Jupiter, the solar system’s reigning gas giant, may have been even more massive in its youth—a bloated behemoth 2.5 times its current volume with a magnetic field 50 times stronger, according to groundbreaking research published today in Nature Astronomy.
By analysing the tilted orbits of Jupiter’s tiny inner moons, Caltech and University of Michigan astrophysicists have reconstructed the planet’s explosive infancy, offering new clues about how our cosmic neighbourhood took shape.
The team studied Amalthea and Thebe – two potato-shaped satellites orbiting closer to Jupiter than volcanic Io -whose odd orbital tilts preserve a record of the planet’s early gravitational influence. “These moons are like ancient hieroglyphs,” said co-author Fred Adams.
“Their crooked paths let us decode Jupiter’s physical state just 3.8 million years after the solar system’s first solids formed.” The calculations suggest baby Jupiter ballooned at a staggering rate, guzzling 1.2 to 2.4 Jupiter masses of gas and dust every million years during this growth spurt.
This rapacious appetite left Jupiter as a diffuse, low-density world before it began contracting under its own gravity. Like a spinning ice skater pulling in their arms, the shrinking planet accelerated its rotation while intensifying its magnetic field, a dynamo effect that still drives Jupiter’s spectacular auroras today. Remarkably, this shrinkage continues at a glacial pace as Jupiter slowly radiates heat into space.
The findings bolster Jupiter’s reputation as the solar system’s prime architect. Its gargantuan gravity likely sculpted the asteroid belt and prevented Mars from growing larger, critical factors that allowed Earth to become habitable. “Jupiter was the solar system’s first planet,” explained lead author Konstantin Batygin. “Understanding its birth helps explain why we find ourselves on a rocky world with a stable climate rather than a gas-shrouded hellscape.”
While the young Jupiter was impressively bulky, it never approached stardom, requiring 85 times its current mass to ignite nuclear fusion. The research provides planetary scientists with a vital benchmark for modelling other gas giants, both in our solar system and among the thousands discovered orbiting distant stars.
