Which planet is known to rotate on its side?

Uranus is known to rotate on its side because it has an extreme axial tilt of about 98 degrees. This unique tilt means that its rotation axis is almost parallel to its orbit around the Sun, which is significantly different from most planets in the solar system. As a result, Uranus experiences unusual seasons because its poles are directed almost directly at the Sun during various parts of its orbit. This tilted rotation also affects its atmospheric dynamics and temperature distribution across the planet.

The other planets mentioned have much smaller axial tilts compared to Uranus. Earth, for instance, has a tilt of about 23.5 degrees, giving rise to seasons but maintaining a more upright position relative to its orbit. Mercury has a negligible tilt of around 0.034 degrees, while Venus has a tilt of about 177.4 degrees, which technically means it is rotating retrograde rather than on its side. However, its rotation is not considered significant enough to classify it as "on its side" like Uranus. Thus, Uranus stands out as the planet that rotates in such an unusual manner.