A massive moon that orbited Uranus in the past may explain the planet's extreme tilt (Image: NASA/ESA/M. Showalter/SETI Institute)
From New Scientist:
Please try to resist the childish jokes, but the fact is that the odd tilt of Uranus may be the result of a particularly large moon.
Uranus spins on an axis almost parallel with the plane of the solar system, rather than perpendicular to it – though why it does this nobody knows. One theory is that the tilt is the result of a collision with an Earth-sized object, but this "hasn't succeeded in explaining much of anything", says Ignacio Mosqueira of the SETI Institute in Mountain View, California. Why, for example, are the orbits of Uranus's 27 known moons not also tilted?
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