Asked by AP News to describe Uranus, scientist Scott Sheppard of the Carnegie Institution gave a three-word answer. “It’s very strange.” And his description is apt, since Uranus is the only planet in our solar system that is actually spinning around on its side. But what’s the explanation of this weird phenomenon?

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Scientists at the University of Durham in the north of England have been searching for an answer to that very question. Before we get to the results of their research, let’s get to know Uranus a little better. Butt of a thousand schoolboy jokes, Uranus is the seventh planet from the Sun, 30 times farther from that star than our planet. Some 63 Earths could fit inside the space occupied by Uranus. So it’s big.

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It’s also unremittingly hostile. A massive toxic sea of icy ammonia, methane and water surrounds its relatively small rocky core. Temperatures hover at about -371 °F, making it the Solar System’s coldest planet. Uranus is encircled by 13 rings and has, at the last count, no fewer than 27 moons. But what intrigues astronomers most is its rotation on its side.

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A team of scientists from Durham University’s Institute for Computational Cosmology, led by Jacob Kegerreis, think they just might have the answer. Speaking to the New York Post newspaper, Kegerreis described how they’d approached the problem. “We ran more than 50 different… scenarios using a high-powered supercomputer to see if we could recreate the conditions that shaped the planet’s evolution,” he explained.

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And the experiments showed that the most likely chain of events was that three to four billion years ago, a massive rock – perhaps twice Earth’s size – crashed into Uranus while it was still an infant in planetary terms. That “cataclysmic collision,” as Kegerreis describes it, knocked the planet off its original axis, and it has stayed on its side ever since.