Uranus appears to be a featureless blue ball at first glance, but upon closer inspection, this gas giant of the outer solar system is quite strange. For starters, the planet rotates on its side for reasons that scientists are still trying to figure out. The most likely explanation is that it was involved in one or more titanic collisions in the distant past. In any case, Uranus' tilt distinguishes it from the other planets in the solar system.