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Why are planets so far from each other?

Well, the planets are pretty far apart compared to their sizes - the size of a planet is generally a few thousand miles (maybe 10s of thousands for the big ones) while the distances between them are tens or hundreds of millions of miles. Why that huge factor of a thousand or so? It probably has a lot to do with the average density of matter in the original dust-cloud that (we think) formed the solar system and the sun itself. There really is not much matter out there in space (it is "empty"!) and even the big nebulas astronomers like to photograph are still very much less dense than the planets. If there are nebulas that are much denser than the one our solar system started, they might have planets closer together... On the other hand, there could be something else going on. The gravitational interactions between the planets even as far apart as they are still pretty complex, and the solar system is really not all that stable - maybe if we had extra planets closer together long ago, they would have been ejected from the solar system by now.

Arthur Smith

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