How is the size defined for a gas planet? The gas density just keeps dropping, where do you draw the line (isosurface, rather)? Earth's radius is always the one without earth's atmosphere.
As others note, the definition of Jupiter’s radius is set by where the pressure is 1 bar. This is somewhat arbitrary, but the arbitrariness doesn’t matter much: the pressure drops to 1 microbar just 320 km higher, which is <0.5% of Jupiter’s ~70,000 km radius.
The most important thing about definitions is that we apply them consistently. A different definition might give different answers, but it's fine as long as it does so uniformly.
I've always wondered about the core of these gas giants. I assume it is some liquid form of light elements. What is puzzling is the presence of the gas giants in the middle of solar system's planetary line up: why are they in the middle and the ones closer or further away from the central star are not like them? Is it the temperature gradient?
I am not an astronomer (save in the very amateur sense), but I think it has to do with Jupiter forming both early in the history of solar system, and, as you guess, beyond the Sun's 'snow line'.
Wikipedia is a good place to start getting a feel for the possible history of the Solar System:
> Data from the Juno mission showed that Jupiter has a diffuse core that mixes into its mantle, extending for 30–50% of the planet's radius, and comprising heavy elements with a combined mass 7–25 times the Earth.
> This has resulted in the theory that Jupiter does not have a solid core as previously thought, but a "fuzzy" core made of pieces of rock and metallic hydrogen.
It's made from something which can generate magnetic fields, since Jupiter has a very strong magnetic field with a lot of distinct inhomogeneous features, resulting in some interesting radio emissions:
Because of solar wind. After the sun formed from the material of the proto solar system it started producing solar winds. This pushes light elements to the edge of the solar system but heavy elements stay. So rocky planets form. Then the light elements collect as well and reenter the interior solar system as comets which redeposit light elements on the surface of rocky planets. In the mean time the light elements that collected together in great quantities formed the gas planets.
This is all a very traditional view afaik and doesn’t explain where mantle light elements come from. For example there is a great deal of water that is in the mantle that drives geochemical changes in the mantle rocks. Was that there originally? Or was it put their after plate tectonics started and subduction sucked water into the mantle? I don’t know but I would assume there are plenty of geodynamics people who would have opinions more deeper than mine on the topic.
Found this on the wiki for metalic hydrogen. Apprently there is a liquid phase as well. Apparently there is debate as to whether there is a solid core besides the hydrogen (thought shown in the pic):
They aren't in the middle of the “planetary lineup”.
The four inner planets are all terrestrial. The four outer planets are gas (the last fwl sometimes distinguished instead as “ice”) giants.
There are dwarf planets (a separate category from, rather than sibcategory of, planets) closer and farther, but no known rocky planets beyond the gas/ice giants.
> Because Amalthea and Thebe have slightly tilted orbits, Batygin and Adams analyzed these small orbital discrepancies to calculate Jupiter's original size…
This seems like a non-sequitur. What do tilted orbits have to do with size?
So is this a significant new finding, changing previous assumptions, or is it part of the "evolutionary history" meaning it was assumed before, that in early times it was bigger?
The new part is probably the precise details about size and strength of the magnetic sphere in the past and that they used a different mechanism to fill in gaps in existing theories.
> Importantly, these insights were achieved through independent constraints that bypass traditional uncertainties in planetary formation models—which often rely on assumptions about gas opacity, accretion rate, or the mass of the heavy element core
> The results add crucial details to existing planet formation theories, which suggest that Jupiter and other giant planets around other stars formed via core accretion, a process by which a rocky and icy core rapidly gathers gas. These foundational models were developed over decades by many researchers, including Caltech's Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, Emeritus. This new study builds upon that foundation by providing more exact measurements of Jupiter's size, spin rate, and magnetic conditions at an early, pivotal time
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...
> 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?
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.
Not larger mass. Simply, was less dense. In layman terms, and if I understand correctly, was the result of the interactions of Jupiter, Jupiter's magnetosphere and Jupiter's circumplanetary disk.
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Wikipedia is a good place to start getting a feel for the possible history of the Solar System:
https://en.wikipedia.org/wiki/Jupiter#Formation_and_migratio...
https://en.wikipedia.org/wiki/Grand_tack_hypothesis
https://en.wikipedia.org/wiki/Nice_model
https://en.wikipedia.org/wiki/Jupiter#Internal_structure
> This has resulted in the theory that Jupiter does not have a solid core as previously thought, but a "fuzzy" core made of pieces of rock and metallic hydrogen.
https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_r...
https://www.ebsco.com/research-starters/science/jupiters-mag...
https://radiojove.gsfc.nasa.gov/library/sci_briefs/decametri...
The rotation of these features is the basis for the "system III" definition of longitude on Jupiter.
This is all a very traditional view afaik and doesn’t explain where mantle light elements come from. For example there is a great deal of water that is in the mantle that drives geochemical changes in the mantle rocks. Was that there originally? Or was it put their after plate tectonics started and subduction sucked water into the mantle? I don’t know but I would assume there are plenty of geodynamics people who would have opinions more deeper than mine on the topic.
Why would you assume that? The heavier elements such as iron are likelier to move to the center of gravity.
https://upload.wikimedia.org/wikipedia/commons/b/b5/Jupiter_...
The four inner planets are all terrestrial. The four outer planets are gas (the last fwl sometimes distinguished instead as “ice”) giants.
There are dwarf planets (a separate category from, rather than sibcategory of, planets) closer and farther, but no known rocky planets beyond the gas/ice giants.
"As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve."
https://en.wikipedia.org/wiki/Jupiter
So is this a significant new finding, changing previous assumptions, or is it part of the "evolutionary history" meaning it was assumed before, that in early times it was bigger?
> Importantly, these insights were achieved through independent constraints that bypass traditional uncertainties in planetary formation models—which often rely on assumptions about gas opacity, accretion rate, or the mass of the heavy element core
> The results add crucial details to existing planet formation theories, which suggest that Jupiter and other giant planets around other stars formed via core accretion, a process by which a rocky and icy core rapidly gathers gas. These foundational models were developed over decades by many researchers, including Caltech's Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, Emeritus. This new study builds upon that foundation by providing more exact measurements of Jupiter's size, spin rate, and magnetic conditions at an early, pivotal time
https://www.caltech.edu/about/news/jupiter-was-formerly-twic...
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...
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.
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.
[1] https://arxiv.org/abs/2505.12652
As its rotation slows, it will shrink even further.