Beyond Pluto, a new frontier of discovery

It's an old trap in science - to look at a field of research and conclude that all the big questions have been answered, leaving little to do but cross the t's and dot the i's.

Robert Millis nearly stumbled into that trap.

"I've been a ground-based astronomer my whole career, and I have to tell you, 10 or 15 years ago I was becoming a bit discouraged," he recalls. When it came to the solar system, "the period of genuine exploration - pushing back frontiers and finding totally new things - seemed to me to be over."

Now, he and his colleagues find themselves on the vanguard of a group of astronomical pioneers pushing deeper into a frontier known as the Kuiper Belt.

Stretching for tens of billions of miles beyond Neptune, the belt is thought to hold at least 100,000 icy relics from the solar system's birth. Dr. Millis, director of the Lowell Observatory here, heads a group that is systematically hunting for these elusive objects in a kind of cosmic census. Their work, along with similar efforts by a handful of other groups, is expected to lay the foundation for building a more complete picture of conditions in the early solar system and explaining how the planets migrated into their current orbits. The work also is expected to provide data that will help astronomers interpret what they see as they look at solar systems forming around other stars.

Kuiper Belt objects "have the ability to tell us things we'd never learn otherwise, because they've been sitting out there largely undisturbed for a long period of time," Millis says.

"The Lowell group is really at the forefront of charting this third zone of the solar system for us," says Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "When I went to grade school, all of us were taught that there were four rocky inner planets, four gas giants, and one oddball, Pluto. It looked like a misfit, but it's a perfect fit in a different context - the Kuiper Belt. Now we get it." Pluto, long regarded as a planet, has finally found a home as a Kuiper Belt object (KBO).

Today, roughly half of the 1,000 known KBOs have been discovered by Millis and his team in a project known as the Deep Ecliptic Survey. The team is in the project's homestretch, sampling the night sky in the northern and southern hemispheres in a band of sky that reaches seven degrees above and below the ecliptic - an imaginary path traced by the sun and the planets as they move across the sky. Unlike the Oort Cloud, a halo of extremely distant icy objects whose orbits trace a rough sphere around the sun, most KBOs appear to orbit the sun roughly in a plane along the ecliptic.

Finding the objects is painstaking work. The objects are small, distant, and thus extremely faint. Many are discovered only to be lost because weather, hardware failures, or other factors conspire to prevent timely follow-up observations. Without the orbit information these follow-ups provide, astronomers don't know where to point their telescopes the next time they want to study the object in more detail.

One measure of the Kuiper Belt's importance lies in the generous amounts of precious telescope time astronomers are given on some of the world's largest telescopes to explore it. Another measure of intense interest is the New Horizons mission to Pluto and the Kuiper Belt, which the National Aeronautics and Space Administration currently has scheduled for launch in January 2006.

The search for KBOs began, if unwittingly, here at the Lowell Observatory. In 1930, Clyde Tombaugh discovered Pluto after carefully photographing the night sky along the ecliptic for a year. By comparing the plates, he spotted an unknown object that moved against the stellar background.

"The discovery was ahead of its time," Dr. Stern says, noting that another 62 years would pass before detector technologies advanced enough to allow astronomers David Jewitt of the University of Hawaii and Jane Luu of the University of California at Berkeley to spot the first "official" Kuiper Belt object.

The belt's existence had been the subject of speculation for years prior to the duo's discovery. Astronomers Kenneth Edgeworth and Gerard Kuiper had grumbled that it seemed too odd for the distribution of planetary mass in the solar system to rise with distance, only to suddenly plummet after Neptune.

They basically concluded that the observation "didn't make much sense" and that there must be other "stuff" farther out, Millis explains.

Then in the late 1980s, a team of solar-system modelers decided to test the idea that so-called short-period comets came from the Oort Cloud and were tugged into their relatively quick circuits around the sun by Jupiter's gravity. The modeling team threw Oort Cloud objects at Jupiter from every direction, let the simulation run, but couldn't duplicate the population of short-period comets astronomers actually see today.

"So they predicted that there has to be another source region - it's likely to be a disk - and coined the term Kuiper Belt," Millis says.

Since the first widely recognized Kuiper Belt object was discovered, the list has grown to include several objects with diameters of 1,000 kilometers or more. This includes Sedna, discovered last year in an orbit that takes it out to nearly 1,000 astronomical units (nearly 93 billion miles) from the sun. These objects are of particular interest because they are bright enough to allow astronomers to make detailed measurements of their properties. These measurements could lead to a better understanding of how the objects formed and what kind of abuse they've taken during their wanderings.

The discoveries have led to follow-up observations that raise fascinating questions, notes John Davies, an astronomer at the Royal Observatory in Edinburgh, Scotland. Scientists are trying to see if the population can be sorted by color - some KBOs appear a faint gray, others a faded red. The origins of the colors are a topic of debate. So are the origins of binary KBOs - large objects with a smaller companion orbiting it. Finally, is the far edge of the belt really an edge, or merely a gap?

"I keep thinking there could be significant objects farther out there - clearly planet-size objects - but beyond the current reach of our telescopes and cameras," Millis says. "There's no evidence that they're not out there; we just haven't been able to look."