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This is one of the great unanswered questions about our solar system.
Some astronomers think that this is just the way Uranus developed, but that
doesn't seem to me to be consistent with how most scientists think that the
solar system developed. The other widely-held belief is that the tipping is the result of a collision
between Uranus and some other body long ago. There are some problems with this
explanation too: if such a collision took place, why is Uranus' orbit so nearly
circular (just like the orbits of most of the other planets, which presumably di d not suffer the
same sort of collision)? Similarly, assuming Uranus' moons formed before the
collision, why are their orbits also nearly circular? They orbit Uranus in
essentially the same plane as Uranus' equator.
As you can see, we don't have to go to the ends of the universe to find puzzling
questions.
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