What is a planet?

The International Astronomical Union (IAU) came up with three criteria that a celestial body has to meet to qualify as a planet:

1 The body must orbit the Sun. A tug from the Sun's gravity causes the current eight planets to revolve around the fiery body along distinct paths.

2 The body must have enough mass to become shaped into a sphere. As a planet spins around its axis, gravity's center-pulling force molds the planet into a sphere. The greater an object's mass, the greater its gravity, and the more likely it is that it will form into a sphere.

3 The body's path around the Sun must be clear of other celestial bodies. Scientists think that as a planet forms, its gravity pulls in surrounding material, such as dust and gas, causing the planet to grow. Pluto does not meet this standard. As a resident of the Kuiper belt, it occupies the same area of space as many other large rocky bodies.