A new study published in Icarus strengthens the theory that our solar system once contained a fifth giant planet—an extra ice giant that was later flung into interstellar space. Using 122 dynamical simulations based on the well-established Nice Model, researchers explored how gravitational instabilities among the giant planets during the early solar system could have shaped today’s planetary architecture. The model suggests that close encounters between Jupiter, Saturn, Uranus, Neptune, and a now-missing ice giant led to a chaotic period that disrupted the formation and orbits of moons, especially around Uranus.

Uranus appears particularly vulnerable in these simulations. Its extreme axial tilt and the unusual characteristics of its moons—especially Miranda, which is small and icy—may stem from multiple disruptions: one during the planet’s initial tilt and another during planetary migration. In over 85% of simulations, Uranus’s regular moons do not survive the gravitational chaos intact, suggesting that they may have reformed from debris after violent collisions.

The study supports a primordial setup with four ice giants—two where we now see Uranus and Neptune, plus two others of similar mass. One was ejected, leaving behind only indirect clues in the current layout of planets and moons. While the models can’t capture every possible scenario, the consistency with observed features strengthens the case for a more crowded and violent early solar system.