What are some plausible formation explanations of a non-hot "Fluffy" Gas Giant planet?

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)?

posted over 5 years ago

Dalila‭

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