Jupiter, the largest planet orbiting the sun, used to be much bigger and stronger when the solar system was just beginning to take shape, a pair of astronomers say.
Two scientists at Caltech and the University of Michigan suggest that early Jupiter was at least double its contemporary size. The primitive version of the gas giant could have held some 8,000 Earths within it, said Konstantin Batygin, lead author of the new study.
What's more, young Jupiter probably had a magnetic field 50 times more powerful. A magnetic field is an invisible force surrounding a planet that interacts with charged particles coming from the sun and cosmic rays.
To calculate those measurements, the scientists looked at how Jupiter’s moons move through space and how the planet spins. This unconventional approach, which didn't rely on traditional models, may fill gaps in the solar system's history. Many scientists refer to Jupiter as the "architect" of the solar system because its immense gravity influenced the orbits of other planets and carved up the cloud from which they all emerged.
"More than any other planet, Jupiter played a key role in shaping our solar system," Batygin said in a post on X. "Yet details of its early physical state are elusive."
The paper, published in the journal Nature Astronomy, rewinds the clock to just 3.8 million years after the first solid objects formed in the solar system and the cloud of gas and dust from which everything formed started to evaporate. This period — when the building materials for planets disappeared — is thought to be a pivotal point, when the general design of the solar system was locked in.
Jupiter, roughly 562 million miles from Earth today, has nearly 100 moons. But Batygin and his collaborator Fred Adams' research focused on two of the smaller ones, Amalthea and Thebe. Both are inside the orbit of the much larger moon Io, the most volcanically active world in the solar system, according to NASA.
These smaller moons have curiously tilted orbits, and their paths around the planet seem to hold clues about how Jupiter and its bevy of moons moved in the past, Batygin told Mashable. As Io migrates away from Jupiter, its gravity causes a kickback — sort of like how a gun recoils when it's fired — that has contributed to the tilts of the smaller moons.
"Similar to how our moon gradually moves away from Earth due to tides, Io is slowly drifting outward from Jupiter," Batygin said.
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By measuring Amalthea and Thebe's tilted orbits, the scientists reconstructed Io’s previous position. That location, they said, should help determine the outer edge of the disk of gas and dust that once surrounded the planet. Based on where they believe the disk ended, the researchers extrapolated how fast Jupiter was spinning back then: about once per day, comparable to its spin now.
Knowing Jupiter's early spin also helped them calculate its size. By applying the physics rules of spinning objects, they figured out how big Jupiter had to have been to match that rotation. The size of a young planet sheds light on its heat and interior dynamics as well. The scientists have concluded that early Jupiter must have started out extremely hot — about 2,000 degrees Fahrenheit. That's a far cry from its modern average temperature of about -170 degrees.
The heat suggests Jupiter had a much stronger magnetic field. That allowed the team to calculate how fast Jupiter was collecting gas and growing — about the weight of one modern-day Jupiter every million years.
"It's astonishing," said Adams in a statement, "that even after 4.5 billion years, enough clues remain to let us reconstruct Jupiter's physical state at the dawn of its existence."
