Director of UNT astronomy lab program comments on proposed redefinition of what constitutes a planet

DENTON (UNT), Texas -- For years, astronomers said the solar system has nine planets. Meetings currently underway by the International Astronomical Union (IAU) could expand the number of planets to twelve, and change the definition of what is considered a planet. The director of the astronomy lab program at the University of North Texas, Ron DiIulio, says the potential is there for the further expansion of what constitutes a planet.

DiIulio says, "The definition that the IAU is considering doesn't include factors like position from the Sun, atmosphere, and geology. It does away with ‘terrestrial' and ‘Jovian' (gaseous planets like Jupiter), which were the traditional planet definitions."

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. In addition to Pluto, they are located in the Kuiper Belt just beyond the orbit of Neptune.

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 beginning August 28.

"We were just finishing up work on the planetary lab for the upcoming semester. We'll have to rewrite parts of it after August 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," DiIulio says.

DiIulio and the UNT planetarium program have been cited for their participation in NASA's Space Place public education and outreach program. He is also overseeing the establishment of the Rafes Urban Astronomy Center near Denton Municipal Airport. It will house the lab portion of UNT's astronomy program, which is the largest non-science major program of its kind in the nation. For more information, contact DiIulio at (940) 369-7655, or starman@unt.edu.