Why Uranus is twisted

Uranus is twisted in a unique way among the planets of our solar system. Uranus’ moons and rings are also aligned in this way, suggesting that the system formed after a catastrophic impact. (Image: Lawrence Sromovsky, University of Wisconsin-Madison / W.W. Keck Observatory / NASA)

Astronomers have been looking at number seven in our planetary system for a long time: Why does Uranus have an axis of rotation tilted over 90 degrees and so strange small moons? Now a simulation shows for the first time that a primeval collision could have given the Uranus system these bizarre features: An icy celestial body of around one to three times the mass of the earth crashed into the youthful Uranus and caused the twisted system.

Usually in our solar system the following applies: The axes of rotation of the planets tend to be oriented perpendicular to their orbit. As a result, most sunlight hits the equatorial region. But a planet is completely out of line – Uranus orbits the sun as it were lying on its side: its axis of rotation is rotated by 98 degrees. As a result, the sun is almost at the zenith for half a year of Uranus over each of the poles of the planet. The ring system and the 27 moons of the ice giant are also tilted accordingly: they circle the equator of the icy planet, which is about four times the size of the earth.

For some time now, astronomers have suspected that a primeval collision with a protoplanet caused the tilt. So far, however, there has been a problem: Simulations of corresponding scenarios could not coherently represent the creation of today’s Uranus system. Above all, the models led to an excessive mass of the debris disc after the impact – it did not match the total mass of today’s Uranus moons.

Inconsistencies resolved

The researchers led by Shigeru Ida from the Tokyo Institute of Technology, on the other hand, have now factored in factors that lead to a coherent model of the origins of Uranus and its moons. Her model now plausibly shows that in the early history of our solar system, Uranus was hit by an ice planet of one to three times the mass of the earth. As a result, the young planet overturned and the moon and ring system developed as we see it today.

To explain their changed model approach, the scientists first turn their eyes to the earth: It is currently assumed that 4.5 billion years ago the impact of a Mars-sized rock planet led to the formation of our planet and its moon. Appropriate models explain the compositions of both celestial bodies and the way in which the moon orbits the earth. Astronomers assume that such massive collisions occurred more frequently in the early solar system and shaped the characteristics of some planets and their satellites. But in the case of Uranus, special factors played a role because it was so far from the sun, Ida and his colleagues say.

Mass loss through evaporation

Since the earth was created near the sun in comparatively hot conditions, it largely consists of substances that are not gaseous under the prevailing pressure and temperature conditions. In contrast, the outer planets are largely made up of volatile substances such as water or ammonia. Due to the low temperatures, these substances form ice there. As Ida and his colleagues explain, the impact of protoplanets on these ice planets was therefore completely different than in the case of rock planets. At Earth, the heat caused a mass that collapsed relatively quickly due to the collision. This allowed the moon to capture a large amount of the debris due to its own gravity.

In contrast, in the case of Uranus, a large amount of matter evaporated after the collision because water and Co remained gaseous for a long time. In concrete terms, this means that the collision with the protoplanet tilted Uranus and allowed most of the material to evaporate. His body, ring system and moons were then formed from this cloud. However, because a large part of the cloud was lost in space, the moons remained small. This effect explains why the mass ratio of Uranus to its moons is more than a hundred times greater than the mass ratio of the earth to its moon.

“Our model is the first to explain the formation of the Uranus lunar system,” sums up Ida. “It could also help explain the characteristics of other systems of icy planets in our solar system like Neptune,” said the scientist. And he looks even further into space: thousands of planets are now known about distant stars, which are probably icy gas giants like Uranus. So the new model could also shed light on its history, says Ida.

Source: Tokyo Institute of Technology, technical article: Nature Astronomy, doi: 10.1038 / s41550-020-1049-8

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