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A planet is a planet by any name

A recent e-mail message I received from a local student prompted another answer to the concern about the status of the distant planet Pluto. Kids seem to be fascinated with what is and isnt a planet. So, lets revisit the astronomical question of the decade: What happened to Plutos status as planet? Permit me to playfully skew Gertrude Steins 1913 poem (Sacred Emily) with the famous line, A rose is a rose...: A planet is a planet is a planet is a planet. Or is it? There appears to be no end in sight to the current controversy over the demotion of Plutofrom the ninth planet of the solar system since first discovered in 1930to mangy dwarf planet. When a few members of the International Astronomical Union (IAU) met in Prague to consider the question, their deliberations created a firestorm among astronomers and planetary scientists; many, notably in the United States, protested loudly that they werent invited to be part of the discussion. Among the protesters is Alan Stern, principal investigator of NASAs New Horizon probe now on its way for a fast flyby of distant Pluto. A portion of the space science community was upset over how the question was handledand by most accounts, the Pluto decision was handled badly. As a result, the IAU shockwaves promise to provide saucy headlines at least until next year when the IAU will reopen the discussion. Science can be messy and theres nothing messier than the current tug of war over planetary semantics. At the heart of the storm is the definition of just what a planet should be. To be classified as a plant, must a celestial body be a certain size? Must it have a traditional orbit around its sun? Must it be composed of rock or ice (or both)? According to Internet science writer Robert Roy Britt, The IAUs final proposal was lambasted by many astronomers for having been slapped together at the last minute and for not adhering to recommendations from two separate committees. NASA astronomer David Morrison attended the August 2006 IAU meeting and was one of the few Americans in attendance for the final vote demoting Pluto. The definition of a planet is not primarily a science issue. Scientists can use all sorts of jargon, Morrison said in a news story about the IAU vote. This issue is of interest because non-scientists, including writers of science textbooks, want a definition. Now they have one. But it is not obvious to me that planetary scientists will adjust their terminology because of the IAU votes. So, then, dwarf or otherwise, isnt Pluto still a planet? Yes and no, Morrison said. The answer is semantic, based on whether dwarf planets are planets, just as dwarf pines are pines. I would say that Pluto is a planet, but it is a dwarf planet, and the first example of the (new) class of trans-Neptunian dwarf planets. Ultimately, the definition of a planet will come through common usage and scientific utility. There is no need to throw away current school texts; Pluto has not gone away. In the meantime, according to Alan Stern, the New Horizons primary mission to Pluto hasnt changed. When the probe arrives at Pluto in July 2015 it will unlock one of the solar system's enduring planetary secrets. The spacecraft will skim the orbits of all eight planets, from Earth to Neptune, and then fly by Pluto and its large moon Charon. Seven science instruments on the probe will shed light on the dwarf planets geology, interior and atmospheres. Lou Varricchio, M.Sc., lives in Vermont. A former NASA senior science writer, he is a member of the NASA/JPL Solar System Ambassador program in Vermont.

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