Astronomers identify a new class of comet.

When astronomer Uwe Fink began studying the pattern of light emissions from nearly 30 comets a few years ago, he suspected some of the spectra might yield a mild surprise or two. In particular, he reasoned that a few comets might contain slightly lower concentrations of two compounds commonly found in high abundance in these solar system bodies--molecular carbon and cyanogen, a daughter product of either hydrocarbon dust grains or hydrogen cyanide. But he never expected to find a comet containing no traces of either compound.

These spectral studies now indicate that the comet known as Yanaka (1988r) belongs to a new class of comets -- one that originally may have orbited a Milky Way star other than the sun. Fink and his colleagues at the University of Arizona in Tucson presented their findings in Palo Alto, Calif., last week at the annual meeting of the American Astronomical Society's Division for Planetary Sciences.

Using the University of Arizona's 61-inch telescope atop Mt. Bigelow, near Tucson, Fink photographed and collected three light-emission spectra from Comet Yanaka during the early morning hours of Jan. 15, 1989. At the time, the object was a relatively close 34 million miles from Earth and just 86 million miles from the sun -- roughly 93 percent of Earth's distance from the sun.

Though the three visible-light and near-infrared spectra indicate that the comet contains normal concentrations of ammonia, the data reveal no evidence of cyanogen or carbon. Even at much greater distances from the sun, the vast majority of comets - including Halley -- show detectable levels of both compounds, Fink observes. "No other comet in our spectral library has shown this behavior," he says.

"It's an anomalous finding," agrees Paul R. Weissman of the Jet Propulsion Laboratory in Pasadena, Calif. Moreover, he adds, "It's intriguing to speculate about how the comet formed."

Weissman notes that Yanaka's orbit, while still somewhat unclear, indicates this object belongs to a group of long-period comets that pass near the sun every 10,000 years or so. Astronomers believe such comets spend much of their time in the Oort cloud, a huge spherical region postulated to exist near the edge of the solar system (SN: 4/21/90, p.248). Such a cloud would appear too faint for observation from Earth.

Yanaka may have emigrated to the cloud from a region of the solar system that contained relatively little carbon, Weissman speculates. Alternatively, he says, the comet may represent the remnant of an icy body that one orbited a planet and lost most or all of its carbonaceous compounds by chemical processes.

In a more exotic but less likely scenario, Yanaka may have resided in an Oort-like cloud orbiting another Milky Way star until our sun gravitationally captured the comet. However, Weissman notes, calculations indicate that such kidnappings have occurred only rarely during the life of our solar system. Fink adds that since most stars contain lots of carbon, this scenario still does not readily explain Yanaka's lack of carbon compounds.

Yanaka, which as a headed away from the sun since the 1989 observations, now appears too dim for researchers to take further spectra. Fink told SCIENCE NEWS that an additional group of about 30 comets his team has studied seems to contain normal amounts of carbon compounds. He and his colleagues are now comparing the relative abundances of carbon and ammonia compounds in these comets with those of better-studied comets, such as Halley. In addition, the team has begun to take spectra from another group of comets not previously studied, he says.