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|Posted: Wed Sep 13, 2006 11:57 am Post subject: In The World of Science VII
|Pluto’s Exotic Playmates
By KENNETH CHANG
Published: September 12, 2006
With a quick vote last month, the International Astronomical Union decreed that Pluto was no longer the ninth planet, but just a dwarf planet — and not even the largest dwarf — orbiting in a distant ring of icy debris.
But perhaps that should not be seen as a slight to Pluto.
For many astronomers, that ring of icy debris, known as the Kuiper Belt, has become an exciting spot for innovative research and has changed how they view the solar system.
“It’s a lot bigger now,” said Marc W. Buie, an astronomer at the Lowell Observatory in Flagstaff, Ariz. “For me, it’s like somebody invented a new field of science.”
Harold F. Levison of the space studies department in the Southwest Research Institute in Boulder, Colo., said, “The more we learn, the weirder it looks.”
More than 1,100 Kuiper Belt objects have been found so far. Astronomers estimate that half a million bodies larger than 20 miles wide are floating out there. At least one appears to be mostly rock with a coating of ice. Some are mostly ice. Some are less dense than ice, indicating a Swiss-cheese-like structure. A surprising number of them have moons.
Some move in clockwork with Neptune; Pluto, for example, is in what is called a 3:2 resonance, taking 1.5 times as long as Neptune to loop the Sun. Many Kuiper Belt objects have been flung into orbits crazily tilted to the rest of the solar system.
“This is really a very exotic zoo out there,” said S. Alan Stern, executive director of the space science and engineering division at the Southwest Research Institute and principal investigator of NASA’s New Horizon spacecraft, which is currently heading to Pluto.
The distribution of Kuiper Belt objects has already provided decisive evidence that Neptune was once perhaps nearly a billion miles closer to the Sun and was then gravitationally nudged outward. Astronomers also hope that the Kuiper Belt preserves a frozen record of the earliest building materials of the solar system.
“It’s kind of like the solar system’s attic,” Dr. Stern said. “It’s like an archaeological dig into the history of our solar system.”
Scientists had initially expected a simple structure for the belt: a thin disk of objects traveling in circular orbits in the plane of the solar system. Some Kuiper Belt objects do fit that profile, and those are called the classical Kuiper Belt objects. (One mystery is why there appears to be a sharp edge at about 4.5 billion miles, with no classical Kuiper Belt objects beyond that distance. Some think a passing star did that.)
Other Kuiper Belt objects share orbits similar to Pluto’s, in resonance with Neptune. Those in the same 3:2 resonance as Pluto have been called the Plutinos.
Still others are called the scattered-disk Kuiper Belt objects. These appear to have been tossed into highly elliptical orbits, often at a sharp angle to the rest of the solar system. Surprisingly, these include some of the larger Kuiper Belt objects, including 2003 UB313, nicknamed Xena, which is larger than Pluto.
An estimated 15 percent of Kuiper Belt objects are binaries — pairs of bodies of similar size and mass. Among some classical Kuiper Belt objects, that fraction may be as high as 30 percent — possibly higher, because even the Hubble Space Telescope cannot distinguish two separate objects if they are too close to each other.
Theorists puzzled about how such small bodies, with weak gravitational pull, could have paired up so often. The answer, it turns out, is that as two objects flew past each other, the gravitational drag generated by many other much smaller Kuiper Belt objects slowed them enough to capture each other.
That mechanism requires a fairly dense Kuiper Belt with a total mass of at least 10 Earths. But while Kuiper Belt objects are many, they do not amount to much today. Adding the masses of Pluto, Xena and the half million other objects, even those not yet seen, gives an estimate of just one-tenth the mass of Earth.
“The mass we measure is pathetic,” said David C. Jewitt, a professor of astronomy at the University of Hawaii.
That, in turn, produces a quandary. Where has 99 percent of the Kuiper Belt gone? This is, as the planetary scientists quaintly put it, the cleanup problem. (One popular idea: repeated collisions smashed most of them to dusty bits, and the bits were blown away by solar radiation.)
Just 15 years ago, the Kuiper Belt was not on the map at all. The known solar system essentially ended at Neptune, except for the occasional comet interloper from far away. (Fifteen years ago, Pluto was traveling along the inner part of its eccentric orbit, closer to the Sun than Neptune.)
Gerard Kuiper, a prominent astronomer for whom the belt is named, speculated about the possibility of it in 1951. Kuiper was prescient, but wrong on a major point. He thought that the belt had existed early in the solar system’s history but that Pluto, then thought to be a more massive planet, had scattered it away.
Some people have suggested that the belt should instead be named after Kenneth Edgeworth, an Irish astronomer who vaguely hypothesized an icy disk a few years before Kuiper. But that was just a guess.
“The outer solar system was just this sort of empty space,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology.
Astronomers did have a few clues that something was out there. One was Pluto, an oddball. Unlike the four rocky inner planets or the four outer gas giants, Pluto is half ice, and its orbit is quite elliptical and tilted 17 degrees to the solar system’s ecliptic plane. It did not fit in.
A second clue showed up in 1977 with the discovery of Chiron, an icy body between 90 and 130 miles wide that loops around on an elliptical path taking it as close to the Sun as Saturn and as far out as Uranus. Astronomers argued over whether it should be called an asteroid or a comet. In the end, they decided both, and created a new category, Centaurs, to describe small bodies orbiting among the giant planets.
More interestingly, Chiron’s orbit is unstable, meaning that within a million years or so, it will probably swing too close to Saturn and be tossed out of the solar system or onto a cometlike trajectory passing closer to the Sun. That also means that Chiron entered its current orbit in astronomically recent times. Like a lamb wandering the streets of Manhattan, it had to have come from somewhere else. No one knew where.
A third, crucial clue came from comets. Some comets, known as long-period comets, visit the inner solar system once in thousands or millions of years, or simply once. Others like Halley’s Comet are short-period comets that swing by more frequently, every few decades or centuries or so.
For a long time, most astronomers theorized that short-period comets were long-period comets that had been deflected by the gravity of a planet.
In 1988, computer simulations by three Canadian astrophysicists — Martin J. Duncan, Thomas R. Quinn and Scott Tremaine — showed that the orbits of deflected long-period comets would not match those of observed short-period comets and that their more likely origin was a ring of debris by Neptune. Those computer simulations spurred several teams to start searching more actively the outskirts of Neptune.
Dr. Jewitt, then at the Massachusetts Institute of Technology, and his graduate student Jane X. Luu had already started looking.
“Our search was very simply motivated by the surprising emptiness at the edge of the solar system,” Dr. Jewitt said. “We would have been happy with either answer: empty because it was empty, or empty because no one had looked.”
For years, they found nothing. To find moving objects requires taking repeated photographs of a region and looking for the points of light that moved between the images. The limits of digital technology stymied the early searches.
Finally, in 1992, they found 1992 QB1, probably about 100 miles wide, the first Kuiper Belt object.
Dr. Brown had a reaction similar to many astronomers. “It’s like, ‘Oh my god, Pluto finally makes sense,’ ” he said. “It’s no longer an oddball at the edge of the solar system.”
Pluto was instead the harbinger of many properties now seen in Kuiper Belt objects: the resonant orbits, the moons, the icy ingredients.
About that time, Renu Malhotra, a professor of astronomy at the University of Arizona, was calculating the effects of Neptune’s migrating outward, as some had hypothesized, early in the history of the solar system. Her calculations indicated that Neptune would effectively snowplow smaller objects into resonant orbits.
The first Kuiper Belt objects in resonant orbits were discovered in 1993. The distribution of resonant Kuiper Belt objects fit with what she predicted, and now planetary scientists uniformly agree that the giant planets were not born in their current orbits but migrated there.
Scientists are looking for more clues about the early history of the solar system. Eugene Chiang, a professor of astronomy at the University of California, Berkeley, theorizes that the solar system used to contain several more gas giant planets that were subsequently ejected, but that their gravitational effects remain imprinted in the Kuiper Belt.
Dr. Levison of the Southwest Research Institute, however, offers a different account. He and his collaborators have created a model where the solar system was initially much more compact, with all the giant planets forming well within the current orbit of Uranus.
That hypothesis sidesteps the cleanup problem, because shrinking the Kuiper Belt increases its density. His model predicts that gravitational wobbling between Jupiter and Saturn created wild oscillations in the orbits of Neptune and Uranus, with the two swapping places repeatedly.
Dr. Levison said he could not prove that his model was correct, just that it reproduces what is seen today in the solar system. And the Kuiper Belt was a key component in creating his model.
“Sometimes how the blood is splattered on the wall tells you more about what happened than the body,” he said.
The Kuiper Belt, Dr. Levison said, is “the blood splattering on the wall,’’ adding, “If we’re going to understand what happened, it’s going to be by studying the Kuiper Belt.”
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