Our solar system, she ain't what she used to be

WASHINGTON — Move over, Copernicus. Your once-revolutionary idea — that the Earth revolves around the sun rather than the other way around — has been eclipsed.

Recent years have brought a sweeping new revolution in solar system astronomy. The Earth still orbits the sun, as Copernicus declared 400 years ago, but the planetary system in the textbooks you studied is now out of date.

"The entire view of astronomy you learned in high school has changed dramatically," said Alan Stern, NASA's associate administrator for science. "We're really in a new age of discovery."

The changes go well beyond the International Astronomical Union's controversial decision to demote Pluto, the baby of the solar system, from the familiar list of nine planets. The organization ruled that Pluto is now a "dwarf planet" or "Plutino." The decision, announced on Aug. 24, 2006, upset millions of children and their parents.

"For six years I got hate mail from children who loved Pluto," said Neil deGrasse Tyson, the director of New York's Hayden Planetarium, who'd moved Pluto out of his solar system display years ago.

Stern and other astronomers offered a revised description of the solar system at a meeting of the American Association for the Advancement of Science last month in Boston. Stern said it differed from the previous understanding in several major ways:

• First, until recently, people thought that there were two parts to the solar system: four small, rocky inner planets — Mercury, Venus, Earth and Mars — and four gas giant outer planets — Jupiter, Saturn, Neptune and Uranus. Then there was Pluto, a "lone misfit," Stern said, with a highly eccentric orbit and a rakish tilt of its axis.

"That was the old view," he said. "Now, there are no more misfits. Plutos abound."

Under the new definition, the International Astronomical Union has officially recognized 11 planets: eight traditional ones plus three "dwarf planets." The dwarfs are Pluto; Ceres, which was thought to be an asteroid between Mars and Jupiter; and Eris, an object that's slightly larger than Pluto and farther from the sun.

At least 40 more dwarfs have been spotted even farther out and are awaiting official recognition. They bear names such as Quaoar, Sedna, Orcus, Varuna and Ixion. Dozens of others are known only by code numbers.

Stern said the solar system now is thought to be composed of three zones instead of two.