Lost Ice Giants May Have Shattered Uranian Moons, New Simulations Reveal

Using over 100,000 early-Solar-System models, Johns Hopkins researchers focused on 122 runs that reproduced today’s giant-planet orbits to test how one or two extra ice-giant planets would affect Jupiter’s and Uranus’s moons.

Live ScienceThe Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the

The Nice model (2005) explains the current spacing of Jupiter, Saturn, Uranus and Neptune via a planetesimal-driven instability; 2011 updates permitted one or two additional ice giants that would later be ejected. Lead author Matthew Clement (Johns Hopkins University Applied Physics Laboratory) and co-author Nathan Kaib (Planetary Science Institute) led the study.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the

Each run tracked 1,000 planetesimals, the four giants and any extra bodies for 20 million years. Roughly 40 % of the selected runs began with five giants; the rest with six. In five-planet cases the extra planet’s mass matched Neptune’s; in six-planet cases masses ranged 1–17 M?, classifying them as super-Earths.

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With two extra planets, Jupiter’s Galilean moons stayed stable while Uranus’s moons suffered collisions and ejections; with a single extra planet the opposite pattern emerged. Only one specific trajectory preserved both satellite systems, suggesting that versions of the Nice model requiring multiple extra ice giants may be untenable without invoking a rare dynamical path or post-instability reshaping of Uranus’s moons.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of theLive Science

Clement described the work as a systematic test of how giant-planet close encounters affect satellite stability. Kaib noted that the missing planets’ masses would have been similar to Uranus and Neptune, implying comparable physical properties.

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The authors note that the difficulty of preserving both moon systems may require revising the Nice model or invoking an unlikely evolution with few deep encounters involving Uranus, suggesting alternative instability timelines.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the

The simulations yield opposite outcomes: two-planet scenarios protect Jupiter’s moons but not Uranus’s, while one-planet scenarios do the reverse. Only a single trajectory reconciles both, leaving open whether the Solar System followed that unlikely path, and no direct evidence of the lost planets exists.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the

• "systematically tested the effects of giant-planet close encounters on the orbital stability of their satellites," — Matthew Clement, Johns Hopkins University Applied Physics Laboratory.

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• "Given that the masses are not too different from Uranus and Neptune, [the long-lost planets'] physical properties probably resembled those planets," — Nathan Kaib, Planetary Science Institute.

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• "The simulations showed that the extra giant planet (or planets) ping-ponged among their neighbors, before finally being kicked out of the system." — Authors, Johns Hopkins University.

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• "Only one scenario allowed the moons of the two planets to consistently survive." — Authors, Johns Hopkins University.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the

The team will broaden initial-condition ranges and examine Uranus’s moons for collision signatures. Additional high-resolution simulations will test alternative instability pathways that might preserve both satellite systems, potentially refining the Nice model.

The Solar System may have lost 2 giant planets, and new simulations now raise an even bigger problem involving Uranus’s moons, which may not have survived intact through the gravitational chaos of the