By Marlene Cimons, National Science Foundation
For the longest time, most astronomers assumed that the outer solar system was empty. They believed nothing existed in the region beyond Neptune’s orbit. Moreover, they didn’t even try to look.
But not every scientist felt that way. “It’s kind of remarkable that the solar system is our cosmic backyard, and yet it’s obvious that we still don’t know what’s in it,” David Jewitt says.
Jewitt, professor of astronomy at the University of California, Los Angeles, and Jane Luu, an astronomer with the Massachusetts Institute of Technology’s Lincoln Laboratory, always thought it unlikely that the outer solar system would be bare “when the inner solar system was so full of objects,” Jewitt says.
So in the middle 1980s, the two astronomers—Jewitt had been Luu’s doctoral thesis advisor when he was at MIT--started peering through telescopes at swaths of sky, itself a major challenge. They needed good-sized telescopes that could provide a large field of view, technology that wasn’t available three decades ago. The charge-coupled devices (CCDs) used on telescopes at that time had less than 512 pixels by 512, yielding a very small field of view, meaning it would take a long time to cover a significant area of the sky.
As a result, they couldn’t scan large areas of the sky, only tiny patches at a time, a problem when “trying to find faint objects in a big sky,” Jewitt says. “It’s hard to see things when they are very faint, as the brightness of an object decreases with distance.”
A given body at 10 AU--an AU is the distance between the Earth and the Sun--will appear 10,000 times fainter than it would at 1AU, he says. Objects could exist out there, but be so faint as to have escaped detection.
“We looked for a couple of years using different techniques, and we found nothing,” Jewitt says. “We had a big sky with a small telescope and a small field of view. We couldn’t see anything. We talked about giving up. Other people told us to stop. Telescope time was hard to get. Money was hard to get. It was kind of painful. But the detectors were getting better every year, so every year I thought, maybe we should try again. There could’ve been nothing to see, but we kept on trying. Our instincts told us that we just hadn’t looked hard enough.”
Their persistence paid off. In 1992, they discovered the first known object, QB1, in the area they were to call the Kuiper Belt, a region distant from the Sun by between 30 and 50 times the Earth-Sun distance.
“It was a faint moving dot,” Jewitt says. “Then, six months later, we found a second one, then four more, then ten more, then many tens. We had broken a barrier: after finding the first one, finding others was easy and more than 1,300 are now known.”
The name QB1 reflects the order of its discovery in the lists of small solar system objects in 1992. The researchers named the collection of the bodies in the region after Gerard Kuiper, a Dutch-American astronomer who had discussed the outer solar system in a 1951 paper. However, the scientists later realized they had made an error in choosing the name.
“If you don’t read his paper carefully, you’d think he had suggested that there are objects to find in the space beyond Neptune,” Jewitt says. “But if you read it carefully, you realize that he concluded there should be no objects beyond Neptune because they would’ve been ejected by Pluto’s gravity. It was a mistake.”
The objects in the Kuiper Belt likely are made up of ice and dirt, like comets. In fact, “we think this is the birthplace of comets,” Jewitt says, adding: “The Kuiper Belt is the solar system’s deep-freeze storage reservoir for the icy nuclei of comets.”
Recently both Jewitt and Luu received the 2012 Kavli prize for astrophysics, with Michael Brown of the California Institute of Technology, who built upon their work.
“I started in studying the Kuiper Belt pretty soon after Jewitt and Luu discovered it,” Brown says. “It seemed new, exciting and potentially extremely important.” Before Jewitt and Luu’s breakthrough, however, “I was initially taught that there was nothing else out there,” Brown adds. “Or, more specifically, I was not taught to even consider that there might be something else out there.”
The Kavli award recognizes scientists for seminal advances in astrophysics, nanoscience and neuroscience, and includes a $1 million cash prize for the field. Jewitt, Luu and Brown were selected for their fundamental contributions to the understanding of the outer solar system.
The same week, Jewitt and Luu also received the Shaw prize--also with a $1 million award--for their discoveries. The Shaw Prize citation describes their work as “an archaeological treasure dating back to the formation of the solar system” and says that the objects “provide our best record of the early stages of planet formation.”
The National Science Foundation (NSF) has been a supporter of Jewitt’s research into comets, themselves former residents of the Kuiper Belt. NSF also has funded Brown’s research, which involves trying to understand the surfaces and atmosphere of satellites of the giant planets.
QB1 is 200 kilometers across, and the biggest objects are about the size of Pluto, which is about half the size of the moon. “We know already there are about 70,000 that are bigger than 100 kilometers, and about 1 billion bigger than a kilometer,” Jewitt says.
Also, there is at least one member, Eris, discovered by Brown, as big as Pluto and possibly slightly larger. Eris, in fact, was the largest object found in the solar system in 150 years, and the object leading to the debate, and eventual demotion, of Pluto from a real planet to a dwarf planet. Pluto now is regarded as part of the Kuiper Belt.
Jewitt and Luu now are studying the attributes of the objects, including the shape of their orbits, their sizes and composition. “The idea is to find out what the Kuiper Belt is like, what’s in it and how it relates to the origins of the solar system, “Jewitt says. “How does it produce comets? What is the relationship between objects in the Kuiper Belt, and objects in the inner solar system?
“We’ve discovered the biggest piece of real estate in the solar system, with 1,000 times more objects than in the asteroid belt,” he adds. “It really makes you wonder what else is out there.”
Brown is interested in what lies beyond the Kuiper Belt. “These most distant regions are essentially unchanged from the earliest time of the birth of the sun, so studying them gives us a window back into that time period,” he says.
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