. Some astrophysicists predict that it will break apart and become a ring over the next 50 million years.

Good question. The answer seems to be that there are small moons orbiting ringed planets at or near the distance from the planets as the rings themselves, and that these moons are needed to keep the rings from flying apart. So, basically, having enough moons the right distance from the planet makes rings possible.

I don't think we know why some planets have lots of moons while others have only a few, although I do notice that the gas giant planets in our solar system (Jupiter, Saturn, Uranus, and Neptune) all have many moons, while the smaller rocky planets (Mercury, Venus, Earth, Mars) have only one or two, if any. I guess it has to do with mass. However, Pluto and Eris, the two largest of the ice planets, have moons, despite both being smaller than Mercury, which has none.

That’s a really good question! I had to do some research on this question. I wasn’t able to find a conclusive answer, but I have a hypothesis (an “educated guess”). There are a few important observations:

1) The “inner planets” (Mercury, Venus, Earth, Mars) don’t have rings and all of the “outer planets” (Jupiter, Saturn, Uranus, and Neptune) do.

2) The outer planets are much more massive than the inner planets. Uranus is the smallest of the outer planets, yet it is nearly 15 times more massive than Earth, and Jupiter is over 300 times more massive than Earth (NASA; click here).

Objects with a lot of mass have big “gravity” wells, meaning that they exert a bigger “pull” on other objects. Because the outer planets are more massive, they have bigger “gravity wells” than the smaller inner planets. Space debris, such as small bits of rock and ice, get caught in the big gravity wells of the outer planets. When they are in the gravity well of an outer planet, like Saturn, they start to orbit the planet. The debris have enough momentum that they don’t get pulled all the way into the planet, instead they orbit around it. Perhaps the smaller planets don’t have large enough gravity wells for big ring of debris to form. The sun makes up over 99% of the mass of our solar system and has a huge gravity well. This is why all of the planets orbit around it.