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Jupiter was formerly twice its current size, had a much stronger magnetic field
- rwmj 1y agoCouldn'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 1y agoA prepublished version of the paper is available on arxiv [1] [1] https://arxiv.org/abs/2505.12652 https://arxiv.org/abs/2505.12652
- lazide 1y agoIt 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.html 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.
- HappMacDonald 1y ago> 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 agoThey’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 agoNeither 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.
- greggsy 1y agoCould 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 1y agoLike 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 1y agoThat'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 1y agoI 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) 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:Huygens_surface_color.jpg https://en.wikipedia.org/wiki/Titan_(moon)#/media/File:Huyge...
- anton-c 1y agoYeah this is gonna be awesome, appreciate the heads up!
- lazide 1y ago
- queuebert 1y ago> ... 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...
- lazide 1y agoIn the sense that higher gravity increases density all other things being equal. yes, absolute size will slowly increase - until it collapses, anyway.
- lazide 1y agoGas 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 https://en.m.wikipedia.org/wiki/Gas_giant] So generally, gas giants don’t get much bigger than Jupiter.
- Zardoz84 1y agoNot 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 1y agoaka "fluffier"
- exe34 1y agoProbably contracted as it's mostly gas.
- adrian_b 1y agoThe mass did not differ much from the present mass, but the planet was less dense and with a more rapid rotation.
- est31 1y agoHow 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 1y agoIt'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 1y agoIt's a matter of definitions, so we skip them and just choose something that makes sense to humans.
- philipov 1y agoThe 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 1y ago> 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 1y agoThe 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 1y ago
- yubblegum 1y agoI'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 1y agoI 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_migration https://en.wikipedia.org/wiki/Jupiter#Formation_and_migratio... https://en.wikipedia.org/wiki/Grand_tack_hypothesis https://en.wikipedia.org/wiki/Grand_tack_hypothesis https://en.wikipedia.org/wiki/Nice_model https://en.wikipedia.org/wiki/Nice_model
- Tepix 1y ago> 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__ 1y agoI'd assume these elements would be created in the sun and keep close to the sun, because of their higher mass.
- fasteddie31003 1y agoI 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 1y agoObservations are inherently always about the past.
- fasteddie31003 1y agoThat’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 1y agoUsually 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.
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- bethekidyouwant 1y agoWhen 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 1y ago> 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?
- curtisszmania 1y ago[dead]
- Civitello 1y agoIt shrank because of all the boys going there to get stupider.
- lukan 1y agoThat 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 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?
- vlovich123 1y agoThe 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 https://www.caltech.edu/about/news/jupiter-was-formerly-twice-its-current-size-and-had-a-much-stronger-magnetic-field https://www.caltech.edu/about/news/jupiter-was-formerly-twic...
- WalterBright 1y agoPresuming 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.