1. Orbits a star: Earth orbits the Sun.
2. Has enough mass for its own gravity to pull it into a nearly round shape: Earth's gravity has pulled it into an almost perfectly spherical shape.
3. Has cleared its neighborhood: Earth's gravity has cleared its orbital path of most other celestial bodies.
The IAU defines a planet as a celestial body that:
* Is in orbit around the Sun.
* Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
* Has cleared the neighbourhood around its orbit.
Earth fulfills all three criteria, making it a planet.
