# Size of our planets

Is there any answer to "what determines the size of each planet"?

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As far as I know there is no theory that determines the size of a planet.  Any such theory would probably have been shredded shortly after planets were discovered outside our solar system.

The Roche Limit gives us a basic guideline as to the size of a planet, its tempure, and density,and how closely it may orbit its parent without being torn apart.  Unfortunately is is over a page long and has so many permutations and derivitives that I hesitate to post it here.

Basically, the denser the material a planet is made from, its mass and orbital energy, the longer is has been accreting, and the distance from its star, dictate the size of the planet.  That makes calculating the mass and temperature of exoplanets rather difficult, but we can make an approximate educated guesstimate of how we are always completely wrong.

I can still remember a paper written by Fred Hoyle saying that Jupiter's moon Io would be one of the coldest bodies in the solar system, when it turns out to be one of the hottest.  Hoyle also said that Titan would be at least as hot as earth because it had an atmosphere.  As it turns out, Titan is one of the coldest things in the solar system.

Science has to prove to itself constantly that we still don't know very much, and most of what we think we know turns out to be wrong.

Even so, the more we know, the better it gets, even if we know virtually nothing.

http://en.wikipedia.org/wiki/Roche_limit

Think it through, work the problem, never guess, sharpen your intuition.

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