Why aren't planets compressed by gravity like stars are?

06 September 2009

Question

As I understand it the nuclear fusion of stars stops gravity from compressing them until they cannot be compressed anymore because of the force of the neutrons repelling each other and when stars run out of fusible materials they are compressed to this point. What keeps the planets from being compressed by gravity as the stars are when they run out of fusible materials?

Answer

Dave - Well, yeah. Star, it's not actually the fusion which is holding the stars up directly. It's actually their temperature. If you had a gas, the hotter it is the more pressure it will exert, the harder it would push out. So stars are basically supported, they're basically made out of very, very hot gas - plasma that are supported by their temperature. So if a star gets hotter it will expand, star cools down it will shrink. A planet doesn't have to be supported like that, planets are made out of solid, lumps of things they're basically supported by the repulsion between atoms and molecules, in the same way as the table is supported or you're supported. So they are not big enough for the need the temperature to support them and basically just molecules and atoms are strong enough.Chris - Because planets like Jupiter are just around the threshold of what we call brown dwarfs, aren't they, they're failed stars are not quite big enough to squeeze themselves enough to trigger a fusion to actually get going.Dave - Yes, small stars it can also be supported just by this molecular strength basically.

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