Why are Uranus and Neptune so different?


Uranus and Neptune are similar in many ways, yet so different from each other in terms of how they orbit and how their satellites move around them. Astronomers from the University of Zurich have studied the solar system’s two giant ice planets and discovered their main distant characteristics.

In their new study, published in the journal Monthly Notices of the Royal Astronomical Society,  the scientists conducted computer simulations which demonstrated that a series of collisions with large, rocky bodies could be the reasons why these two planets differ from any other. 

How are they alike?

Uranus and Neptune have masses, sizes and distances similar to the sun and for a long time, scientists believed that these two planets also formed themselves in a similar way. 

"However, there are also striking differences between the two planets that require an explanation," notes Christian Reinhardt, who has studied both planets along with PlanetS members Alice Chau, Joachim Stadel and Ravit Helled, who are all working at the University of Zurich.

Stadel explains that “Uranus and its major satellites are tilted about 97 degrees into the solar plane and the planet effectively rotates in retrograde with respect to the Sun.”

The most obvious difference between the two outermost planets in the solar system is their rotation angle. Unlike Uranus, Neptune and most of the rest of the planets have rotations that are aligned with their orbits.

Difference in satellites

The main satellites of Uranus move in regular orbits that are inclined with the planet, suggesting that they were formed from a disk, similar to the Earth's Moon. Triton, on the other hand, which is the largest satellite in Nepture is very steep and therefore, very likely to be an object captured by gravity. 

The rest of Neptune’s moons have an irregular shape and appear to have an internal heat source.

Planetary scientists conducted simulations to test how collisions could diversify planets starting in a similar way. Thus, each planet was hit by a planetoid between one and three times the mass of the Earth, a common event in the formation of star systems. The key difference was in how these collisions developed.

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

Researches are still in a very immature state, as scientists still need to connect these possible collisions with simulations of the solar system’s formation.

Reference: "Bifurcation in the History of Uranus and Neptune: The Role of Giant Impacts," Christian Reinhardt et al., 2019 Nov. 22, Monthly Notices of the Royal Astronomical Society [https://doi.org/10.1093/mnras/ stz3271, preprint: https://arxiv.org/abs/1907.09809].

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