What’s Round and Reflective? Maybe It’s a Planet, Maybe Not

What’s a planet anyway?

The International Astronomical Union, the group charged with coming up with definitions and names for astronomical objects, has never had a formal definition for “planet.” It’s never really needed one. Many textbooks and encyclopedias don’t even formally define the word. Why should they? It’s always been so simple.

For centuries, planets were considered to be big, round objects that orbited a sun and reflected light but did not give off light of their own. And we knew of only nine: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

But after the discovery of several strange, rule-breaking objects, the old definition was clearly inadequate. The first planet outside of our solar system was found in 1995. Scientists have now found 100 extrasolar planets. They have found nearly 20 objects that look just like planets, but float free in space.

They have found tiny, failed stars called brown dwarfs that resemble Jupiter a lot more than they resemble stars. Pluto, once alone in its neighborhood, now has several objects nearby that approach it in size and behave much more like planets in the way they orbit the sun.

Some people now call comets and asteroids “minor planets.” And small bodies that may one day coalesce into planets go by the moniker “planetesimals.”

“It’s not that easy anymore,” said astronomer Brian Marsden, an International Astronomical Union director who wrangles with such issues.

A few scientists have tried to come up with some scientific basis for determining what is or isn’t a planet. A theory by the Southwest Research Institute’s Alan Stern suggests that anything big enough to become round through gravitational forces is a planet.

By that measure, anything more than about 185 miles across--including Quaoar--would be deemed a planet. UC Berkeley’s Gibor Basri suggests that any spherical object not capable of nuclear fusion--a “non-fusor”--circulating around an object like a star that is capable of fusion--a “fusor”--should be called a planet.

Confused? You’re not alone. The IAU spent months trying to perfect the definition of a planet. In the end, it came up only with what they call a “working definition” that it will revise as needed. A planet: “Any non-fusor object that orbits a star.”