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Why are the planets different sizes?
mt mrtha, victoria, australia
The short answer is 'We don't know'. It is still being debated in the astronomy community and all the answers are not yet in. Only in the past year has Pluto been down-graded from "Planet" to "Dwarf Planet" amidst much controversy. Most scientists agree that the initial solar system consisted of a mixture of gas, mainly hydrogen and helium, and with a sprinkling of metalic dust. Most of this material collapsed to form the sun but a small fraction was left over and formed a disk-like spinning cloud. The material in the disk then started to coagulate into various clumps due to gravitational attraction. These clumps interacted with each other, sometimes sticking together, sometimes smashing each other apart. The end result is that we ended up with eight planets (nine if you include Pluto) of various sizes. There is also a lot of junk left over in the form of asteroids and comets which add to the rich complexity of our current solar system.
In sum, we do not have an exact theory of the distribution of sizes of the planets. There is a web site: http://en.wikipedia.org/wiki/Solar_nebula that has a lot more information.
(published on 10/22/2007)
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