"Uranus" by NASA/JPL-Caltech / Public Domain

Origins of Uranus’ oddities explained by Japanese astronomers

Apr 7, 2020

By Tokyo Institute of Technology

The ice giant Uranus’ unusual attributes have long puzzled scientists. All of the planets in the solar system revolve around the sun in the same direction and in the same plane, which astronomers believe is a vestige of how our solar system formed from a spinning disc of gas and dust. Most of the planets also rotate in the same direction, with their poles orientated perpendicular to the plane in which the planets revolve. However, uniquely among all the planets, Uranus is tilted at about 98 degrees.

Instead of thinking about the reality of stars spread in all directions and at various distances from the Earth, it is easier to understand by envisioning the celestial sphere. To picture what the  is, look up at the night sky and imagine that all of the stars you see are painted on the inside of a sphere surrounding the . Stars then seem to rise and set as the Earth moves relative to this “sphere.” 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. Uranus also has a ring system like Saturn’s, and a slew of 27 moons that orbit around its equator; thus, they are also tipped relative to the plane of the ecliptic. The origins of Uranus’ unusual set of properties has now been explained by a research team led by Professor Shigeru Ida from the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology. Their study suggests that early in the history of our solar system, Uranus was struck by a small, icy planet roughly one to three times the mass of the Earth, which tipped the young planet over and left behind its idiosyncratic  and ring system as a smoking gun.

The team came to this conclusion while constructing a novel computer simulation of moon formation around icy planets. Most of the planets in the solar system have moons of different sizes, orbits, compositions and other properties, which scientists believe can help explain how they formed. There is strong evidence that Earth’s own 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.

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