You call that a planet?

Answers to your questions about the news.
Aug. 2 2005 6:28 PM

You Call That a Planet?

How astronomers decide whether a celestial body measures up.

Plain as day. Click image to expand.
Plain as day

Last Friday, astronomers at Caltech declared the discovery of a 10th planet. The new celestial body, which the International Astronomical Union has dubbed 2003UB313, is larger than our solar system's smallest planet, Pluto. Some scientists, though, don't think 2003UB313 should be classified as a planet. What makes a celestial body a planet?

It depends on who you ask. In simple terms, a planet is a sphere that materializes from the gas and dust that's left behind when a star gets formed. The IAU, the organization charged with categorizing celestial bodies, has a Minor Planet Center that distinguishes planets from comets and the like based on differences in size and orbit. Because astronomers thought until very recently that there weren't any more Pluto-sized bodies left to discover in our solar system, there's little official protocol for determining whether an object is a "major planet." The IAU's Working Group to Define a Planet, which was founded last year, is now hurrying to make a recommendation about minimum size requirements in the face of Friday's discovery.


When the IAU designated Pluto a planet in 1930, it was estimated to be the size of Mars. But by the 1970s, astronomers learned that Pluto was, in fact,  around half the size of the then-tiniest planet, Mercury. * Pluto's stature diminished even further in the 1990s, when scientists found several objects in the Kuiper Belt, Pluto's region of the solar system, that were nearly the same size.

Many astronomers consider Pluto (and presumably 2003UB313) a little cousin to Jupiter and Saturn. The Harvard-Smithsonian Center for Astrophysics calls objects larger than 10 meters in diameter and smaller than 4,000 kilometers in diameter "minor planets." According to this definition, both Pluto (2,274 kilometers in diameter) and 2003UB313 (estimated to be one to one and a half times the size of Pluto) are no different than tens of thousands of other minor planets that have been discovered and named.

Others yield to cultural preferences. Since Pluto's discovery, children have sung songs and learned mnemonic devices to remember the order of the nine planets. Because people have become attached to Pluto, these astronomers argue, its diameter should be a planet's minimum size. In that case, 2003UB313 is a planet; anything smaller is a "planetoid."

There is a generally accepted rule of thumb of a planet's upper limits, but it relates to mass rather than diameter. A planet must not exceed 13 times the mass of Jupiter—the size at which a body exerts enough pressure at its core to fuse deuterium. *

Bonus Explainer: When a celestial body is discovered, the IAU gives it a temporary name based on the date it was found. Once the object's orbit is determined, its discoverer has 10 years to submit a permanent name. The nine planets are named after Greek or Roman gods. The IAU has no further rules for naming major planets but does regulate minor planet nomenclature. Objects in the Kuiper Belt, for example, are named after figures in creation mythology.

Explainer thanks Dan Green of the Harvard-Smithsonian Center for Astrophysics, Andrew Pickles of Caltech Optical Observatory, David Rabinowitz of Yale's Center for Astronomy and Astrophysics, and Oddbjorn Engvold and Iwan Williams of the International Astronomical Union.

Correction, August 3, 2005: This piece originally and incorrectly stated that the thermonuclear fusion of the hydrogen isotope deuterium is what makes stars shine. It's the fusion of different hydrogen isotopes. (Return to corrected sentence.)

* Correction, August 4, 2005:This piece originally stated that Pluto is "two times smaller" than Mercury. It's half the size of Mercury. Click here to return to the corrected sentence.

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