"Drunken" Uranus got sideways tilt from several hits

Credit: www.ifa.hawaii.edu

Uranus has an uncanny characteristic of rotation axis making an angle 98 degree with the axis perpendicular to the solar system plane. In other words, it turns sideway. No other planet has such a huge angle like this, for example, Jupiter is at 3 degree, Earth is at 23 degree, and Saturn and Neptune rotates at 29 degree. How could Uranus get this special configuration?

Theory for forming a standard planet says that Uranus, Neptune and cores of Jupiter and Saturn, are formed by accreting mass from only small objects in their accretion disk. That is why they should not have experienced major collision. However, recent studies might make us re-think about the formation of giant gas planet over again.

Scientists have long suspected that Uranus experienced a big collision in the past that tilted it axis. The collision should have a size many times larger than the Earth. The impact should occurred in the beginning phase of solar system when Uranus was still covered with disk of dusts and gases. Also, the collision creates moons of Uranus. The disk turn into a fat donut shape around the new equatorial axis which is tilted 98 degree. Other minor collisions inside the disk should make the donut more flat and create moons that are also tilted.

However, the computer simulation shows another result. From just only one major collision, moons of Uranus should have retrograde motion which is totally opposite of what they actually are. If Uranus were hit at least 2 times, motion of the moons are better explained and simulated. That is, early state of solar system might be a busier and more violent place than we expected.

From:

space.com: planet Uranus got sideways tilt from multiple impacts

astronomy.com: series of bumps sent Uranus into its sideway spin