Cataclysmic Collisions Have Caused Uranus And Neptune To Look Different

Uranus and Neptune, respectively, photographed by Voyager 2. Credit: NASA/JPL

The ice giant planets of the Solar System, Uranus and Neptune, are similar in many ways but also surprisingly different. Astronomers have spent a long time trying to figure out where their peculiarities come from – and now they think the source is two different giant impacts.

The study, published in the Monthly Notices of the Royal Astronomical Society, investigated how cosmic collisions might have separated these planets. Astronomers knew that Uranus must have had a major impact given that it orbits on its side, but the same has not been studied for Neptune. Both planets have similar bulk compositions, size, and mass, but Neptune is about 18 percent heavier.

“However, there are also striking differences between the two planets that require explanation,” lead author Christian Reinhardt, from the University of Zurich, said in a statement.

Uranus has a regular set of moons formed in an accretion disk like our Moon, seems to be in equilibrium with the energy it receives from the Sun, and its 97-degree tilt is unique among the planets. Neptune, on the other hand, has captured its largest moon, Triton, from a population of objects that orbit around and beyond its orbit, snagging it within the pull of its gravity. The other moons of Neptune are also captures, have an irregular shape, and seem to have an internal heat source.

In the study, the planetary scientists ran simulations to test how impacts can diversify ice giant planets that start out similar to each other. Each planet was hit by a planetoid between one and three times the mass of Earth – an event believed to be rather common in the formation of stellar systems. The key difference, however, was how the collisions unfolded.

An oblique impact is the best explanation for the formation of Uranus, according to the researchers, as it would throw material into orbit that would later coalesce into fully-fledged moons. For Neptune, the collision is likely to have happened head-on, mixing its interior.

“We clearly show that an initially similar formation pathway to Uranus and Neptune can result in the dichotomy observed in the properties of these fascinating outer planets,” co-author Ravit Helled, from the University of Zurich, explained.

The two ice giants were visited once by humanity when the Voyager 2 probe flew by them in 1986 and 1989, respectively. New missions could help refine these models and tell us more about how these two planets came to be what they are today.

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