Comment by lazide | Hacker News Reader

Comment by lazide | Hacker News Reader

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It was hotter, a lot hotter.

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.

> ... decreased by increased mass.

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...

In the sense that higher gravity increases density all other things being equal. yes, absolute size will slowly increase - until it collapses, anyway.

Gas giants generally only get slightly larger than Jupiter (even with adding a lot of mass), until they start to shrink - and eventually with enough mass, turn into actual stars [https://en.m.wikipedia.org/wiki/Gas_giant]

So generally, gas giants don’t get much bigger than Jupiter.

Could it cool and crystallise?

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.

Like the interior of the planet, the atmosphere is overwhelmingly hydrogen and helium. And helium is liquid even at 0 temperature unless under pressure, so presumably (?) would be liquid on the surface. These materials are mechanically very different than the silcates and metals dominating the Earth’s crust, and I don’t think we even have well measured bulk properties? Not sure what erosion processes would look like.

That's wild to think about. My mind is struggling to picture 'liquids on the surface' of Jupiter. No idea what that would look like.

I am deeply looking forward to the dragonfly mission to Titan, since we'll finally get high-resolution color images from the surface, which has liquid seas of hydrocarbons like methane and ethane at -290 F.

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...

Yeah this is gonna be awesome, appreciate the heads up!

At the point hydrogen, helium, ammonia, etc. have cooled to solid ‘rock’, chemistry and weather as we’re familiar with it doesn’t really apply anymore. Pluto has been that way for a long time though, albeit good luck spending enough time there to get very familiar with it.

> Like stars, radius for a gas giant is [..] decreased by increased mass.

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.

They’re all made out of different mixes of gases and other elements, and are different distances from the sun, and any number of other variables.

Neither Jupiter nor Saturn is close to thermal equilibrium, whereas the sun is. Bounded self-gravitating gas spheres in thermodynamic equilibrium can show negative specific heats [The classic LBLB, Lynden-Bell & Lynden-Bell, 1977 <https://adsabs.harvard.edu/full/1977MNRAS.181..405L>]. A negative specific heat capacity reducews the gas's volume as its temperature increases. Temperature in stars and gas giants is mostly lowering due to outward radiation driven by internal processes. Unlike a star, Jupiter's specific heat capacity is positive. Very roughly the sun's excess power output will cause it to grow (this handwaves a complex balance of temperature, pressure, mass, and nuclear fusion as it rises in the main-sequence part of the H-R diagram <https://chandra.harvard.edu/graphics/edu/formal/variable_sta...> -- as it climbs in that region with similar temperature the sun gets brighter because it gets bigger), while Jupiter's power output has been higher (presently about 2.5x) than its solar radiation input yet the planet has probably been shrinking.

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.

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