Scientists gain key insights on comets by mining old data from now-silent satellite

Comet science has entered a new era. Now scientists can study a star system and its comets from the outside -- seeing it as a whole -- rather than just probing a star system from within, as they have been doing with our solar system.

This is part of the continuing fruitage from studying data gathered by the now-silent Infrared Astronomy Satellite (IRAS).

These data already have revealed something like 36 nearby stars surrounded by diffuse material suggesting clouds of comets, similar to one thought to accompany our solar system.

Comets are believed to represent material left over from formation of the solar system. Understanding their nature and origin is essential to understanding how the solar system formed and developed.

``We have the very exciting prospect right now that we're looking at other solar systems perhaps forming or already formed,'' says Paul Weissman of the National Aeronautics and Space Administration's Jet Propulsion Laboratory.

He adds that this illustrates the value of continuing to mine data from past space-science missions. It is a relatively cheap yet underfunded aspect of space research that can return big scientific dividends, he says.

Dr. Weissman was one of several scientists who reviewed the fast-developing knowledge of comets at the recent annual meeting of the Astronomical Society of the Pacific here.

Scientists generally believe comets consist of ice and dust with a nucleus of asteroid-like material -- the so-called ``dirty snowball'' theory.

Comets may also contain enough organic material to have seeded the primitive Earth with chemical precursors of organic life.

Such icy bodies would have formed from material that condensed relatively far out as the solar system formed. Perturbed by the outer planets, this cometary material settled into the so-called Oort cloud, named for Dutch astronomer Jan Oort who developed the theory.

Weissman noted that the latest data indicate there are something like 1 trillion to 2 trillion comets in that cloud between 100 and 10,000 astronomical units (AU) from the sun. An AU is the radius of Earth's orbit, or about 93 million miles.

Weissman and other speakers noted that recent data, especially from the IRAS, which detected material by the infrared (heat) radiation it emits, suggest there is also an inner cometary cloud. This is perhaps 40 to 80 AU from the sun. It may contain 10 to 100 times as many comets as the outer Oort cloud. Also, it tends to be concentrated in a belt, whereas the Oort cloud envelops the solar system.

Material orbiting the stars Vega and Beta Pictoris, two of the first such systems to be found, may represent just such a cometary belt.

William A. Baum of the Lowell Observatory said it now looks as though our solar system would be very peculiar if it did not have such material beyond the orbit of Neptune.

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