Eli5 Are the outer planets really only made up of gas?

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I just saw a post showing that Jupiter’s red eye has been shrinking over time, which lead me to thinking about what the core of the gas giants must be like. I still can’t wrap my head around the idea that the outer planets (except Pluto), are just made up of gas. Wouldn’t at some point within the planet the pressure would just be so high that a solid/liquid core would form? Or does the immense pressure mean that the core is way to hot for something to form?

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Anonymous 0 Comments

We really don’t know. There’s a lot of theories but nothing confirmed.

The commonly accepted theory these days is a rocky metallic core (mostly iron/nickel), like the inner planets. But right above it is basically an ocean of liquid metallic hydrogen. Heat and pressure are essentially opposing forces when it comes to what state matter will take. Heat pushes the element toward gas, pressure pushes it toward solid. In the center of planets like Jupiter the pressure wins, hence the liquid hydrogen ocean. The only reason we don’t think there’s solid hydrogen there is that there isn’t enough pressure for hydrogen to solidify at the temperatures in Jupiter’s core.

Note that gas giants can get big. There’s been some that we’ve seen that are borderline stars. If they got just a little bit more matter they would have enough gravity, and therefore pressure, to kick off fusion and turn the core into plasma from the heat.

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