Why Uranus and Neptune are different

According to Researchers at the University of Zurich, the two outermost giant planets of the solar system, Uranus and Neptune, have similar masses but are strikingly different.

In size, possibly bulk composition, and their large distance from the Sun they are similar and clearly segregated from the inner terrestrial planets and the gas giants Jupiter and Saturn. "However, there are also striking differences between the two planets that require explanation," said researcher Christian Reinhardt.

Joachim Stadel, another researcher who was part of the study, explained: "For example, Uranus and its major satellites are tilted about 97 degrees into the solar plane and the planet effectively rotates retrograde with respect to the Sun." Also, satellite systems are different. Uranus' major satellites are on regular orbits and tilted with the planet, which suggests that they formed from a disk, similar to Earth's Moon. Triton instead, Neptune's largest satellite, is very inclined and therefore most likely a captured object. Finally, they could also be very different in terms of heat fluxes and internal structure.

Scientist Alice Chau said: "It is often assumed that both planets were formed in a similar way."

Since impacts are common during the formation and early evolution of planetary systems a giant impact was proposed as the origin of this dichotomy. But prior work either only investigated impacts on Uranus or was limited due to strong simplifications in the impact calculations.

For the first time, the team of scientists at the University of Zurich investigated a range of different collisions on both planets using high-resolution computer simulations. Starting with very similar pre-impact Uranus and Neptune they showed that an impact of a body with 1-3 Earth masses on both planets can explain this dichotomy.

In the case of Uranus, a grazing collision can tilt the planet but does not affect the planet's interior. On the other hand, a head-on collision for Neptune strongly affects the interior but does not form a disk, and is therefore consistent with the absence of large moons on regular orbits. Such a collision, which remixes the deep interior, is supported by the largely observed heat flux of Neptune.

Ravit Heeled summarised and said: "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."

Future NASA and ESA missions to Uranus and Neptune can provide new key constraints on such a scenario, improve our understanding of the formation of the solar system, and provide a better understanding of exoplanets in this mass regime.