Definition of "planet" changing, says director of astronomy laboratory program

Thursday, August 17, 2006

For years, astronomers have said the solar system has nine planets. Meetings currently underway by the International Astronomical Union (IAU) could expand the number of planets to 12, and change the definition of what is considered a planet.

The director of the astronomy lab program at the University of North Texas says the potential is there for the further expansion of what constitutes a planet.

Ron DiIulio says the definition that the IAU is considering doesn't include factors like a possible planet's position from the sun, atmosphere, and geology.

"It does away with the traditional planet definitions of 'terrestrial' and 'Jovian' - a term for gaseous planets like Jupiter," he says.

Instead, DiIulio says, the planets will be divided into two groups; eight classical planets and three planets in a category called "plutons" that would include the planet Pluto. Ceres - the largest object in a body of asteroids located between the planets Mars and Jupiter - will now be considered a planet.

"Ceres has long been thought of as a minor planet, going nearly back to when it was discovered on January 1, 1801," DiIulio says. "The fact that it doesn't easily fit into the two new planetary definitions will certainly lead to more questions, and more interest in it. I don't think we're anywhere near done exploring this."

The two additional "plutons" that will be considered planets if the IAU definition is approved are Charon, a moon of Pluto that also rotates around the sun, and 2003 UB313, which was nicknamed "Xena" by the astronomer who discovered it. Both are located in the Kuiper Belt just beyond the orbit of Neptune, along with Pluto.

DiIulio says the proposed new definition of a planet - a round object larger than 800 kilometers in diameter that orbits the sun and has a mass of one-12,000th of the Earth - will force scientific theories to be reconsidered and educational materials to be rewritten. He adds that process is already underway at UNT in advance of the fall 2006 semester, which begins Aug. 28.

"We were just finishing up work on the planetary lab for the upcoming semester. We'll have to rewrite parts of it after Aug. 24, when the IAU meeting ends. Science is always developing; we have to accept that. What we are seeing is what science is about - it's the dynamic nature of change," he says.

UNT News Service Phone Number: (940) 565-2108

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