In the early days of our solar system, when the Sun was still young and the planets were barely formed, Jupiter wasn’t the gas giant we see today. In fact, today’s Jupiter would look like a midget in comparison. It was a bloated, tempestuous sphere — twice its current size, glowing with internal heat, and wrapped in a magnetic field fifty times stronger than what it possesses now. This is according to a new study, which employed the orbits of Jupiter’s tiny inner moons like fossil records etched into space.
The study by Konstantin Batygin of Caltech and Fred Adams of the University of Michigan pulls off a rare feat in planetary science. They have reconstructed Jupiter’s physical state at a precise moment in cosmic time — only 3.8 million years after the first solid particles in the solar system formed.
The Bloating Giant
In this epoch, known as the “end of the protoplanetary disk,” the swirling cloud of gas and dust around the Sun was beginning to dissipate. The last building blocks of planets were being swept away. And Jupiter, having recently finished its phase of ravenous gas accretion, was undergoing a dramatic transformation.
According to Batygin and Adams, Jupiter’s radius at that time was between 2.0 and 2.5 times its current size. That would have made it large enough to contain over 2,000 Earths by volume.
This size can tell scientists a great deal about how the planet formed. “Our ultimate goal is to understand where we come from,” Batygin said. “And pinning down the early phases of planet formation is essential to solving the puzzle.”
A Planet’s Past Etched in Moons
But how can scientists know what Jupiter looked like billions of years ago?
Surprisingly, the answer lies not within Jupiter itself but in its moons — specifically, Amalthea and Thebe. By the latest count, Jupiter has 97 confirmed moons. These two small satellites orbit close to the planet, even closer than Io, one of the larger and better-known Galilean moons. Their slightly tilted orbits, it turns out, are owed to ancient forces that can be deconstructed.
During the solar system’s infancy, Jupiter’s larger moons were migrating outward due to tidal forces. As Io moved, it swept through a series of orbital resonances with its inner neighbors. These gravitational interactions gently nudged the orbits of Amalthea and Thebe, leaving behind orbital inclinations — small tilts — that have persisted for over four billion years.
The team used those inclinations as a kind of archaeological tool. By modeling how those resonances would have played out, they could infer where Io used to be. This, in turn, told them how large Jupiter must have been at that moment.
“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.
Ice Skating Jupiter
To refine their model further, Batygin and Adams tapped into another rich data source: Jupiter’s angular momentum. Like a cosmic figure skater pulling in her arms, Jupiter’s spin has accelerated over time as the planet contracted. By calculating how it would have spun back when it was twice as large, the researchers could back-calculate the planet’s original size and interior structure.
That spin also hints at something else: the strength of Jupiter’s early magnetic field.
Newly formed Jupiter was still accreting gas through a swirling circumplanetary disk. The friction between the disk and the planet’s powerful magnetic field created torques that regulated its rotation. Based on these interactions, the team estimates that Jupiter’s magnetic field strength was about 21 millitesla — around 50 times stronger than today.
Such a field would have carved out a wide magnetosphere, shielding the planet from solar winds and likely shaping how its moons and rings formed.
A Rare Snapshot in Time
Perhaps most remarkable about this study is that it doesn’t rely on the usual models of planetary formation, which depend on assumptions about how quickly planets accrete gas or how transparent their atmospheres are to radiation. Instead, Batygin and Adams base their work on orbital mechanics and conservation laws — principles as old as Newton and as precise as clockwork.
Their findings also coincide with an independent line of evidence: a 2023 study of meteorite magnetism, which dated the dissipation of the solar nebula to 3.8 million years after the formation of the first solids. That’s the same timestamp this new model settles on for Jupiter’s swollen phase.
“This brings us closer to understanding how not only Jupiter but the entire solar system took shape,” Batygin said.
Jupiter is often called the “architect” of the solar system, and for good reason. Its massive gravity shaped the orbits of other planets, directed comets inward and outward, and likely prevented the formation of another planet in the asteroid belt.
Understanding its early evolution gives scientists vital clues about how our planetary neighborhood was constructed — and, by extension, how planetary systems might form around other stars.
The core accretion model, the dominant theory of gas giant formation, proposes that giant planets form from rocky cores that later suck up massive gas envelopes. This new study supports that model but sharpens it by pinpointing actual physical characteristics — like size and magnetism — at a key evolutionary milestone.
It also adds fuel to the ongoing debate about “hot start” versus “cold start” planets. The terms describe how much heat newborn planets retain after formation. Batygin and Adams find that Jupiter’s early structure was consistent with a “warm start,” threading a middle path through decades of theoretical disagreement.
The findings appeared in Nature Astronomy.
