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How might a giant collision have tilted Uranus onto its side?
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Uranus
Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17, 1986. In colours visible to the unaided human eye, Uranus is a bland, nearly featureless sphere (left). In a colour-enhanced view processed to bring out low-contrast details, Uranus shows the banded cloud structure common to the four giant planets (right). From the polar perspective of Voyager at the time, the bands appear concentric around the planet's rotational axis, which is pointing nearly toward the Sun. Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera.
The extreme tilt of Uranus, which causes its spin axis to lie nearly within the plane of its orbit, is widely attributed to violent events during the final stages of the solar system's formation.1
There are two primary theories regarding how this occurred:
- Giant Impacts: The prevailing hypothesis suggests that during the final stages of planetary accretion, Uranus was struck by one or more bodies comparable in size to the present-day planets. These violent collisions are thought to have knocked the planet onto its side.1
- Moon Collision: An alternative theory proposes that a Mars-sized moon orbiting Uranus in a retrograde direction—opposite to the planet's own spin—eventually crashed into it, forcing the planet into its current orientation.1
These theories reflect the chaotic nature of the early solar system, where collisions were instrumental in shaping the physical characteristics of the planets we observe today.1