Curiosities: Why is Pluto not considered a planet?

Nov. 15, 2010

Until 2006, astronomers had not carefully defined “planet,” says James Lattis, director of the UW Space Place. Asteroids were not considered planets because they are too small and numerous. Likewise, comets were not considered planets because they are too small and have noncircular orbits that go far outside the plane of the solar system (location of Earth, Jupiter, Saturn, and the other "real" planets).

Although astronomers recognized nine planets, “Pluto had always been suspect because we knew it was small and followed a noncircular orbit that deviates far above and below the plane of the solar system,” says Lattis. By 2006, Pluto was demoted because it was clear that it is one of thousands of objects that occupy the distant Kuiper Belt.

Pluto is too small and its orbit too elliptical to fit that “planet” category, Lattis says. “To be consistent, we’ve developed a new category of ‘dwarf planets’ that includes Pluto and two similar objects.”

Eight planets still satisfy the more rigorous definition of “planet” — a large object with an orbit that is fairly circular and within the plane of the solar system. “Astronomy is a science of discovery, and it’s only fair to expect that we will expand our list of objects — and perhaps our categories as well,” says Lattis.

 “I was surprised at the hubbub raised by Pluto’s reclassification,” says Lattis. “It makes sense to clearly define a scientific term, but in retrospect people are understandably attached to their language, and suddenly telling native speakers that they’ve been misusing a common word is asking for trouble. It would have made more sense to abandon (for scientific purposes) common words rather than to redefine them to contradict their common meaning. Many scientific fields have specialized, clearly defined terminologies, and astronomy should do the same. If you told people they can no longer call a tomato a vegetable (because it’s technically a fruit), you would have a similar problem.”