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According to more than a hundred computer simulations, two additional planets once circulated in the outer regions of the solar system before being ejected into interstellar space. Their passage would have profoundly reshaped the orbits of their neighbors.
This hypothesis is based on the famous "Nice model," proposed in 2005. It describes a great instability of the current giant planets — Jupiter, Saturn, Uranus, and Neptune — which would have migrated from their original positions to reach the orbits we know today. This migration, caused by close encounters, would thus be at the origin of the current configuration of the solar system.
A revised version of the model already considered the presence of a fifth giant, later ejected. The new study goes further by systematically testing the effects of these encounters on the moons of the current giants.
To gain clarity, the researchers reviewed a database of 100,000 simulations of the young solar system's evolution. They extracted 122 that replicated the final configuration of the current giant planets. In about two-fifths of these scenarios, the solar system had five giants, and six in the others, making the latter the most likely case. By "replaying" planetary encounters with the moons in place, they were able to observe the fate of these satellites.
The results obtained are surprising. Simulations with two additional giants stabilized Jupiter's moons, while those with only one additional giant preserved Uranus's moons. In other words, the satellites of Jupiter and Uranus appear to have been preserved during distinct events. The orbital dance of Jupiter's Galilean moons — Io, Europa, and Ganymede — currently in a 1:2:4 resonance, argues in favor of two additional giants.
These additional planets, migrating between the gas giants and Uranus, would have destabilized Uranus's moons. The result: fragmentation of its satellites and vaporization of volatile materials like ice. This ice would then have accumulated on the remaining orbiting debris, explaining why Miranda, a moon of Uranus, contains 50% more ice than the others. But this is not the only clue: two simulations managed to preserve the satellites of both planets with only one additional giant. Future work will distinguish between these hypotheses.
What do we know about these lost planets? Little, except their mass. In the five-giant scenario, the single additional planet had a mass similar to Neptune's. With six giants, the two additional planets were "super-Earths," with masses between those of Earth and Neptune. Their physical properties were likely similar to those of Uranus and Neptune. The researchers will continue to study Uranus's moons for traces of these distant disturbances.