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> 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

> 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/ 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.

skywhopper 1y ago
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.
raattgift 1y ago
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 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_stars/evolutionary_track.jpg 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 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.

skywhopper 1y ago

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.

raattgift 1y ago

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 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_stars/evolutionary_track.jpg 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 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.