How unusual is it
Most planets have only a slight axial tilt: Earth's is 23 degrees, which is enough to create the seasons. Uranus is tilted almost 98 degrees, meaning its axis lies nearly in the plane of its orbit, and its poles take turns pointing straight at the Sun.
These numbers lead to the strangest part. At either pole of Uranus, the Sun does not set for 42 years straight, then does not rise for another 42. No other planet in the Solar System has seasons like this.
23-degree tilt. A top: the axis points to the side, and the seasons are mild
98-degree tilt. A wheel: the pole points at the Sun, and the seasons last 42 years
Where did the tilt come from
The main theory is an impact. The young Solar System contained many large bodies, and a collision with an object the size of Earth could have knocked the planet over. This theory has one weak point: a single impact should have scattered the moons, but they orbit neatly.
The behavior of the moons and rings offers the main clue. They all orbit in the plane of the equator, meaning they are tilted along with the planet. So the tilt appeared early, while there was still a disk of material around Uranus from which the moons later formed.
That leads to refined versions of the theory: not one impact but several, or a slow tipping caused by the influence of a moon that later disappeared. Calculations show that each scenario is possible, but the available evidence cannot distinguish between them.
Why we know so little
Only one spacecraft has approached Uranus, and that was in 1986. It flew past in a matter of hours and managed to capture just one snapshot. Everything else is known from observations made at a great distance.
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Sources
- Uranus's axial tilt and orbital parameters are precise measurements. Hypotheses about the tilt's origin have been debated since the 1990s
- the only flyby was carried out by the Voyager 2 spacecraft in 1986