All planets tilt on their axes. Earth's own tilt of 23-1/2 degrees is the reason people at extreme Northern and Southern hemispheres experience seasons, and why summer days in Alaska are 23 hours long.

The planet Uranus is an extreme example of this tilting, however. Uranus tilts at 98 degrees, essentially rolling along like a bowling ball. Scientists now think they know why.

Originally, the theory was that Uranus had been knocked over in a collision with a small, rocky planet during the early years of the solar system. However, recent observations of other solar systems indicate that the planets in our solar system - particularly the gas giants - were much close together than they are now. Each one's powerful gravitational pull had a profound effect on the others. These gravitational interactions caused the planets to tilt on their axes as they moved into their present positions. What's more, the tilts of the gas giants tend to remain stable over time, while those of the inner "rocky" planets can shift. Earth' own tilt is held in place by the gravity of its moon, which is larger in proportion to its parent planet (25% the size of Earth) than any other moon in the solar system.

Mars' tilt, however, could very well change in the coming eons, since its moons are quite tiny - only about 7 - 15 kilometers across.