A new study reveals that about 3.8 million years after the solar system's first solids formed, Jupiter was [1]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.

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> 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.

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> 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 [2]published in the journal Nature Astronomy .



[1] https://phys.org/news/2025-05-jupiter-current-size-stronger-magnetic.html

[2] https://www.nature.com/articles/s41550-025-02512-y