Which planet rotates on its side?

Uranus is the planet that rotates on its side.

Its rotational axis is tilted about 82 degrees relative to the plane of its orbit, making Uranus appear to roll around the Sun rather than spin upright like Earth. Astronomers sometimes quote the tilt as about 98 degrees under a different pole convention, but both descriptions refer to the same extreme orientation.

This unusual tilt gives Uranus extraordinary seasons. Near a solstice, one pole can face the Sun continuously while the opposite pole remains in darkness. Each pole experiences approximately 42 Earth years of sunlight followed by about 42 years of night. Around the equinoxes, sunlight reaches the equator more directly and day-night cycles become more familiar.

The cause of Uranus’s tilt is not known with certainty. A leading idea is that a collision with an Earth-sized protoplanet during the Solar System’s formation knocked it sideways, although models involving multiple impacts or other gravitational effects have also been studied. Uranus is therefore not literally lying flat; its axis is simply almost parallel to its orbital plane.