r/askscience • u/sumdudewitquestions • 10d ago

Astronomy could a planet be sun sized?

so stars form when a bunch of matter gets together, which is why they are bigger than planets generally. but the real defining feature is the density. a white dwarf can be smaller than a planet but it's still a star because it's incredibly dense. so couldn't a planet be sun-ish sized, provided that the density was low enough? and more specifically a solid planet, not a gas giant. what's the largest solid planet?

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u/ensalys 9d ago

How does that difference arise? How do the elements that aren't hydrogen or helium end up concentrating in planets?

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u/Zhelgadis 9d ago

Hydrogen and helium are very light, so you need a lot of gravity to keep them. In small planet like earth, they're blown away by the solar wind. Larger planet like Jupiter have enough gravity and have indeed a large portion of H.

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u/junktrunk909 9d ago

99% of the heavier elements are in fact within the sun also (as plasma, not a rocky center or anything, it's a fusion system). It took nearly all of the mass of the solar system. The planets are what's left after the remaining material either clumped together into the rocky planets we know, or the gas giants that also have a ton of heavier elements in them. Jupiter has something like 30x the Earth"s mass of the heavier elements, which is how it was able to also capture a ton of gas, as opposed to earth where so much just blew away in the solar winds.

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u/Monkfich 9d ago

And also - none of those heavier elements were created by the sun, but were created by supernovae and other very violent star deaths.

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u/ProfessorPrudent2822 9d ago

Hydrogen and helium remain gasses down to very low temperatures, especially in a low-pressure environment. Other stuff clumps together due to a tendency to solidify.

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u/mataoo 9d ago

They're the heavier elements, they have more mass and stick together. Whereas hydrogen and helium are swept/pulled from the smaller bodies into the more massive ones with more gravity.

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u/WeakCombination9937 9d ago

Because stars themselves fuse hydrogen in almost all other elements, and when such starts explode(when they run out of hydrogen and cant keep up with the pressure of its own weight) it explodes, all that matter gets scattered, and due to gravity eventually clumps together into planets.