A Moment of Science

The Tenth Planet?

On last count, there were nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Soon, however, we might need to add a tenth planet to the solar lineup. Astronomers have announced the discovery of what they have exotically named 2003 UB313, an object that might be a planet orbiting the sun beyond Pluto. The name isn’t quite as catchy as Saturn, but it will have to suffice until astronomers decide whether what they’ve found is, in fact, a planet.

What the term “planet” means, exactly, is not entirely clear. In the most general sense it is an orb, consisting of various combinations of rock, gas, ice, and other materials that orbit a star. Since not all large bodies that orbit a star are planets, like asteroids and comets, the main criterion for what makes a planet is size. In terms of size, 2003 UB313 makes a decent showing. According to the astronomers’ measurements, its bigger than Pluto. That’s not saying much, though. Pluto is the runt of the solar system, smaller even than our moon. Some astronomers argue that Pluto is really a comet, or just a good-sized chunk of rock and ice. This would mean that 2003 UB313 is also more like a comet or large asteroid than a planet.

Astronomers are being cautious because what’s at stake is nothing less than our understanding of what a planet is. If the newly found object is determined to be a planet, then there might be many more orbital objects of similar size that would also demand planet status. It’s hard enough remembering the names of the nine planets, let alone a hundred more.

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