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Ask a Scientist: Difference Between Planets and Comets - Transcript

Hi, my name is Kamrin and my question is how are comets and planets different from each other?

Steve Chesley, EPOXI, NASA/JPL:
There's two big differences between comets and planets, how they orbit the sun and how big they are. The planets are much larger. The smallest planet, Mercury, is about 3,000 miles in diameter while a big comet nucleus like Halley's comet is only about 7 miles in diameter. And then they orbit in a different way. The planets go more or less in circles staying about the same distance from the sun all the time while the comets spend most of their lives at a great distance from the sun and occasionally they swoop in close to the sun where the gases start to cook off and form these fantastic tails that comets are so famous for.

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