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

Traditional realm is dwarfed by Kuiper Belt and Oort Cloud

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A cluster of nearly 1,000 newly formed stars is captured in this undated infrared photograph as it emerges from the gaseous womb from which it was recently born. It contains numerous brown dwarfs.

A cluster of nearly 1,000 newly formed stars is captured in this undated infrared photograph as it emerges from the gaseous womb from which it was recently born. It contains numerous brown dwarfs.

Associated Press

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orbits the sun, as Copernicus declared 400 years ago, but the planetary system in the textbooks you studied is now out of date.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.

The four rocky planets make up the inner zone. The gas giants form a "middle solar system." Beyond them lies an enormous third zone composed of the Kuiper Belt and the Oort Cloud, both named for the astronomers who predicted their existence.

"This third class of planets vastly outnumbers the terrestrial planets and gas giants," Stern said.

The Kuiper Belt, which was discovered in the 1990s, is a ring of dwarf planets, including Pluto, and smaller icy objects that range from 3 billion to 5 billion miles beyond the sun.

More than 1,000 Kuiper Belt objects have been detected, and astronomers think that there may be 50,000 to 100,000 more. Most are small, but some rival Pluto in size. Some have atmospheres and moons of their own, and some may have warm, wet interiors.

Far outside the Kuiper Belt looms the Oort Cloud, which Stern calls "the solar system's attic." The cloud is a gigantic sphere with an outer edge almost 5 trillion miles from the sun and is nearly a quarter of the way to the next nearest star, Proxima Centauri.

"If the Oort Cloud were the diameter of a football stadium, the inner solar system would be the size of a washer one-eighth of an inch in diameter," said David Aguilar, a spokesman for the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The Oort Cloud is thought to contain at least 1,000 planetary bodies, some as large as Earth or larger, and as many as a trillion comets. Periodically, a passing star knocks an Oort comet loose and sends it diving toward the sun.

• A second upheaval in planetary science is the realization that most members of the solar system weren't born where they are now. Instead, gravitational forces forced them to migrate from their birthplaces to their present homes.

"This is a true revolution," Stern said.

"The planets didn't necessarily form where we see them today," said Douglas Lin, an astronomer at the University of California-Santa Cruz. "They move all over the place."

For example, the giant planets Neptune and Uranus formed where Saturn now lives and drifted outward to their present orbits.

Michael Meyer, an astronomer at the University of Arizona in Tucson, said "forensic evidence" proved that there used to be large objects in the solar system that no longer are there.

For example, Uranus was struck and knocked on its side by a long-gone object that must have been as large as Earth or bigger. Triton, Neptune's largest moon, revolves backward around its planet, evidence that it formed elsewhere, probably in the Kuiper Belt, and that Neptune's gravity later captured it.

"There were many more planets that were born and swept away," Meyer said. "The Oort Cloud was entirely formed by objects not born there but ejected outwards."

• The third revolution in planetary science is the realization that making planets is a common process in the universe. In the past dozen years, 276 planets — some of them forming miniature solar systems — have been detected orbiting other stars. One such system has at least five planets. Astronomers discover an average of 25 planets each year.

According to Meyer, dusty disks that may be nurseries for planets surround many, perhaps most, sunlike stars. He estimated that as many as 60 percent of these stars have rocky planets orbiting them, and 20 percent host gas giants.

"I think every low-mass star (like the sun) will have rocky planets around it," he said.

NASA will launch a new spaceship, named Kepler, next year hoping to find hundreds more of such faraway planets.