Planets, dwarf planets, and Makemake
July 22nd 2008
In August 2006, the International Astronomical Union (IAU) voted on a definition of “planet”. To be considered a planet,
- The object must be large enough to be spherical under its own gravitational force. This is a general principle: if a chunk of rock or a ball of gas has enough mass, it tends to become spherical just because that’s the way gravity works. Smaller objects, like pebbles or baseballs, can be spherical for other reasons, namely erosion or forces other than gravity, so some Solar System bodies lie on the boundary of this criterion. Two examples of this are the asteroids Vesta and Pallas, which appear to be roughly spherical, but might not quite be massive enough to count.
- The object must orbit the Sun directly. Certain moons, like Ganymede (which orbits Jupiter) and Titan (which orbits Saturn), are actually more massive than Mercury, and Titan has a thicker atmosphere than Earth. However, since they orbit planets rather than following a direct orbit around the Sun, they don’t count as planets in their own right.
- The object must dominate its orbit. In other words, the body must be the most massive object in its neighborhood. It also, by virtue of its gravity, cleared most of the smaller objects out of its path, so that impacts won’t substantially change its orbit.
- The object must be small enough that it doesn’t emit its own light via nuclear fusion. Planets don’t emit their own light: we see Venus, Jupiter, and the like by the light they reflect from the Sun. Stars like the Sun, on the other hand, compress the hydrogen atoms in their cores so much that they fuse into helium, releasing a huge amount of energy in the form of light. This criterion isn’t terribly important for our Solar System, since Jupiter (by far the largest planet) comes nowhere near the threshold for becoming a star.