Couldn't read the actual paper as it is paywalled, but does "twice its current radius" mean that it had a larger mass, and if so what happened to all that extra mass?
Like stars, radius for a gas giant is increased by heat, and decreased by increased mass.
These two factors are rarely completely independent, of course, so it gets complicated. Especially in a star where masses are large enough to result in densities sufficient to cause fusion - and large releases of heat, which then cause decreased density, etc.
But all other factors being constant, the volume of a gas increases (and density decreases) as temperature increases.
See page 6 and the first couple paragraphs of page 7 in the paper for a breakdown.
Eventually Jupiter will cool enough it will be a small fraction of it’s current size, assuming that our understanding is correct and it doesn’t have enough mass to meaningfully result in fusion regardless of how dense it gets. [https://www.pas.rochester.edu/~blackman/ast104/jinterior.htm...]
In theory, it will even eventually cool to the point all those clouds and atmosphere are liquid (or even solid!) gas oceans. That is going to take awhile.
I don't think this is in general true for planets or stars. You're confounding multiple effects. For a fixed number of particles, increasing metallicity, which follows average particle mass, should reduce radius, but for a fixed metallicity and temperature, increasing particles will increase radius. Temp has the effects stated. You can roughly validate this by the fact that massive planets and stars are bigger than less massive ones. Obviously many other things start happening as stars reach end of life...
So generally, gas giants don’t get much bigger than Jupiter.
Could those crystals then erode and reform again as sedimentary rocks to be come a solid planets like earyh?
I understand that’s not how earth itself came to be, but it’s an interesting metamorphosis that I hadn’t previously considered.
https://en.wikipedia.org/wiki/Dragonfly_(Titan_space_probe)
The single image from the surface by the Huygens probe leaves a lot to be desired.
https://en.wikipedia.org/wiki/Titan_(moon)#/media/File:Huyge...
If this is the case then do you have any intel on why do the gas giants in our system appear to more closely directly correlate mass with radius instead of inversely?
https://nssdc.gsfc.nasa.gov/planetary/factsheet/ Mass: Jupiter = 3.3 x Saturn = 22 x Uranus = 19 x Neptune Radius: Jupiter = 1.2 x Saturn = 3 x Uranus = 3 x Neptune
I mean Saturn's density is far less than either of the other three planets, despite being smaller and less massive than Jupiter but larger and more massive than Uranus/Neptune, as well as slightly cooler than Jupiter and far warmer than Uranus/Neptune. And Saturn has the lowest angular velocity among the four, which it would make sense might have the opposite relative effect on density.
The energy input and internal heat budgets are under active study for Jupiter <https://www.nature.com/articles/s41467-018-06107-2> (open access), and will supply further evidence for various hypotheses about "primordial Jupiter", one of which is the topic here. One of the major points of comparison with a star here would be how the former is much more like an ideal blackbody than our local gas giants. And of course there is a dark side of Jupiter, while there is no dark side of the sun.