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Astronomers Ponder an Unusual Object’s Behavior on the Edge

TIMES SCIENCE WRITER

A new kind of object orbiting the outer limits of the solar system in a wildly eccentric path threatens to disrupt the well-ordered vision of our local universe, according to astronomers at Harvard and University of Hawaii.

Known only as 1996TL66, the 300-mile-diameter body appears to swing well out of the boundaries of the previously known solar system. Its highly unusual behavior suggests that far reaches of the sun’s planetary family “may be far more chaotic than previously thought,” said astronomer Renu Malhotra of the Lunar and Planetary Institute in Houston. “It has extended the boundaries of our solar system,” she said.

The findings, published today in the journal Nature, suggest that a huge population of such objects may buzz around “like a swarm of bees” at the fringes of the solar system, according to astronomer David Jewitt of the University of Hawaii, one of the paper’s authors. “The point is,” he said, “we have no idea of what is in the outer solar system.”

Coincidentally, the existence of such peculiar interplanetary interlopers is predicted--by astronomers who used computers instead of telescopes--in a paper to be published in the journal Science next week.

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“I was ecstatic,” said astronomer Harold Levison of the Southwest Research Institute in Boulder, Colo., an author of the Science paper, which was released early. He added that he was “very surprised” that the anticipated object had already been found.

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Blacker than coal and about the size of Texas, the new object is too big to qualify as a comet but too small to be a proper planet (although many astronomers feel the same way about Pluto, the farthest planet from the sun.) It’s the biggest rock discovered at the frontier of the solar system since Pluto’s companion, Charon, was found in the 1970s.

At its closest approach to the sun, the planetessimal passes about 500 million miles outside the orbit of Neptune, then speeds out into the unknown to distances 130 times the distance between the Earth and the sun--or about 12 billion miles.

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Probability alone suggests it is just one of thousands of such objects, according to Jane Luu of the Harvard Smithsonian Center for astrophysics, a co-discoverer. “Unless we were extremely lucky and found the only one of its kind, we think there are a lot of these,” she said.

In 1992, Luu and Jewitt discovered the first object other than Pluto beyond Neptune’s orbit. Astronomers theorize that leftover chunks of the cloud that formed the solar system probably orbited there in the so-called Kuiper belt. Indeed, many astronomers consider tiny Pluto to be simply the biggest Kuiper belt object, rather than a full-fledged planet.

The first Kuiper belt objects found by Luu and Jewitt five years ago orbited in circles, much closer to the sun than the new body at its farthest point. Their findings extended not only the physical reach of the solar system, but also its history, because the hunks of ice and rock in that region are thought to be the most primitive in the solar system.

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Astronomers believe these primitive rocky snowballs have been circling outside Neptune’s orbit more or less undisturbed since the planets were formed, and thus hold clues to our earliest origins. When the sun and planets condensed out of the primordial cloud, the Kuiper belt objects were planetary wannabes that never got involved in the planet-building process.

This new object, however, appears to be of a different sort--with a far more interesting history. Perhaps it began as a standard Kuiper belt object that got caught in Neptune’s gravitational grasp and slung outward, Jewitt said. In fact, all the giant planets are surrounded by mostly empty space because any rocks that wander within their gravitational clutches are either captured or flung far off into space.

Many of these far-flung rocks are thought to accumulate in a huge spherical halo of matter called the Oort cloud that hovers far outside the solar system, and is thought to be the storage area for comets that occasionally come crashing toward Earth.

The puzzle is: How did such a rock, propelled by the gravity of Neptune as if by a slingshot, manage to settle into a stable orbit instead of careening right out of the solar system or into the Oort cloud?

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While Luu and Jewitt were searching with telescopes for new Kuiper belt objects, Levison and astronomer Martin Duncan of Queen’s University in Ontario, Canada, were using computers to track the paths of thousands of objects wandering into Neptune’s path over 4 billion years of virtual history. “You might call it very long-range forecasting,” said Duncan.

They found that as many as 1% of these objects might settle into surprisingly stable orbits. “Most people assumed these objects would have been ejected long ago,” he said. The new-found object fits into this pattern. If Duncan and Levison had gotten their paper out a few months sooner, he said, “we could have said that we’d predicted it.”

This new planetessimal--or planetary building block--was probably involved in planetary formation, even though it didn’t wind up as part of a giant planet, said Duncan. “It’s almost certainly an object that took part in the dance that formed the planets.”

How these highly eccentric orbits can remain stable is still unknown, but it probably has to do with the power of sympathetic vibrations to shape the distribution of matter in the solar system. Just as a clock pendulum falls into a natural pattern of vibration, so does anything following a regular rhythm, including planetary orbits.

When the rhythmic patterns of two or more objects resonate, their behavior is more stable. In fact, the only reason that Pluto hasn’t been tossed out of the solar system by Neptune, astronomers say, is that it revolves around the sun exactly two times for every three orbits of Neptune, putting them in a stable 3-2 resonance. A whole family of similar objects--which Luu calls Plutinos--also populate the Kuiper belt in the same 3-2 resonance.

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Still, the discovery of this eccentric new object may imply “that things haven’t settled down” at the edges of the solar system, said astronomer Brian Marsden of the Harvard center.

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Detecting what amounts to a large lump of coal 12 billion miles out into space required a special kind of image processing that Luu compared to children’s flip books, where characters seem to move when the pages are shown in quick succession. Using the University of Hawaii’s 2.2-meter telescope, the astronomers snapped electronic images of the sky every few hours, stored them, then displayed them quickly one after another.

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Background stars and galaxies stayed fixed, while moving objects stood out. The distance to the object was determined by how fast it was moving; nearby objects appear to fly past an Earth observer’s field of view more quickly than distant ones.

One of the authors of the paper is an amateur astronomer, Warren Offett of Arizona, who had constant access to a large telescope to track the object.

Luu attended Ventura High School after arriving in the United States on one of the last airlifts out of South Vietnam as it fell to the North in 1975. She learned English initially at refugee camps, studied physics at Stanford, and became entranced by the solar system during a nine-month stint at the Jet Propulsion Laboratory in Pasadena.

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Solar System Zooms Out

A newly discovered planetoid is the most far out member of the solar system yet seen by astronomers. It approaches the Sun only as near as the orbit as the orbit of Pluto, then swings outward in a highly stretched orbit.

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Red: Orbit of newly discovered planetoid.

Black: The Kuiper belt, material leftover from when solar system was formed.

Green: Orbits of Pluto and its companions.

Light Blue: Orbits of Jupiter, Saturn, Uranus and Neptune.

Yellow: Planetary chucks that were probably knocked out of the Kuiper belt and are on their way to become comets.

Note: At this scale, the orbits of Mars, Earth, Venus and Mercury would be too small to see.


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