ON THIS PAGE

    What is a Planet?

    This seemingly simple question doesn't have a simple answer. Everyone knows that Earth, Mars and Jupiter are planets. At least, they are for now. Both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

    1. It must orbit a star (in our cosmic neighborhood, the Sun).
    2. It must be big enough to have enough gravity to force it into a spherical shape.
    3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

    Discussion—and debate—will continue as our view of the cosmos continues to expand. › More

    Eyes on the Solar System Ad

    Explore in 3D—Eyes on the Solar System

    Eyes on the Solar System lets you explore the planets, their moons, asteroids, comets and the spacecraft exploring them from 1950 to 2050. Ride with the Curiosity Rover as it lands on Mars or fly by Pluto with the New Horizons spacecraft all from the comfort of your home computer.

    Eyes on the Solar System ›

    Solar System News