Comet Reservoir Detected at Outer Reaches of Solar System

Hubble Telescope helps astronomers confirm existence of Kuiper Belt

by Harold F. Levison, Ph.D.      image of PDF button


Senior Research Scientist Dr. Harold F. Levison works in the Space Sciences Department Boulder, Colorado, office, where he specializes in celestial mechanics and the origins of the solar system.


NASA’s Hubble Space Telescope has helped scientists detect a long-sought population of comets dwelling at the icy fringe of the solar system. The observation, which is the astronomical equivalent of finding the proverbial needle in the haystack, bolsters proof for a primordial comet reservoir just beyond Neptune, at present the planet farthest from the Sun.


Above is an artist's concept of the Kuiper Belt of comets outside Neptune's orbit (planets are not drawn to scale). The comets are debris remaining from the formation of the solar system 4.5 billion years ago.

Based on the Hubble observations, a team of astronomers including principal investigator Anita Cochran of the University of Texas in Austin; the author and Dr. Alan Stern of Southwest Research Institute’s Boulder, Colorado, office; and Martin Duncan of Queen’s University, Ontario, Canada, estimate the belt contains at least 200 million comets — the remnants of the birth of our solar system 4.5 billion years ago. The region is thought to be the source of comet Shoemaker-Levy 9.

“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,” Cochran says. “We now know where these short-period comets formed, and we have a context for their role in the solar system’s evolution.”

The existence of a comet belt encircling our solar system — like the rings that wrap around Saturn — was hypothesized more than 40 years ago by astronomer Gerard Kuiper. The so-named Kuiper Belt remained just a theory until ground-based telescopes began detecting about 30 large icy objects ranging from 60 to 200 miles in diameter. The planet Pluto, with a radius of 1,200 km, is considered by astronomers to be the largest member of the Kuiper Belt region. However, researchers had to wait for the Hubble Space Telescopes’ high spatial resolution and sensitivity before they could search for an underlying population of much smaller bodies assumed to be present —just as there are more pebbles on the beach than boulders.

“This is a striking example of what the Hubble can do well,” continues Cochran. “We can at last identify small comet-sized objects that are just a few miles across, about the size of New York’s Manhattan Island.” The team discussed their findings at the 186th meeting of the American Astronomical Society in Pittsburgh, Pennsylvania, in June 1995.

The findings apparently close the mystery of the source of short-period comets that orbit the Sun in less than 200 years, including such members as comet Encke, Giacobini- Zinner, and the infamous comet Shoemaker-Levy 9, which collided with Jupiter in July 1994. The comet belt lies just beyond Neptune and may stretch 500 times farther from the Sun than Earth. This is 100 times closer to Earth than the hypothesized Oort cloud, commonly thought to be a vast repository of comets that were tossed out of the early planetary system. Despite their close proximity to the sun, the Kuiper belt comets don’t pose any greater threat of colliding with Earth than comets that come from much farther out.

Scientists believe the Kuiper Belt is the best laboratory in the solar system for studying how the planets formed, because, unlike the planets, the comets have remained largely unaltered since the solar system was born.

Comet nuclei are considered the primordial building blocks that condensed out of the cloud of gas, dust, and ices that collapsed to form the Sun. Knowing where comets come from will help define and develop models for the formation of the solar system and will reveal something new about our origins.

The icy nuclei are too far away to have the characteristic shell (coma) and tail of gasses and dust that are a comet’s trademarks when it swings close enough to the Sun to warm up and sublimate. Detecting these bodies in their “deep freeze” state, at the dim horizon of the solar system, pushed the Hubble Space Telescope to its performance limits.

The team used the Hubble’s Wide Field Planetary Camera 2 to observe a selected region of the sky in the constellation Taurus, which had few faint stars and galaxies to confuse the search. The detection is based purely on a statistical approach, because the objects being discovered are so faint (see box below).

The search for objects continues, and more images have already been collected with the Hubble. These images will allow scientists to better quantify the number and sizes of comets in the Kuiper belt, as will additional Hubble observations slated for the future.

Published in the Fall 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.


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