Jupiter, the behemoth of the Solar System, wasn’t always the planet we see today. New research reveals that early in its formation, Jupiter was more than twice its current volume. It boasted a magnetic field 50 times stronger than today’s. The findings provide new insights into the planet’s growth and the broader story of how the Solar System evolved.
Astronomers Konstantin Batygin of Caltech and Fred Adams of the University of Michigan made the discovery. They examined two of Jupiter’s inner moons, Amalthea and Thebe. Their work, published in a recent study, reveals that around 3.8 million years after the Solar System’s first solid materials formed, Jupiter had already ballooned to an enormous size.
A MASSIVE BEGINNING
Jupiter is already massive—its current mass is 2.5 times greater than all other planets in the Solar System combined. But the study suggests the gas giant once held two to 2.5 times its current volume, confirming theories that gas giants undergo a phase of rapid, early growth via core accretion.
“Our ultimate goal is to understand where we come from,” said Batygin. “Pinning down the early phases of planet formation is essential to solving the puzzle. This brings us closer to understanding how not only Jupiter, but the entire Solar System took shape.”
TRACKING CLUES THROUGH JUPITER’S MOONS
Instead of relying solely on traditional planet formation models, Batygin and Adams took an innovative approach. They analyzed the orbital tilts of Amalthea and Thebe, two small moons that orbit even closer to Jupiter than Io. These orbital inclinations act like fingerprints, recording the effects of Jupiter’s early gravitational and magnetic forces.
“It’s astonishing that even after 4.5 billion years, there are enough clues. These clues let us reconstruct Jupiter’s physical state at the dawn of its existence,” said Adams.
The team traced the moons’ orbital history backward, revealing the size and magnetic strength of early Jupiter. Their calculations show that Jupiter accreted mass at a furious rate—between 1.2 and 2.4 Jupiter masses per million years—as it pulled in material from the surrounding protoplanetary disk.
FROM GIANT TO GIANT STILL SHRINKING
After this phase of explosive growth, Jupiter contracted under its own gravity, gradually shrinking to the size we see today. This process of gravitational compression continues slowly, causing the planet to heat internally and spin faster.
Despite its former size, the planet was never close to becoming a star. To achieve hydrogen fusion in its core, it would need to be much more massive. A star’s hallmark, hydrogen fusion, requires a mass at least 85 times greater than Jupiter’s current mass.
THE BIGGER PICTURE: UNDERSTANDING OUR COSMIC ROOTS
Jupiter’s early expansion and magnetic field may have played a vital role in shaping the Solar System. Its gravitational influence could have stabilized planetary orbits. This stabilization potentially created a more hospitable environment for life to arise on Earth.
“What we’ve established here is a valuable benchmark,” Batygin said. “A point from which we can more confidently reconstruct the evolution of our Solar System.”
The study adds a powerful new tool to the planetary science toolkit. It offers a clearer picture of how giants like Jupiter formed. Their growth shaped everything around them, including us.
