Planets distances from each other
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.
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Update: June 2012