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Sudarsky's gas giant classification predicts the visual appearance of gas giant planets based on their temperatures.

But what determines their temperatures in the first place? Is it just the irradiation from their star, or is there more to it than that? If there's more to it, what else is involved?

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The other thing(s) involved are:

  1. Contraction and the release of gravitational potential energy. Gas giants are born bigger than they are now. They gradually contract towards a completely degenerate configuration, and as they do so half the released potential energy is radiated away, whilst the other half heats the planet.

  2. Gravitational diffusion and separation also releases gravitational potential energy. Heavier elements tend to settle towards the centre, falling through the majority hydrogen.

  3. Radioactive decay of heavy elements that may form part of the giant planet's core.

  4. If the "planet" is more than about 13 times the mass of Jupiter, then it will likely attain a central temperature sufficient to fuse its deuterium. Some would refer to such objects as brown dwarfs at that point.

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    $\begingroup$ There's also the possibility of deuterium fusion in the most massive giant planets, see e.g. Mollière & Mordasini (2012). This does bring up the issue of whether to call such objects planets or brown dwarfs, but I prefer to regard Upsilon Andromedae as a 3-planet system! $\endgroup$ – mistertribs Dec 26 '18 at 23:28

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