To the eye alone, Uranus is a dull planet. It looks like a featureless blue-green ball, with few of the dynamic cloud patterns that decorate Jupiter and Saturn. Yet Uranus may have the most interesting history of any planet in the solar system: A collision soon after its birth may have knocked

The poles of most planets aim away from the plane of the solar system; like Earth's poles, they point roughly north and south. But the poles of Uranus point east and west, close to the plane of its orbit around the Sun. As a result, its north pole aims at the Sun at the beginning of northern summer, and away from the Sun at the start of southern summer. That gives the planet's northern and southern hemispheres 42 years of sunlight followed by 42 years of darkness. This sideways tilt also creates weather patterns unlike those on any other planet, with winds and clouds changing directions with the seasons.