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The definition of a planet is going under the telescope
By Dan Vergano, USA TODAY
The simple word "planet" may soon be lost in space.
Scientists have long wrestled with this question: When is a planet really a planet? Apparently, there is no easy answer.So the International Astronomical Union (IAU) panel, headed by Iwan Williams of the University of London, is proposing that astronomers use the label "planet" only with an added descriptive term.
Astronomers have long disagreed on what makes a celestial body a planet.
At a Space Telescope Science Institute symposium in Baltimore this summer, scientists were split on whether planets should be designated by location, size, characteristics, how they formed or according to historical standards.
This fundamental disagreement has been magnified by the gold rush in planetary discoveries in recent years, says IAU panelist Brian Marsden of the Harvard-Smithsonian Center for Astrophysics.
And the status of Pluto has been a bone of contention among scientists.
Initially thought to be larger than it actually is, Pluto has been revealed to be smaller than our moon, and just one among a handful of similar icy objects lingering in a comet belt more than 6 billion miles away from the sun.
UB313, the solar system's so-called 10th planet whose discovery was announced this summer by astronomer Mike Brown of the California Institute of Technology in Pasadena, is larger than Pluto.
The discovery of planets orbiting nearby stars and even floating freely in space has further muddied the planetary picture. Some of those objects, for example, are the size of Jupiter but orbit closer to their stars than Mercury in our solar system, earning them the nickname "roasters."
The ancient Greeks first dubbed some stars "planets," meaning "wanderers," because the objects seem to move erratically among the fixed stars.
In the 1800s astronomers began to describe some asteroids as "minor" planets after realizing that Ceres, located between Mars and Jupiter, was just part of an asteroid belt.
Under the new proposal, astronomers would deal with the new zoo of worlds swimming into view of telescopes by describing them as "planetary objects."
The proposal would formally settle only the issue of planets beyond Neptune in our solar system, Boss says, but astronomers have other qualifiers to add: Rocky planets like Earth could become "terrestrial" planets; Jupiter and Saturn "gas giants"; Uranus and Neptune "ice giants."
Marsden would like planets closer to the sun than Jupiter designated "cis-Jovian" planets.
Pluto and UB313 would become "trans-Neptunian planets," a demotion for Pluto.
Details of the Sept. 12 proposal, which had been circulating quietly among astronomers, are reported today in the journal Nature.
If the group develops a consensus on the proposal, it will go to the IAU executive committee for a vote. The organization certifies claims of new planet discoveries, so its views on matters astronomical are influential but not binding.
"We couldn't come up with a definition of planets in splendid isolation," says panel member Alan Boss of the Carnegie Institution of Washington, D.C.
"There are a whole bunch of things people want to use the 'p-word' for, so we have to be careful."
The 19-member IAU panel has been considering the issue for a year and even there, disagreement remains over the proposal, Boss adds. "There are a lot of good arguments on all sides."
Brown was not available to give his views on the proposal Wednesday. His voice-mail, however, noted he is out of the office, "observing the 10th planet," this week at Hawaii's Keck Observatory.