No longer nine planets?

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17 planets? Astronomers' heads spinning

The discovery of new objects in the icy junkyard called the Kuiper Belt forces science to rethink the definition of a planet.

BY ROBERT S. BOYD
Washington Bureau

WASHINGTON — The familiar solar system that you learned about in school — nine well-behaved planets, from Mercury to Pluto, circling sedately in tidy paths around the sun — isn't what it used to be.

Astronomers recently have discovered a flock of at least eight other planet-like objects in sometimes wildly eccentric orbits. Four new "ice dwarfs," plus two more probable moons around Pluto, were announced in the last six months.

The latest mini-world, temporarily nicknamed "Buffy" and more than 5 billion miles from the sun, was revealed Dec. 13. The object, about half the size of Pluto, was spotted roaming through the so-called Kuiper Belt, a vast junkyard of icy, rocky bodies stretching for billions of miles beyond the orbit of Neptune.

The first scientific mission to explore Pluto and the Kuiper Belt, a nine-year voyage to the outer solar system, is set to be launched in January.

"Next month, we set sail for Pluto," said Alan Stern, chief scientist at the Southwest Research Institute in Boulder, Colo.

Unlike the rest of the planetary family, Pluto resides in the Kuiper Belt. But it's not alone there. More than 1,000 frozen chunks of debris left over from the formation of the solar system have been found since 1992. Astronomers expect at least half a million more pieces are out there.

"The discovery of the Kuiper Belt in the 1990s has given Pluto a place to call home, with icy brethren to call its own," said Neil deGrasse Tyson, the director of the Hayden Planetarium at the American Museum of Natural History in New York, in an e-mail.

"The Kuiper Belt is the largest structure in the solar system," Stern said recently. "We used to think Pluto was a misfit," he added. Now Earth and the other inner planets are the oddballs.

Even the inner solar system looks different. Astronomers no long believe that the four biggest planets have always been in their present orbits.

Instead, they now say, Jupiter has moved toward the sun from its original home. Saturn, Uranus and Neptune have slid outward from their birthplaces, according to Donald Yeomans, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., "The field of planetary science is currently enjoying an intense period of readjustment and discovery," Yeomans wrote in the journal Science.

Scientists no longer are sure what a planet is and how many reside in our system. In addition, 160 extra-solar planets have been discovered around other stars, not the sun, in the last 10 years.

The International Astronomical Union, a worldwide alliance of astronomers, has been struggling for about two years to agree on a definition for planets. Three proposed definitions are being studied, but a decision isn't likely until spring, according to Robert Williams, an astronomer at the Space Telescope Science Institute in Baltimore.

Depending on what definition is adopted, Pluto may be demoted from its status as the solar system's smallest planet to what astronomers call a "trans-Neptunian object." That's a fancy name for any world beyond Neptune, which is 2.8 billion miles from the sun or 30 times farther than Earth.

Teachers are scrambling to stay current.

"It's a very exciting and fast-changing area," said Richard Pogge, an astronomy professor at Ohio State University in Columbus. "It requires us to get out and do research on our own, since the textbooks are behind."

Pogge uses classroom computers to connect his students to NASA's Web site and other Internet pages to view the latest findings.

Science museums and planetariums have special problems since they can't afford to update expensive exhibits when a new object is found.

"It's hard to hit a moving target," said Tyson of the Hayden Planetarium, who's writing a book on the difficulty of defining Pluto.

Pluto would remain a planet if the International Astronomical Union accepts a definition that would declare a planet to be any round object larger than 1,000 kilometers — 625 miles — across that's orbiting the sun.

So far, nine such mini-worlds, including Pluto (diameter 1,430 miles), are known to dwell in the frigid Kuiper Belt.

They are "a completely different type of object that predominates in the outer solar system beyond Neptune," Pogge said.

The largest and most distant of the ice dwarfs is nicknamed Xena after the television warrior princess. Discovered in 2003, it's 1,600 miles across and 20 percent bigger than Pluto is. Xena has a moon of its own, named Gabrielle after the TV Xena's sidekick.

If Xena and Pluto are counted, our solar system has 10 planets; if they're not, it has eight. But if all the known objects larger than 625 miles across are included, there would be 17 planets with more expected soon.

A small Kuiper Belt mini-worlds is nicknamed Santa and has a moonlet named Rudolph. Until the International Astronomical Union assigns official names, the others are known as Easter Bunny, Orcus, Quaoar, Ixion and now Buffy. The most distant such object is called Sedna; its elliptical orbit carries it more than 9 billion miles beyond the sun.

As an added complication, new moons keep popping up around planets, and some old moons are found to be performing unusual tricks. Even some asteroids have tiny moons of their own. One asteroid has two moonlets.

In November, two possible new moons were sighted around Pluto. If confirmed, that would mean Pluto has three satellites, counting its previously known moon, Charon.
"The total number of natural satellites orbiting the major planets has grown to more than 150, with more than 50 percent of these discoveries occurring within the last six years," said Yeomans of the Jet Propulsion Lab.

Unlike our own moon, which is geologically dead, at least four moons show signs of activity.

Titan, Saturn's largest moon, has rainfall and rivers of liquid methane (natural gas), and renews its atmosphere by venting more methane from below the surface.

Enceladus, another of Saturn's moons, spews huge jets of icy particles, probably driven by subsurface radioactivity. Jupiter's moon Io is wracked by volcanic explosions and gigantic lava flows. Neptune's moon Triton has active geysers of dust and gas.

"For planetary explorers like us, there is little that can compare to the sighting of activity on another solar system body," said Carolyn Porco, an astronomer at the Space Science Institute in Boulder.