Halley’s Comet May Be a Deep Space ‘Interloper’

Halley’s comet, which has fascinated scientists and lay people alike because of its periodic and sometimes spectacular return to the Earth’s neighborhood, may have originated somewhere beyond the solar system, making it truly a visitor from deep space, scientists reported Wednesday.

It has long been thought that the periodic comet came from a cloud of “dirty snowballs” on the fringe of the solar system, but a new chemical analysis suggests that Halley may be different from all other objects in the solar system and could have been an “interloper” that was captured by the sun’s gravity as it passed nearby.

That scenario offers one explanation for why Halley now appears to be somewhat different than scientists had thought, said astronomer Susan Wyckoff of Arizona State University.

Using a technique that has allowed astronomers to examine the chemical composition of a comet for the first time, Wyckoff and a team of researchers discovered that the ratio between carbon 12 and carbon 13 in Halley “differs from all other solar system objects examined, including terrestrial and lunar rocks, meteorites and the atmospheres of large planets,” she said.

The National Science Foundation, which sponsored the research along with the National Aeronautics and Space Administration, announced the results Wednesday. The findings are also reported in the April 1 issue of the Astrophysical Journal.

The scientists used a sensitive instrument that reveals the chemical composition of a celestial body through a process called “spectral analysis.”

She said even an instrument as sensitive as the one used by the team is effective only on bright objects. The big question now remaining is whether any other comets have the same composition as Halley.

“There’s another bright one coming in sometime this summer,” Wyckoff said. That comet, called Brorsen-Metcalf, should be bright enough for chemical analysis, and like Halley, it passes this way rarely.

If it turns out that Halley is different than all other comets, “it might mean that it was a captured comet,” she said. “The sun just ventured too close to an interloper comet” and the comet became trapped in the sun’s gravitational field.

That would explain one thing that has mystified scientists for centuries: Halley travels in the wrong direction as it orbits the sun.

“Only five out of 124 (periodic comets that return again and again to the inner solar system) go around the sun backwards,” she said. “People have been puzzled by that.”

That could be explained if Halley was an interloper captured by the sun’s gravity and it entered the solar system in such a way that it ended up going around the sun in the opposite direction of nearly everything else.

An alternative reason could be that a star exploded nearby just as the our sun was forming, “splattering parts of the solar system with carbon 12 material,” Wyckoff said. Some materials, possibly similar to whatever formed Halley’s comet, may have been somehow shielded from the blast and ended up with less carbon 12 than others.

That could explain why the ratio of carbon 12 to carbon 13 is 89 to 1 throughout most of the solar system, including on Earth, and only 65 to 1 in Halley’s comet, according to the new findings.

Whatever the explanation, Halley’s comet continues to engage the astronomical community despite the fact that it long ago disappeared from the southern sky on its journey back toward the outer edge of the solar system. It takes about 76 years for the comet to complete one orbit, and it has been observed for more than 3,000 years.