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Why is Jupiter made up of gas? If the gravitational pull was strong enough would it compress to form a solid body?

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The pull of gravity is strong, but not strong enough to make a solid layer on the surface. When forming, a gas giant collapses until it reaches an equilibrium point where, at each radial "layer", the inward force due to the weight of the gas further out is compensated by the gas pressure. Until this equilibrium is achieved, the pressure, density and temperature are all changing as the system tries to "find" its equilibrium.

Once equilibrium is established it's just a matter of seeing whether the temperature and pressure are such that material will exist in a solid, liquid or gas phase. Near the surface of Jupiter (temperature $\approx 200\,{\rm K}$; $\log_{10}(T/{\rm K})\approx 2.3$), the pressure is much too low for hydrogen gas: Hydrogen phase diagram

There is no exact surface, but depths one can see to have pressures on the order of hundreds of ${\rm kPa}$ ($\log_{10}(P/{\rm Pa})\approx 5$; the pressure would need to be about $100\,000\times$ higher for solid hydrogen to form at this temperature. The core of Jupiter is thought to be hot and dense enough that it may be a metallic liquid or perhaps solid, but as far as I know this has not been conclusively measured.

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  • $\begingroup$ Jupiter quite probably has a core that isn't hydrogen at all, since it likely formed by core-accretion onto a rocky body. $\endgroup$ – ProfRob May 8 '18 at 10:04
  • $\begingroup$ @RobJeffries - Is this a new idea from the Juno mission? (I had not heard that hypothesis, thus why I guessed the new mission results.) $\endgroup$ – honeste_vivere May 9 '18 at 13:26
  • $\begingroup$ @honeste_vivere The core-accretion model is the favoured model for how gas giants can form. It is already known that Saturn, Neptune and Uranus have rocky/icy cores. $\endgroup$ – ProfRob May 9 '18 at 13:31
  • $\begingroup$ @RobJeffries - Ah, okay I do recall learning about the other outer planets being "ice giants" but I was not aware that Jupiter was thought to have a rocky core. I thought it was still considered to have a potential metallic hydrogen core. $\endgroup$ – honeste_vivere May 9 '18 at 17:06
  • $\begingroup$ @honeste_vivere If it forms by core-accretion it has to have a rocky/icy core. That is one of the things Juno was sent to find out. $\endgroup$ – ProfRob May 9 '18 at 17:15

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