1. In orbit around the Sun.
2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
3. Has cleared the neighbourhood around its orbit.
Pluto, previously considered a planet, was demoted in 2006 because it doesn't meet the third requirement. It shares its orbital space with other icy bodies.
So, there are still eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
