Jupiter, already the behemoth of our solar system, was once even larger and more powerful than it is today, according to a groundbreaking study published in Nature Astronomy on May 20, 2025. Just 3.8 million years after the first solid materials formed in the solar system, Jupiter had a volume 2 to 2.5 times greater than its current size and boasted a magnetic field at least 50 times stronger. At that stage, its enormous radius would have allowed it to contain over 2,000 Earths—compared to about 1,321 today.
These findings, led by planetary scientist Konstantin Batygin of Caltech and astrophysicist Fred C. Adams of the University of Michigan, challenge many long-standing assumptions about the gas giant’s early development and offer a rare glimpse into the chaotic infancy of our solar system.
Different Approach to Planetary History
To reach their conclusions, Batygin and Adams used a novel method. Rather than relying on traditional planetary formation models—which often depend on assumptions like gas opacity or accretion rates—the researchers focused on two of Jupiter’s small, inner moons: Amalthea and Thebe. These lesser-known satellites, which orbit closer to Jupiter than Io, have slightly tilted orbits that have remained stable since the early days of the solar system.
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By analyzing the tiny orbital tilts of these moons, the team was able to reverse-engineer Jupiter’s size and magnetic strength at the time the protoplanetary disk—an enormous cloud of gas and dust that once surrounded the young Sun—was dissipating. “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,” Adams said.
Rapid Growth, Then Contraction
According to the study, Jupiter experienced an intense growth spurt early in its formation. Once it amassed a core roughly ten times the mass of Earth, it began rapidly pulling in surrounding gas at a rate of 1.2 to 2.4 Jupiter masses per million years. This aligns with the core accretion model of planet formation, which suggests that gas giants begin as rocky cores before growing massive gaseous envelopes.
Eventually, the gas surrounding Jupiter dissipated. With the loss of external material, the planet began to contract under its own gravity. That contraction increased its spin rate and decreased its volume. This process continues today, albeit at a much slower pace, as Jupiter gradually cools and compresses. Despite its once enormous size, Jupiter was never massive enough to ignite hydrogen fusion and become a star. For that, it would need at least 85 times its current mass.
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The findings emphasize Jupiter’s critical role as the “architect” of the solar system. Its gravitational influence helped shape the orbits of other planets and likely played a stabilizing role that made the emergence of life on Earth possible. “This brings us closer to understanding how not only Jupiter but the entire solar system took shape,” said Batygin.