PITTSBURGH -- Pushing the Hubble Space Telescope to its limits, astronomers have detected what they say are more than two dozen Halley-sized comets in a great comet reservoir beyond the orbit of Neptune.
If they're right, it provides important evidence for the existence of the Kuiper Belt, a vast region beyond Neptune long suspected as the source of many familiar comets that periodically swing by the inner planets and around the sun.
It also represents a remarkable performance by Hubble. The comets are incredibly small and dim to be seen from such a distance. It's the equivalent of a detecting a 100-watt light bulb from 4.3 million miles.
"For the first time, we have a direct handle on the population of comets in this outer region. The solar system just got a lot more interesting," said University of Texas astronomer Dr. Anita L. Cochran.
Dr. Cochran headed the space telescope's Kuiper Belt Search Team, with Dr. Harold F. Levison and Dr. Alan Stern of the Southwest Research Institute in Boulder, Colo., and Dr. Martin Duncan of Queens University, Ontario.
They reported their findings yesterday to the 186th meeting of the American Astronomical Society.
The existence of the Kuiper Belt was first theorized in 1951. It remained only theory until 1992, when astronomers at Stanford University reported finding a 100-kilometer (61-mile)-wide object just beyond Neptune's orbit.
Several more like it have been found since. But Dr. Cochran said they're all much bigger than the 3- or 4-mile-wide "Halley-sized" comets that periodically leave the Kuiper Belt and tour the solar system.
"We've seen none of the 100-kilometer comets in the [inner] solar system," she said.
With the Hubble discovery, she said, "we have Halley-sized objects in the Kuiper Belt."
And plenty of them. Based on their observations, the Kuiper search team estimated there are 1 billion to 10 billion comets in the region of the Kuiper Belt they studied, which reaches from Neptune's orbit outward about 1 billion miles.
They are as much as 1,000 times more numerous than the rocky objects in the asteroid belt between the orbits of Mars and Jupiter. Even so, the combined mass of all Kuiper Belt objects may be less than 10 percent of Earth's.
The actual outer limits of the belt are unknown. From Neptune's orbit 3 billion miles from the sun, the Kuiper Belt is thought to extend tens of billions of miles farther out.
Astronomers believe comets are regularly kicked out of the Kuiper Belt, by collisions and the gravitational effects of the large planets. They make their way toward the inner planets and the sun, where they become the familiar tailed comets sometimes visible from Earth.
Dr. Levison said the Kuiper Belt, now that it can be studied, will help scientists learn more about the early solar system. It is "probably the most pristine region of the solar system, affected the least by planetary formation," he said.
In the more densely packed inner regions of the early solar system, the primordial gas, ice and dust were colliding and condensing under the influence of gravity to form the sun and major planets.
Beyond Neptune's orbit, however, the same material was more sparse, and planets never formed. That makes it a valuable laboratory.
"We can look out there now and see the process as it was when it went stillborn," Dr. Cochran said.
Pluto, usually thought of as the ninth and "last" planet, actually spends much of its time inside Neptune's orbit. Many scientists think tiny Pluto, with a diameter of only 1,500 miles, is not really a planet but a product of the Kuiper Belt.
They believe Pluto, and several moons of the outer planets, are actually "refugees" captured after leaving the Kuiper Belt.
The existence of such a region was first suggested in 1951 by Dutch astronomer Gerard Kuiper, who thought it was unlikely the solar system ended beyond Neptune's orbit.
Other astronomers noticed a class of comets that seemed to come from someplace closely aligned with the rest of the solar system.
Called "Jupiter family" comets, they return in periods of 20 years or less along orbits very close to the plane of the major planets, and they circle the sun in the same direction as the planets.
These are the ones thought to come from the Kuiper Belt.
Other comets, called the "Halley family," had periods longer than 20 years. They seemed to come from farther away, and from regions of space at widely varying angles to the plane of the planets.
Astronomers believe Halley family comets come from the Oort Cloud, a spherical halo of comets far beyond the Kuiper Belt and 10,000 times farther from the sun than Earth.