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Space

Jupiter Was Formerly Twice Its Current Size and Had a Much Stronger Magnetic Field (phys.org) 47

A new study reveals that about 3.8 million years after the solar system's first solids formed, Jupiter was twice its current size with a magnetic field 50 times stronger, profoundly influencing the structure of the early solar system. Phys.Org reports: [Konstantin Batygin, professor of planetary science at Caltech] and [Fred C. Adams, professor of physics and astronomy at the University of Michigan] approached this question by studying Jupiter's tiny moons Amalthea and Thebe, which orbit even closer to Jupiter than Io, the smallest and nearest of the planet's four large Galilean moons. Because Amalthea and Thebe have slightly tilted orbits, Batygin and Adams analyzed these small orbital discrepancies to calculate Jupiter's original size: approximately twice its current radius, with a predicted volume that is the equivalent of over 2,000 Earths. The researchers also determined that Jupiter's magnetic field at that time was approximately 50 times stronger than it is today.

Adams highlights the remarkable imprint the past has left on today's solar system: "It's astonishing that even after 4.5 billion years, enough clues remain to let us reconstruct Jupiter's physical state at the dawn of its existence." 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. Instead, the team focused on the orbital dynamics of Jupiter's moons and the conservation of the planet's angular momentum -- quantities that are directly measurable.

Their analysis establishes a clear snapshot of Jupiter at the moment the surrounding solar nebula evaporated, a pivotal transition point when the building materials for planet formation disappeared and the primordial architecture of the solar system was locked in. 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.
The findings have been published in the journal Nature Astronomy.
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Jupiter Was Formerly Twice Its Current Size and Had a Much Stronger Magnetic Field

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  • ...I demand a Congressional investigation!

    Ionization of the atmosphere via the planet's then extra strong magnetic field? That's a lot of material to ionize away. There should be lots of residue left over somewhere. If it blew inward, our moon should have a record. If it blew outward, then we'd have to land on rocky bodies out there to get samples. Could all that "stuff" be what formed the Oort cloud or Kuiper Belt?

  • by Sneftel ( 15416 ) on Wednesday May 21, 2025 @04:05AM (#65392477)

    Batygin, K., and Adams, F.C. You Think Jupiter's Big Now: Lemme Tell You, Back In MY Day. In Nat Astron (2025).

  • by butt0nm4n ( 1736412 ) on Wednesday May 21, 2025 @04:10AM (#65392485)

    Jupiter spins slowly so they can see it all and quietly thanks the weight loss jab.

    I was so .. gassy .. before, always bloated. I feel great.

  • I think it would have been a cool looking "star" that comes and goes.
    • by Tablizer ( 95088 ) on Wednesday May 21, 2025 @04:56AM (#65392587) Journal

      Estimates are about 10 to 15x Jupiter's size to become a "brown dwarf" star, so 2x wouldn't do it. I wonder how bright a brown dwarf at Jup's distance would look from Earth? I guesstimate about as bright as the moon, and it probably wouldn't stand out in the day.

      • A "brown dwarf" isn't really a "star". Its just a class of object where we start considering it too big to be considered a planet. Jupiter would need about ~65 to 70x its current *mass* (not size) to become the smallest type of star (red dwarf).

        Also in this class of object its very, very important to distinguish between mass and size, because they stop scaling anything close to linearly at that scale.

        Jupiter and Saturn are very similar in size. Saturn is 84.3% of the *size* of Jupiter (as it relates to d

        • by Tablizer ( 95088 )

          A brown dwarf generates energy by fusing deuterium, something Jupiter is too small to do. Most the energy of brown dwarfs is in the infrared wavelength such that they are hard for humans to directly spot.

          • A brown dwarf generates energy by fusing deuterium,

            Not untrue, but the following caveat is important :

            briefly.

            As in, maybe a million years during which there is energy generated in the core. But at quite a low rate, because (this is also important) deuterium is a relatively rare nucleus.

            There's a good chance that it could be fusing deuterium in it's core, and if you didn't have a couple of cubic km of water for a neutrino detector, you'd be hard pressed to disentangle the energy production from the deuteri

        • Plus there are multiple ways to look at "size".

          Yeah, it's bloody annoying when writers aren't clear about that. Sq.Rt(2) = 1.418... Cu.Rt(2)=1,2599... ; worth remembering.

          • FTFP :

            we obtain the following range for Jupiterâ(TM)s primordial radius: R_jup_original = 2.02 ~2.59 times R_jup_present (substituting for their typography)

            Which is a lot larger than I'd expected. The journalists have done their job correctly - they do mean upwards of twice the radius (so 8 times to 17.4 times the volume) of present day Jupiter. And correspondingly lower density.

            The surface area would have been between 4 and 6.7 times that of modern Jupiter - which would have helped with shedding heat

    • It won't ignite for a number of reasons that have been documented many times.

      I think it would have been a cool looking "star" that comes and goes.

      Sure. Having 6 months of light 24/7, like in the poles ... Hard to sleep!

      • by Mr. Dollar Ton ( 5495648 ) on Wednesday May 21, 2025 @07:39AM (#65392711)

        It won't ignite for a number of reasons

        The most important being the absence of a monolith in orbit around it.

      • Having 6 months of light 24/7,

        No.

        "Star Jupiter" would be at most (counts-on-fingers.GIF) 28deg from the celestial equator (the Earth's axial tilt, plus the approx 5 difference between Earth's orbital inclination and that of Jupiter). So you'd get ... well, it'd be the interaction of two periods - Earth's axial rotation and Jupiter-orbit-period minus Earth-orbital-period. At inferior opposition (Jupiter near to the Sun in the sky), you'd have approximately our current situation ; at superior opposition (Jupi

    • Bear in mind the theory here is about size (volume), not mass. So it could have been the size of the solar system and still been less likely to turn into a star because there wouldn't have been enough mass for that to happen.

  • Ob. (Score:5, Funny)

    by greytree ( 7124971 ) on Wednesday May 21, 2025 @07:12AM (#65392685)
    Whereas Uranus has doubled in size and is much less attractive.
    • by Anonymous Coward

      I came here for the 'yo mum' jokes. She was also twice the size of jupiter, but still is - and is just as gassy.

    • by Tablizer ( 95088 )

      "Does this asteroid belt make my Uranus look big?"

Reactor error - core dumped!

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