Hidden Ice Giant May Have Shaped Jupiter and Uranus’ Moon Systems

A new study used 122 simulations of the early outer solar system, varying how many giant planets there were. They find that a fifth, long‑lost ice giant could explain the current layout and that Jupiter’s moons survived in less than 15% of scenarios and Uranus’s moons in about 9%, with the surviving cases requiring the presence (and eventual ejection) of the extra planet. The result suggests our solar system’s history involved a stochastic instability that included a vanished planet reshaping the orbits of moons around the gas/ice giants.