Researchers Find Similarities Between Comet Hale-Bopp And Comets Outside Earth's Solar System

April 08, 1997

Cincinnati -- University of Cincinnati astronomer Michael Sitko and his collaborators have found similarities between the composition of Comet Hale-Bopp and what appear to be similar- sized comets in other solar systems.

"They're almost identical," said Sitko, an associate professor in the physics department at UC. "We compared the spectrum of our star with the spectrum of Hale-Bopp. It looks as if the material we see in these stars is exactly equal to the material that's being seen in Comet Hale-Bopp."

Sitko reported his findings during the recent International Conference on the Formation and Evolution of Solids in Space March 10-21 in Erice, Sicily. They're based on infrared observations of evolving solar systems where a new star is surrounded by a great disc of dusty material. His evidence will be published in the formal proceedings from that meeting.

Sitko's collaborators are Carol Brady of Eureka Scientific in Oakland, David Lynch and Ray Russell of Aerospace Corporation in Los Angeles, and Martha Hanner of NASA's Jet Propulsion Laboratory in Pasadena.

Together, they've been trying to understand the changes which take a clump of interstellar dust and turn it into a solar system with a central star and orbiting planets. Their strategy has been to search for star systems which are in the middle stages of development between very early star formation and mature solar systems like our own.

"We can't watch one system go through its evolution, because it takes hundreds of millions of years, but we can look at different objects that we think are in different stages of the same formation process." explained Sitko. "We're trying to find the missing links."

So far, the researchers have identified 15 such systems using earlier ultraviolet spectra obtained with the International Ultraviolent Explorer (IUE) satellite, a forerunner of the Hubble Space Telescope. Although no one has searched the systems for direct evidence of planets, Sitko says there is circumstantial evidence that planets have formed. For example, the team found several star-grazing comets comets which are rushing at tremendous speed (up to 400 kilometers/second) toward a collision with the central star.

"As far as anybody knows, the only way to make an object like a comet hit a star is to have some source of gravity to throw it down there," said Sitko. "In our own solar system, it's things like Jupiter and Saturn. So, we think there's a good chance these systems will have planets circling them."

Because the researchers have seen so many star-grazing comets, it appears the systems are in a heavy bombardment period, similar to the bombardment period which left the great craters on the surface of Earth's moon over four billion years ago. However, the actual process of "star-grazing" makes these objects more visible in the infrared spectrum.

"As it approaches the star, material evaporates off. When it does, we can see it in the gaseous state and that's how we detect it," said Sitko. "We see iron and silicon and magnesium which you normally associate with rocky material, but we also see nitrogen and carbon. The whole thing together suggests we're seeing a comet-like object."

The astronomers were even able to estimate the size of the objects, a few kilometers across. The observations were made using three infrared observing facilities: the Broad Band Array Spectrometer System (BASS) in Hawaii, the Infrared Astronomical Satellite (IRAS), and the Infrared Space Observatory (ISO).

The researchers hope to make additional observations this fall, but they'd also like to conduct what Sitko calls the "reverse experiment." They want to look for similar patterns in systems where astronomers agree planets have been found. If the same combinations of minerals and elements are seen, that would indicate that solar systems tend to follow the same pattern of evolution. If the patterns don't hold up, that would indicate there is more than one way to build a solar system.

University of Cincinnati

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