A world on its side
Uranus is the outlier of the Solar System. While other planets spin like tops with modest inclinations, Uranus rolls along its orbital path, knocked over at an extreme axial tilt of 97.8 degrees. This peculiar orientation means the planet experiences the most extreme seasons known. For 42 consecutive Earth years, one pole is bathed in continuous sunlight, while the other faces a long, dark winter. This cycle then reverses, creating a planetary year that lasts 84 Earth years.
The only spacecraft to ever visit Uranus was Voyager 2, which flew past on January 24, 1986. Its brief, five-and-a-half-hour window of close study provided nearly all the detailed data we have. Voyager 2 revealed a world with a complex ring system, discovered 10 new moons, and measured a strange magnetic field. The magnetic field is tilted 59 degrees away from the rotational axis and is significantly offset from the planet's physical center, a feature that might be a result from the planet's violent past.
A catastrophic collision
The leading explanation for Uranus's tilt is the giant impact hypothesis. This theory suggests that billions of years ago, during the chaotic formation of the Solar System, a protoplanet at least the size of Earth—and possibly twice as large—slammed into Uranus. Computer simulations show that such a grazing blow could have been powerful enough to knock the planet on its side while allowing it to retain most of its atmosphere.
This impact event could also explain other Uranian mysteries. The debris ejected from the collision may have formed a thin shell in the upper layers, trapping the planet's internal heat. This would account for why Uranus is the coldest planet in the Solar System, with atmospheric temperatures dropping to -224°C (49 Kelvin), colder even than Neptune, which is farther from the Sun. The debris from the impactor likely coalesced to form the planet's 28 known moons and its faint ring system, which all orbit along its tilted equatorial plane. Some models even suggest two or more impacts were necessary to produce the current configuration of its moons.
A gradual wobble
The impact theory is popular, it is not the only explanation. Alternative models suggest the tilt could have developed more gradually. One idea involves a "secular spin-orbit resonance," where the planet's axial wobble fell into sync with its orbital precession. This could have been driven by gravitational tugs from a massive circumplanetary disk during its formation, or perhaps by a large, long-lost moon that was later ejected from the system. Simulations show a resonance could have tilted the planet up to 70 degrees, with a smaller, later impact needed to finish the job. This resonance theory avoids some of the challenges of the giant impact model, such as why Uranus and Neptune have similar rotation periods, which would be an unlikely coincidence after such a violent collision.