Kuiper Belt and the Outer Solar System

Kavli prize winners continue research beyond initial discoveries.

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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.”