Largest tilt of any planet

The planet with the greatest axial tilt is Uranus, which spins on an axis that is slanted 97.77 degrees relative to its orbital plane. It has been theorized that Uranus' extreme tilt is the consequence of a collision with an Earth-sized planet, likely during the early formation of the Solar System, which knocked Uranus over on its side.

The term "orbital plane" refers to the ellipse described by a solar-system object as it moves through space. If this is visualized as a flat disc, the orbital axis is a line extending up a right angle from the centre-point.

It can be argued that this record is actually held by Venus. Depending on how you define certain key terms, Venus either has an axial tilt of 2.64 degrees, but spins in retrograde (i.e. backwards) or it spins normally, but has an axial tilt of 177.36 degrees (i.e., it's upside down).

The uncertainty here lies in the two possible definitions of what constitutes a planet's "north" pole.

The first definition uses each planet's direction of rotation as its reference point. According to this rule, you're on the "north" pole if the surface appears to be rotating counter-clockwise, while you're at the "south" pole if it's rotating clockwise. This is often illustrated with a rule of thumb: curl the fingers of your right hand and extend your thumb – your fingers are pointing in your hand-planet's direction of rotation, and your thumb is pointing towards its north pole.

The second, easier to grasp, definition uses something called the "invariable plane" as its reference point. This is the average orbital plane of all the bodies in the solar system. Per this definition, a planet's north pole is simply the pole that is pointing above this solar centreline (i.e., towards the same hemisphere as the Earth's north pole).

For most planets, these two definitions lead to the same conclusion, but for Uranus and Venus things are a little more complicated. The International Astronomical Union – the organization that makes sure the world's astronomers are all on the same page – uses the second definition when talking about planets, so that's the one we've used for this record.