Carbon ratio shows Halley may be alien.

Carbon ratio shows Halley may be alien

Comet Halley is an oddball of the solar system, according to a research report in the April 1 ASTROPHYSICAL JOURNAL. Scientists say the comet's ratio of carbon-12 to carbon-13 deviates from the ratio found in other solar system matter. Susan Wyckoff, Eric Lindholm and Peter Wehinger of Arizona State University in Tempe and colleagues say their analysis suggests Halley may be an alien body, born fairly recently outside the solar system and captured in a close encounter with the sun.

Current theory holds that comets originated near the orbits of Saturn and Uranus as the solar system condensed from a gaseous cloud about 4.6 billion years ago. Gravity flung the comets outward, leaving many in orbits far past the outer planets. Gravity from a passing star can disturb a comet's orbit and send it on a path swinging close to the sun at one end and far away at the other. Halley's comes about as close to the sun as Venus every 77 years.

During its 1986 flyby, Wyckoff's team measured Halley's carbon-12/carbon-13 ratio using a telescope and spectrometer on Australia's Mt. Stromlo. They found it to be about 65 to 1, compared with the 89 to 1 ratio measured in the solar system's planets and meteorites. Halley's higher proportion of carbon-13 is closer to that measured in interstellar gas and dust.

Wyckoff says some scientists believe the interstellar medium keeps getting richer in carbon-13 relative to carbon-12. When the solar system condensed, its ratio would have frozen in place while the ratio beyond it continued to evolve. Wyckoff and her colleagues say Halley may have solidified out of the interstellar gas and dust must more recently than the solar system, explaining its richer levels of carbon-13.

To join the solar system, the comet would have had no drift about as close to the sun as the inner planets -- considered an unlikely event. But Wyckoff says it is also unlikely Halley originated with the rest of the solar system, because the comet revolves around the sun in the opposite direction from the planets.

As an alternative to the capture hypothesis, the researchers say, a supernova might have enriched the rest of the solar system with carbon-12 while relatively little of the isotope reached distant Halley.

Other comet experts remain skeptical of the capture idea. The carbon isotope ratio is difficult to measure in comets, notes astrophysicist Paul Feldman of Johns Hopkins University in Baltimore. He says scientists need more comet measurements to determine if Halley is truly an adopted child.