Source: https://science.nasa.gov/uranus/facts
Title: uranus
Selected source excerpts (not the complete document).

Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees. This may be the result of a collision with an Earth-sized object long ago. This unique tilt causes Uranus to have the most extreme seasons in the solar system. For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter.
