Jupiter Was Formerly Twice Its Current Size and Had a Much Stronger Magnetic Field

Understanding Jupiter's early evolution helps illuminate the broader story of how our solar system developed its distinct structure. Jupiter's gravity, often called the "architect" of our solar system, played a critical role in shaping the orbital paths of other planets and sculpting the disk of gas and dust from which they formed.

In a new study published in the journal Nature Astronomy, Konstantin Batygin (PhD '12), professor of planetary science at Caltech; and Fred C. Adams, professor of physics and astronomy at the University of Michigan; provide a detailed look into Jupiter's primordial state. Their calculations reveal that roughly 3.8 million years after the solar system's first solids formed—a key moment when the disk of material around the Sun, known as the protoplanetary nebula, was dissipating—Jupiter was significantly larger and had an even more powerful magnetic field.

"Our ultimate goal is to understand where we come from, and pinning down the early phases of planet formation is essential to solving the puzzle," Batygin says. "This brings us closer to understanding how not only Jupiter but the entire solar system took shape."

Batygin and Adams approached this question by studying Jupiter's tiny moons Amalthea and Thebe, which orbit even closer to Jupiter than Io, the smallest and nearest of the planet's four large Galilean moons. Because Amalthea and Thebe have slightly tilted orbits, Batygin and Adams analyzed these small orbital discrepancies to calculate Jupiter's original size: approximately twice its current radius, with a predicted volume that is the equivalent of over 8,000 Earths. The researchers also determined that Jupiter's magnetic field at that time was approximately 50 times stronger than it is today.

Adams highlights the remarkable imprint the past has left on today's solar system: "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."

Importantly, these insights were achieved through independent constraints that bypass traditional uncertainties in planetary formation models—which often rely on assumptions about gas opacity, accretion rate, or the mass of the heavy element core. Instead, the team focused on the orbital dynamics of Jupiter's moons and the conservation of the planet's angular momentum—quantities that are directly measurable. Their analysis establishes a clear snapshot of Jupiter at the moment the surrounding solar nebula evaporated, a pivotal transition point when the building materials for planet formation disappeared and the primordial architecture of the solar system was locked in.

The results add crucial details to existing planet formation theories, which suggest that Jupiter and other giant planets around other stars formed via core accretion, a process by which a rocky and icy core rapidly gathers gas. These foundational models were developed over decades by many researchers, including Caltech's Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, Emeritus. This new study builds upon that foundation by providing more exact measurements of Jupiter's size, spin rate, and magnetic conditions at an early, pivotal time.

Batygin emphasizes that while Jupiter's first moments remain obscured by uncertainty, the current research significantly clarifies our picture of the planet's critical developmental stages. "What we've established here is a valuable benchmark," he says. "A point from which we can more confidently reconstruct the evolution of our solar system."

The paper is titled "Determination of Jupiter's Primordial Physical State." Funding was provided by Caltech, the David and Lucile Packard Foundation, the National Science Foundation, the University of Michigan, and the Leinweber Center for Theoretical Physics at the University of Michigan.