• Uranus: The Ringed Planet That Sits on its Side (Space.com)

In new research coming out of the Tokyo Institute of Technology, researchers have published a model that seems to explain Uranus’ bizarre tilt. This 7th planet from the Sun is knocked on its side, with its rotational pole sitting in the plane of the planets and periodically pointing at the Sun. This is not normal. This is not how planets form. To get into this weird inclination, something had to happen to Uranus. Since this pale blue planet is a swirling mass of gas, we can’t directly see scars left by impacts the same way we can on solid worlds like the Moon or Mars. What Uranus has instead are a set of icy rings and icy moons… and a weirdo tilt.

In their new model, scientists led by Sigeru Ida describe how a collision between a young Uranus and a massive ice world could have both tipped Uranus over and also formed its rings and its 27 moons in the process. The colliding body would have needed to be 1-3 Earth Masses in size, which is larger than any icy body so far found, but is consistent with the kind of world we expect to find out on the edge of the Kuiper Belt, as planet 9. 

While this process was similar in many ways to what happened with our moon, the differences between a rocky impactor and an icy impactor left their marks. The press release explained this so well, we’re just going to quote it: 

“Because the temperature at which water ice forms is low, the impact debris from Uranus and its icy impactor would have mostly vapourised during the collision. This may have also been true for the rocky material involved in Earth’s Moon-forming impact, but in contrast this rocky material had a very high condensation temperature, meaning it solidified quickly, and thus Earth’s Moon was able to collect a significant amount of the debris created by the collision due to its own gravity. In the case of Uranus, a large icy impactor was able to tilt the planet, give it a rapid rotation period (Uranus’ ‘day’ is presently about 17 hours, even faster than Earth’s), and the leftover material from the collision remained gaseous longer. The largest mass body, what would become Uranus, then collected most of the leftovers, and thus Uranus’ present moons are small. To be precise, the ratio of Uranus’ mass to Uranus’ moons’ masses is greater than the ratio of Earth’s mass to its moon by a factor of more than a hundred. Ida and colleagues’ model beautifully reproduces the current configuration of Uranus’ satellites.”

In retrospect, the Earth got kind of lucky in how we got knocked around. We could have ended up as tilted as Uranus, which would have made our seasons much more severe and made the kind of ecosystem we enjoy impossible. As it is, we have a nice tilt that gives us reasonable seasons and a giant moon that gives us a small bit of stability in this crazy universe.