[HN Gopher] Jupiter was formerly twice its current size, had a m...
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Jupiter was formerly twice its current size, had a much stronger
magnetic field
Author : pseudolus
Score : 112 points
Date : 2025-05-21 09:26 UTC (3 days ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| rwmj wrote:
| 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?
| elashri wrote:
| A prepublished version of the paper is available on arxiv [1]
|
| [1] https://arxiv.org/abs/2505.12652
| lazide wrote:
| 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.roc
| hester.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.
| HappMacDonald wrote:
| > 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.
| skywhopper wrote:
| 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 wrote:
| 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/va
| riable_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.
| greggsy wrote:
| 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.
| jessriedel wrote:
| 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.
| anton-c wrote:
| 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.
| jessriedel wrote:
| 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_prob
| e)
|
| 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:Hu
| yge...
| lazide wrote:
| 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.
| queuebert wrote:
| > ... 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...
| Zardoz84 wrote:
| 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.
| DonHopkins wrote:
| aka "fluffier"
| exe34 wrote:
| Probably contracted as it's mostly gas.
| adrian_b wrote:
| The mass did not differ much from the present mass, but the
| planet was less dense and with a more rapid rotation.
| est31 wrote:
| 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.
| Maxatar wrote:
| 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.
| amelius wrote:
| It's a matter of definitions, so we skip them and just choose
| something that makes sense to humans.
| philipov wrote:
| 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.
| JumpCrisscross wrote:
| > _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.
| amelius wrote:
| 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.
| jessriedel wrote:
| 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
| _micro_ bar just 320 km higher, which is <0.5% of Jupiter's
| ~70,000 km radius.
| gus_massa wrote:
| For comparison, extracting the numbers from the graphic in
| page 3 of https://projects.iq.harvard.edu/files/acmg/files/in
| tro_atmo_... 1 microbar on Earth is like 50Km, that is
| 50/6400 ~= 0.8%
| queuebert wrote:
| Venus would get a slight radius buff, too, if we applied that
| metric.
| hnuser123456 wrote:
| But Venus has a solid surface.
| layer8 wrote:
| 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-...
| formerly_proven wrote:
| This is because of Newtonian gravity being inversely
| proportional to the square of the radius, right?
| skykooler wrote:
| 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).
| vecter wrote:
| What makes that curve exponential?
| westurner wrote:
| Newtonian gravity (classical mechanics).
|
| Two-body gravitational attraction is observed to be an
| inverse square power law; gravitational attraction
| decreases with the square of the distance.
|
| g, the gravitational constant of Earth, is observed to be
| exponential; 9.8 m/s^2.
|
| Atmospheric pressure: https://en.wikipedia.org/wiki/Atmos
| pheric_pressure#:~:text=P... :
|
| > _Pressure (P), mass (m), and acceleration due to
| gravity (g) are related by P = F /A = (m*g)/A, where A is
| the surface area. Atmospheric pressure is thus
| proportional to the weight per unit area of the
| atmospheric mass above that location._
| marcosdumay wrote:
| It's related to the Boltzmann distribution being an
| exponential. But there are all kinds of effects that make a
| planet's atmosphere deviate from an ideal gas at rest in a
| homogeneous container.
| yubblegum wrote:
| 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?
| GolfPopper wrote:
| 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...
|
| https://en.wikipedia.org/wiki/Grand_tack_hypothesis
|
| https://en.wikipedia.org/wiki/Nice_model
| Tepix wrote:
| > 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.
| k__ wrote:
| I'd assume these elements would be created in the sun and
| keep close to the sun, because of their higher mass.
| frutiger wrote:
| 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.
| pfdietz wrote:
| Some will be ejected back into space when the aged Sun
| becomes a planetary nebula (with a white dwarf at the
| center).
| bell-cot wrote:
| Until several billion years in the future, our sun will
| create no element heavier then helium.
|
| IIR, our sun's mass is far too low to ever create any
| element heavier than carbon.
| mystified5016 wrote:
| The core of Jupiter is thought to contain _metallic hydrogen_.
| yubblegum wrote:
| 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_.
| ..
| twic wrote:
| 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.
| everyone wrote:
| In an Arthur C Clarke story (I forget which one) the core of
| jupiter is a planet sized diamond.
| ReptileMan wrote:
| Space Odyssey one of the sequels.
| teamonkey wrote:
| 2061. Not his best work, alas.
| bell-cot wrote:
| Actually 2010, in the closing words of Chap. 38.
|
| Though I agree that 2061 fell rather short of his usual.
| yencabulator wrote:
| > 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...
| fasteddie31003 wrote:
| 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.
| layer8 wrote:
| Observations are inherently always about the past.
| fasteddie31003 wrote:
| 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.
| layer8 wrote:
| 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.
| drob518 wrote:
| 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.
| iwontberude wrote:
| PC is from the future
| impendia wrote:
| 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?
| iwontberude wrote:
| This is how young earth type misunderstandings begin. Thanks
| for bringing us inside the mind.
| ricksunny wrote:
| parent commenter invokes Popperian epistemology. Your comment
| aligns Popper with flat-earth thinking. One of you is
| engaging in pseudoscience.
| SJC_Hacker wrote:
| So I guess you're skeptical about continental drift theory and
| universal common descent ?
| bethekidyouwant wrote:
| 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.
| waynecochran wrote:
| > 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?
| Civitello wrote:
| It shrank because of all the boys going there to get stupider.
| lukan wrote:
| That seems at odds, with something I previously learned about
| Jupiter:
|
| "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?
| WalterBright wrote:
| Presuming that Jupiter got its start from a cloud of gas and
| dust, far far larger, at what point did its gradual compression
| make it a planet?
|
| As its rotation slows, it will shrink even further.
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