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Somebody Define Planet, Please

2003 UB313
Artist concept of 2003 UB313
(also known as the "10th" planet)
Credit: Robert Hurt (IPAC)

The International Astronomical Union (IAU) will be meeting this August to hammer out the final definition of the word "planet." If approved, the definition will be announced in September.

Webster defines the word planet as: any of the seven celestial bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars b (1) : any of the large bodies that revolve around the sun in the solar system (2) : a similar body associated with another star.

So one might ask, what's the big deal? Apparently it's UB313, an object roughly the size of Pluto that orbits the Sun beyond Neptune. The object's discoverer, Mike Brown of Caltech, has argued it should be called a planet. Some astronomers say if UB313 is a planet, then several similar bodies should gain the same status. The number of planets in our solar system could ultimately climb into the thousands as technology improves.

What will the definition be? Will they scratch Pluto from being a planet? What qualifiers will they use to rewrite this definiton? Will they look at mass? Some have wondered if they in fact would include orbit characteristics and formation scenarios? I suppose we won't know until the new committee that includes historians and educators, make their recommendation to the IAU in September. Mark your calenders.