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.
For comparison, extracting the numbers from the graphic in page 3 of https://projects.iq.harvard.edu/files/acmg/files/intro_atmo_... 1 microbar on Earth is like 50Km, that is 50/6400 ~= 0.8%
Venus would get a slight radius buff, too, if we applied that metric.
But Venus has a solid surface.
The density falls off pretty steeply at the “edge”, so the exact definition only makes little difference for the radius: https://www.researchgate.net/figure/Density-vs-radius-for-a-...
This is because of Newtonian gravity being inversely proportional to the square of the radius, right?
Gravity changes little over that distance - it's more because of the compounding effect of atmospheric pressure (the deeper you go, the more air you have above you which raises the pressure, raising the density and meaning that pressure increases exponentially faster).
What makes that curve exponential?
It's defined as the distance from the center of mass to the point where the pressure is equal to the pressure on Earth at sea level.
It's a matter of definitions, so we skip them and just choose something that makes sense to humans.
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.
The point is that there will be multiple definitions, so which one do you choose? From there your conclusion can be that we just use a loose definition that humans can easily grasp.
> most important thing about definitions is that we apply them consistently
The most important consideration for a definition is its practical consequence.
In this case, whether the line is drawn at 1 bar or an order of magnitude more or less doesn’t materially change that, on the same measure, Jupiter was 2x larger in the past. (Less than 1% in both cases.)
In a different context, that difference may be meaningful and should thus be noted and tested for robustness.
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:
https://en.wikipedia.org/wiki/Jupiter#Formation_and_migratio...
> 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.
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...
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:
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.
> I assume it is some liquid form of light elements.
Why would you assume that? The heavier elements such as iron are likelier to move to the center of gravity.
I'd assume these elements would be created in the sun and keep close to the sun, because of their higher mass.
All the heavier elements were created in a former star that went supernova. The solar system formed from the gas/dust after that.
The heavier elements being formed in our sun now are going to stay there until something can tear it apart.
Some will be ejected back into space when the aged Sun becomes a planetary nebula (with a white dwarf at the center).
The core of Jupiter is thought to contain metallic hydrogen.
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):
https://upload.wikimedia.org/wikipedia/commons/b/b5/Jupiter_...
In an Arthur C Clarke story (I forget which one) the core of jupiter is a planet sized diamond.
Space Odyssey one of the sequels.
2061. Not his best work, alas.
> Because Amalthea and Thebe have slightly tilted orbits, Batygin and Adams analyzed these small orbital discrepancies to calculate Jupiter's original size…
When the solar system was young, Jupiter was hotter and spun faster. (which affects its radius, but not its mass) This is not new information.
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?
A prepublished version of the paper is available on arxiv [1]
The mass did not differ much from the present mass, but the planet was less dense and with a more rapid rotation.
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...
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...
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.
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.
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.
Probably contracted as it's mostly gas.
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.
aka "fluffier"
I am deeply skeptical of any "research" that concludes something in the past. The scientific method relies on observation, experimentation, and replication, but these aren't possible with past events, so we can't directly test or falsify historical claims. Instead, researchers infer conclusions based on indirect evidence like documents, artifacts, or statistical patterns—often without being able to isolate variables or rule out alternatives.
If something is not falsifiable, it is not science in my book. Research that is falsifiable uncovers deep truths of nature that will benefit humanity's progress, which this kind of research will not.
Sorry to be a downer. I haven't had my morning coffee yet.
Observations are inherently always about the past.
That’s true in a narrow sense—every observation records something that has already happened. But in science, observations can be tested, replicated, and used to predict future outcomes. The kind of "research" I'm skeptical of draws broad, causal conclusions about unique, unrepeatable past events where none of that is possible.
Usually these are predictions made by a model that has explanatory power for things that we can observe. The model might be wrong, or there might be a better model. That’s always the case in science. Observations that confirm a model also increase the credence for its predictions that we can’t directly observe. It means that given our best current understanding of X, it also implies Y. Yes, Y might be wrong, but then that implies that something is likely also wrong with our current understanding of X. The predictions (or retrodictions) aren’t black and white. They always have some associated level of credence, which depends on how well we think we understand the kind of system we are talking about.
It’s always a theory, but what choice do you have? You can’t rerun the experiment again under controlled conditions. Your only choice is to theorize or not. Sure, there is more possible error in such theories compared to other theories where you can rerun the experiment multiple times to test it, but that doesn’t mean that a theory that can’t be tested is wrong.
PC is from the future
If we're to take your claims at face value, can we make any conclusions about the past at all?
For example, suppose that I were to claim that the universe is exactly one hundred years old. George Washington, Genghis Khan, Julius Caesar, dinosaurs, etc. are all figments of our collective imagination.
If you deny the validity of research that makes conclusions about the past, on the grounds that such claims can't be tested or falsified -- then have you left yourself any means of making a counterargument?
So I guess you’re skeptical about continental drift theory and universal common descent ?
This is how young earth type misunderstandings begin. Thanks for bringing us inside the mind.
parent commenter invokes Popperian epistemology. Your comment aligns Popper with flat-earth thinking. One of you is engaging in pseudoscience.
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