Why Does Uranus Spin on Its Side?

Step 1 of 4

Planets spin roughly upright.

They all formed from the same flat spinning disc, so they all turn in much the same direction with only a modest lean. Earth's lean of 23 degrees is what gives us seasons.

Step 2 of 4

Uranus is tipped past ninety degrees.

It is not leaning. It has gone right over, so its axis lies almost flat in the plane of its orbit. Technically it is upside down and turning backwards.

Step 3 of 4

The likely cause is an enormous collision.

Something roughly Earth-sized is thought to have struck it while the solar system was still forming, knocking it over. Its moons and rings are tipped to match, which fits a single early shove.

Step 4 of 4

So its seasons are unlike anywhere else.

For a quarter of its orbit one pole points almost at the Sun and the other sits in darkness. Since a year there lasts 84 Earth years, that is a summer over forty years long.

It is just barely visible without a telescope, and was mistaken for a star for centuries before anyone realised it was moving.