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So, in my research and preparation for the question I asked immediately before this one, I was introduced to the concept of "fluffy" gas giants, gas giants with significantly lower density than would otherwise be expected, given their mass. In a comment on my previous question, it was suggested that these are generally due to the gas giant orbiting so close to their star that the heat from the star effectively increases the volume of the Gas Giant atmosphere, which increases the radius, and decreases the overall density.

But, are there any other plausible explanations of why a Gas Giant might have a lower density, higher radius, etc., compared to a "standard" Gas Giant ?

As a side note, I also came across the concept of "Failed stars", that start burning hydrogen, but run out of fuel in relatively short times (less than half a billion, and in some cases, only a few hundred thousand, years), and become brown dwarfs. Could the excess internal heat from the short stage of hydrogen burning 'inflate' the brow dwarf's 'atmosphere' like that, and maintain it over stellar/geological/evolutionary time (2+billion years)?

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    $\begingroup$ "Failed stars" are considerably more heavy (like 10+ times heavier) than Jupiter, and there is no easy way lo lose that mass. $\endgroup$
    – Alexander
    Commented Oct 11, 2018 at 17:37
  • $\begingroup$ A side note - in your previous question, you seem to be willing to made your moon's core out of "handwavium". How scientific do you need to get with the host planet? $\endgroup$
    – Alexander
    Commented Oct 11, 2018 at 21:41
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    $\begingroup$ @Alexander In this case, since 1 Jupiter radius will already meet the needs of my story, but bigger radius would be better, I'm willing to go with "Highly unlikely, but certainly explainable within known physics", or even "theoretically possible if some of the more unlikely physics theories turn out to be true", but no outright "handwavium" since I can get what I need without it using 1 Jupiter radius $\endgroup$
    – Harthag
    Commented Oct 12, 2018 at 12:59

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Heating is really your only option. Jupiter (and Saturn) already have atmospheres comprised mostly of the least dense material of which one can make a gas giant.

While I'm not an astrophysicist, I would speculate that some sort of mechanism that led to convection currents from the core of the planet (Jupiter's core is hotter than the surface of the sun) might be sufficient to heat your giant to a pleasant fluffiness, but you really can't beat proximity to a star as a mechanism for decreased density.

Edit in light of your subsequent edit: brown dwarfs are substantially denser than gas giants, because they're failed stars.

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    $\begingroup$ "some sort of mechanism that led to convection currents" > Do you mean something like the 'perfect combination' of rotation speed, radius, and the resulting Coriolis effects, etc, keeps the gases churning enough to stay (relative to 'standard' gas giants) heated by the core? Maybe I should just spin it up to make Uranus' winds look like a breeze ... $\endgroup$
    – Harthag
    Commented Oct 11, 2018 at 18:30
  • $\begingroup$ It'd need to be more than that - the liquid layer surrounding the metallic core of the giant would also need to be constantly in motion, bringing up the energy from the core. Honestly, it's grasping at straws, since this isn't something that shows up in normal planetary models, so would have to be an extraordinary circumstance. $\endgroup$
    – jdunlop
    Commented Oct 11, 2018 at 18:38
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Make your gas giant not quite so giant.

Scale down the mass and size of your gas giant. The density of the gas giant is due to the large gravity of the high mass object. Or, depending on how you want to look at it, is due to the weight of all the extra mass higher in the atmosphere crushing down on it.

Scale your planet down so that there is less atmosphere to weight down on and squeeze it all into a dense form.

Eventually, if scaled down far enough, the gas will not hold together and will disperse.

Check out the astronomy.SE question Can small gas planets exist? Then make your gas planet as small as possible.

And add heat, as you mentioned.

To make this even better, you could, as you suggested, also add heat at the same time. Getting heat from a star is the easiest way to think of, but I just had another idea. Perhaps the gas planet is early in its formation (but will still hold this form for a long time), and it is in the process of clearing the matter in its orbit. If it attracts flammable materials, they could ignite within your planet to keep it heated.

There are quite a few ways you could claim there are flammable materials available in your planet's orbital path. The obvious ones would be recent nearby supernova. But perhaps there were previously (a) planet(s) nearby that have been destroyed, or maybe they still exist and are ejecting material that is being absorbed by your planet.

It is a bit of a stretch, but you can probably count on a reasonable amount of suspension of disbelief in your audience if you say your planet has a moon which ejects oxygen from geysers or volcanoes which escape the moon's orbit and are captured by the planet. A cloud of oxygen descending from the moon onto the gas planet could then be involved in a reaction that helps to maintain heat. Perhaps it settles to the center since it is lighter than the other elements and a few rocks at the center make occasional sparks, combined with the oxygen and some other element you get heat.

With the previous paragraph, you do need to bank on the audience not wondering "Why isn't that oxygen bonding with the hydrogen before that point?" You could probably use other chemicals instead which would provide a better and more solid excuse for you, but I'm not a chemist. You could also just not explain it fully and leave it at "Elements ejected from the gas planet's moons and attracted to the planet provide a slow, steady exothermic reaction which sustains the planet's temperature and density. This will not last forever, as the material in the moons is not infinite, but this situation will last many lifetimes." This is slightly hand-wavy, but it is not unreasonable and might be possible.

Summary

With the reduced mass this temperature increase might not need to be severe. So with the mass/size reduction combined with temperature increase via chemical acquisition leading to gradual endothermic processes, you should be good.

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  • $\begingroup$ There're a bunch of problems with this - the OP explicitly wanted lower density than a standard gas giant - anything using standard formation processes (as detailed in the astronomy.SE post) is going to be standard density. In addition to this, there is no conceivable chemical reaction that can heat a mass that size for geologic time. Having combustibles (and oxygen from the unrealistic moon) won't add more than a footnote to a page detailing the thermal budget of the planet. Supernovae do not generally eject combustibles. Low-mass gas giants have to have icy, rather than rocky, cores. $\endgroup$
    – jdunlop
    Commented Oct 11, 2018 at 20:20
  • $\begingroup$ @jdunlop 1) The method of formation does not dictate density. The other variables which influence density can in turn be influenced by the formation method, but regardless of the method my main point was "make it smaller and less massive." 2) That astronomy post was about getting smaller gas planets and suggested it was possible to have a smaller gas planet. 3) The chemical reaction does not need to be the only thing keeping the planet warmer, but either way I was just throwing out another idea, and I did specifically state that this part was a stretch but would likely pass most ... (1/2) $\endgroup$
    – Loduwijk
    Commented Oct 12, 2018 at 2:25
  • $\begingroup$ (2/2) ... viewers' suspension of disbelief. 4) I was thinking of the supernova ejecting oxygen to aid in the other chemical processes, but you are right it needs more than that. My whole heating section was a marginal addition and I specified it was a stretch, yet it took up more space than my main point, so perhaps I should shorten or remove it. $\endgroup$
    – Loduwijk
    Commented Oct 12, 2018 at 2:26

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