Japanese astronomers explain the origins of Uranus’ weirdness


The ice-giant planet Uranus has its spin axis tilted by 98 degrees. Its satellite system is equally inclined and believed to be the result of the consequence of a giant impact.

As Uranus rotates and orbits the sun, it keeps its poles aimed at fixed points with relation to this sphere, so it appears to roll around and wobble from an Earth observer’s perspective.

Like Saturn, Uranus also has a ring system and 27 moons that orbit around its equator. The moons are also tipped relative to the plane of the ecliptic.

Now, a team scientists at the Tokyo Institute of Technology led by Professor Shigeru Ida from the Earth-Life Science Institute (ELSI), has explained the origins of Uranus’ unusual set of properties.

Their study recommends that early in the history of our solar system, Uranus was struck by a small, icy planet about one to three times the mass of the Earth, which spilled the young planet and left behind its idiosyncratic moon and ring system as a smoking gun.

Scientists concluded this after building a novel computer simulation of moon formation around icy planets. Most of the planets in the solar system have moons of various sizes, orbits, compositions, and different properties, which scientists believe can help clarify how they formed.

There is strong evidence that Earth’s single moon formed when a rocky Mars-sized body hit the early Earth almost 4.5 billion years ago. This idea explains a great deal about the Earth and the moon’s composition, and the way the moon orbits Earth.

However, such massive collisions are frequent in the early solar system.

Unlike earth, Uranus must have experienced different impacts simply because Uranus formed so much farther from the sun.

Earth formed closer to the sun, where the environment was hotter, it is mostly made of what scientists call ‘non-volatile’ elements, meaning they don’t form gases at normal Earth-surface pressures and temperatures; they are made of rock. In contrast, the outermost planets are composed mainly of volatile elements like water and ammonia. Even though these are gases or liquids under Earth-surface temperatures and pressures, at vast distances from the sun, they are frozen into solid ice.

In the case of Uranus, a vast, icy impactor had the option to tilt the planet, give it a quick rotation period, and the extra material from the collision remained vaporous longer. The largest mass body, which would become Uranus, then collected the vast majority of the extras, and along these lines, Uranus’ moons are small.

More precisely, the ratio of Uranus’ mass to Uranus’ moons’ masses is higher than the ratio of Earth’s mass to its moon by a factor of more than 100.

The model reproduces the current configuration of Uranus’ satellites.

Professor Ida says, “This model is the first to explain the configuration of Uranus’ moon system, and it may help explain the configurations of other icy planets in our solar system such as Neptune. Beyond this, astronomers have now discovered thousands of planets around other stars, so-called exoplanets, and observations suggest that many of the newly discovered planets known as super-Earths in exoplanetary systems may consist largely of water ice, and this model can also be applied to these planets.”

Journal Reference:
  1. Uranian satellite formation by the evolution of a water vapor disk generated by a giant impact. DOI: 10.1038/s41550-020-1049-8
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