Ancient Jupiter was roughly twice as large with a 50× stronger magnetic field
A 2025 Nature Astronomy study using the orbits of Jovian moons Amalthea and Thebe suggests Jupiter was about 2–2.5× its current size and had a magnetic field roughly 50× stronger shortly after the solar system formed (about 3.8 million years in). The stronger early convection would have produced a much larger magnetosphere; the authors infer these conditions from angular-momentum constraints rather than modeling formation directly. The article notes that a separate, older estimate of Jupiter’s current contraction rate (~2 cm/year) exists but is not measured by this study. This finding provides a benchmark for constraining the solar system’s early history.
