Will Pluto lose planet status this week?
By Tom Beal
ARIZONA DAILY STAR
"When they send the mission to Pluto and it arrives there and there is a big sign on it that says 'I am not a planet, Clyde,' then I'll be happy to demote it."
Clyde Tombaugh
Discoverer of Pluto, as told to David Levy
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Pluto has never been much of a planet — small, wired orbit, not much atmosphere — and questions about its status have been raised periodically since its discovery 76 years ago.
This week, the international body with the power to decide what is and is not a planet will consider a new definition that could expand the number of planets in our solar system or demote Pluto to mere "Kuiper Belt object."
Talk about culture shock. Pluto's been a planet for 76 years. It's one of the nine we memorized in school. It's the only planet discovered in the United States — done right here in Arizona by a high school graduate from Kansas named Clyde Tombaugh, who was working at Lowell Observatory in Flagstaff.
The International Astronomical Union, which meets in Prague this week, hasn't tipped its hand. Its committee deliberations on crafting the definition of planet have so far remained secret. It hasn't even revealed the process by which it will make a decision, though it promises to outline that at a press conference Wednesday.
If Pluto doesn't fit the definition, we're back to the pre-1930 count of eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
If the definition is broad enough to include Pluto, other round, sun-orbiting bodies could become planets as well.
The folks at Lowell Observatory, which rocketed to astronomical prominence with the 1930 discovery, are speechless about the whole thing. They met on the subject Friday and decided to make no comment until a decision is announced, said spokesman Steele Wotkyns.
Cal Tech planetary astronomer Mike Brown, co-discoverer of what could become the 10th planet, doesn't think the international group should expand the definition of a planet in a manner that would include his discovery.
"If the IAU had guts, it would stand up and say 'Look guys, there are eight planets,' " Brown said. "That's the best bet for getting a scientific definition. To me, it's a simple concept. The first eight planets are significantly bigger than everything else."
Brown calls the notion of a more expansive definition the "No Ice Ball Left Behind Act."
Brown, in a sense, began this latest round of arguments in 2003 when he and two colleagues discovered a Pluto-style body officially known as UB313.
Called "Xena" for convenience, it's another icy dwarf with a round shape that orbits the sun in an elongated ellipsis. It's just like Pluto, only a little bit bigger and farther out in that region of iced-over spheres, comets and asteroids past Neptune, dubbed the Kuiper Belt.
It would be tough to write a definition of planet that includes Pluto and doesn't include his discovery, Brown said.
But if they call UB313 a planet, said Brown, they might have to add another 20 bodies he's discovered and a few dozen more, many of them found by the astronomers at Lowell Observatory. "I did a count the other day and came up with 53," Brown said.
Alan Stern, principal investigator for a NASA mission to Pluto that should fly by the planet in another nine years, thinks Pluto, Xena and the other orbiting icy dwarfs hidden among the comets and asteroids of the Kuiper Belt are planets.
"Start showing pictures to folks on the street and they'd make the obvious conclusion that they are planets," he said. "A chihuahua may be small but it's still a dog."
Stern, the executive director of the space science and engineering division at the Southwest Research Institute in Boulder, Colo., said the IAU should simply let in a whole new class of planets, calling them and Pluto "dwarf planets" to differentiate them from the big eight.
"A chihuahua may be small but it's still a dog," Stern said. "You wouldn't misclassify the Platte as not a river just because it doesn't remind you of the Mississippi," he said.
Mark Sykes, director of the Planetary Science Institute, with headquarters in Tucson, said he hopes the international body doesn't take the politically easy route of creating a definition that grandfathers Pluto as a planet and rejects the latest discoveries — and new planets awaiting discovery.
Pluto and the other ice dwarfs of the Kuiper Belt aren't meteorites or comets, Sykes said. They are more akin to planets than anything else. "Having sufficient size to be round is the main criteria," he said.
Tucson astronomer David Levy agrees.
Levy was a close friend of Clyde Tombaugh, who died in 1997. He wrote the biography "Clyde Tombaugh: Discoverer of Planet Pluto."
"The Earth has much more in common with Pluto than with, say, Jupiter," Levy said. "You can walk on Pluto, there are three moons in its sky. Land on Jupiter, you'd just fall right in. Jupiter and the Earth have almost nothing in common, yet they're both called planets without any debate."
Levy, science editor for Parade Magazine and discoverer of 21 comets, said he would settle for a definition that a planet is a spherical body orbiting the sun that is "the size of Pluto or bigger."
He is willing to change his mind, however, per his late friend's instructions.
"Clyde himself said, 'When they send the mission to Pluto and it arrives there and there is a big sign on it that says "I am not a planet, Clyde," then I'll be happy to demote it.' "
"When they send the mission to Pluto and it arrives there and there is a big sign on it that says 'I am not a planet, Clyde,' then I'll be happy to demote it."
Clyde Tombaugh
Discoverer of Pluto, as told to David Levy
● Contact reporter Tom Beal at 573-4158 or tbeal@azstarnet.com.
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