Newborn Jupiter Was Twice as Big As It Is Today

A new study of the motion of Jupiter’s moons has revealed that the king of planets, when it was young, was at least twice as large and 50 times more magnetically potent than it is today.

Traditional methods rely on planet-building models to recreate the earliest epochs of the solar system. For example, in the core accretion model, smaller bodies collide to build a small core, which then gathers a thick mantle of most hydrogen gas around it. But models require assumptions, and planetary scientists Konstantin Batygin (Caltech) and Fred C. Adams (University of Michigan) wanted to avoid depending on those assumptions. Instead, they took a more indirect approach, analyzing the subtle orbital shifts of Jupiter’s innermost satellites to reconstruct the planet’s youth, publishing their results in Nature Astronomy.

“[We] traced a clear path backward through time, allowing [the moons’ motions] to pinpoint Jupiter's original size, rotation rate, and magnetic strength at the epoch when the solar system’s disk of gas and dust was evaporating,” explains Batygin.

Batygin and Adams rewound the clock to when the solar system was just 3.8 million years old by decoding the orbital dynamics of two its innermost moons: Thebe and Amalthea.

(Two even closer satellites, Adrastea and Metis, reside within the Roche limit, inside which solid objects are torn apart by the gravitational stresses from the massive planet. So they didn’t form with Jupiter and instead are likely the gravitationally shredded remnants of a larger body that ventured too close.)

The somewhat farther-out Thebe and Amalthea are tiny, not even massive enough to shape themselves into spheres. They swing well inside the orbit of Io, the closest of the greater Galilean moons. Thebe and Amalthea orbit over Jupiter’s equator, but they’re just slightly inclined to it, with orbital inclinations of 0.36° and 1.09°, respectively. These slight inclinations come from past interactions with a migrating Io.

All four Galilean moons are so massive, they probably formed much farther out before migrating inward. Their orbits would have stabilized near the inner edge of the moon-forming disk. So the motions of Thebe and Amalthea, combined with Io’s early migration, shows not only the size of this disk and the planet it was feeding but also the magnetic field that carved out a gap between the growing planet and its disk. The little moons thus show the primordial Jovian system in “unprecedentedly sharp focus,” Batygin says.

The resultant evolutionary snapshot reveals a young Jupiter that was 2 to 2½ times larger than it is now. The planet’s primordial magnetic field was 50 times stronger than in modern times.

“It's astonishing that even after 4.5 billion years,” Adams says, “enough clues remain to let us reconstruct Jupiter's physical state at the dawn of its existence.”

Core Accretion Confirmed

Although the researchers didn’t rely on the core accretion model, their findings align with the model’s predictions, confirming the the framework for gas giant formation.

Under this scenario, Jupiter would have grown at a rapid pace, forming within a million years or so. But even as it rapidly gained mass, gravitational contraction compressed its size to twice its current size within hundreds of thousands of years. “Forming a giant planet is a delicate race against time: Jupiter had to rapidly accumulate its gaseous envelope before the solar nebula dissipated,” Batygin says. “Our results pinpoint Jupiter’s physical state at the end of its formative period, providing a robust constraint that future planet-formation models must satisfy.”

Understanding Jupiter’s history can illuminate the history of other planets, too, Batygin says: “Jupiter's formation had profound ripple effects throughout the early solar system — shaping everything from the arrangement of planets to the distribution of meteorites.” Previous research even suggests that the gas giant’s gravity may even benefited life on Earth by helping stabilize our planet in its current near-circular orbit.

“The authors use a novel approach,” comments Ravit Helled (University of Zurich, Switzerland), who wasn’t involved in the study. “Although their results are consistent with estimates from previous studies, their approach is independent and complementary and provides support to existing planet formation models.”

“At the same time,” she adds, “there is still much more to explore and I hope that this study will open the door to further investigations of the topic.”