|MadSci Network: Astronomy|
Good question - and one it's difficult to give a direct answer to. The problem is that the only solar system we can currently study in detail is our own, so it's difficult to know whether it's just coincidence that all of the large planets currently have rings (Neptune, for example, has rings which are extremely broken up, and it's difficult to say whether this is common, or just something that happens to be there now).The first thing to say is that Saturn's beautiful rings (you can find pictures on sites like this one or this one are probably only a temporary feature, lasting for only a few million years. They're believed to result from the breakup of a moon under the powerful gravitational field of the giant planet. Which brings us to the best answer we currently have to your question - the smaller planets don't have rings because they are smaller, and so their gravity is weaker. They don't attract as much material into the neighbourhood, and once there it is easily disrupted by the motion of nearby bodies, the Sun or even the giant planets themselves. In other words, we're unlikely to have rings because we don't have the gravity to keep them there. Hope that helps! Chris
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