Comet 96P/Machholz (lower left) comes very close to the Sun, whose light is blocked in this 2002 image taken by the SOHO spacecraft (Image: SOHO/LASCO/ESA/NASA)

A comet orbiting our Sun may be an interloper from another star system.

Comet Machholz 1 isn’t like other comets. David Schleicher of the Lowell Observatory in Flagstaff, Arizona, measured the chemical makeup of 150 comets, and found that they all had similar levels of the chemical cyanogen (CN) except for Machholz 1, which has less than 1.5% of the normal level. Along with some other comets, it is also low on the molecules carbon 2 and carbon 3 .

Schleicher suggests three possible explanations.


The simplest is that Machholz 1 could have formed in an extremely cold region of the solar system. The other comets depleted in carbon 2 and carbon 3 are thought to have formed in the chilly outer regions of the Kuiper belt far beyond Neptune, where the low temperatures mean that most carbon gets trapped in other molecules. “In really extreme cold, maybe the cyanogen goes away as well?” says Schleicher.

Hurled out

A second possibility is suggested by the comet’s peculiar orbit. Machholz 1 approaches very close to the Sun on its orbit, closer even than Mercury, so it is possible that repeated baking by the Sun’s heat has removed most of its cyanogen.

But a more exciting idea is that Machholz 1 is an alien. “An extrasolar origin makes it easy to explain the composition – of course we’d expect everything to be different,” Schleicher told New Scientist. “Here, three molecules with carbon are all depleted, so maybe carbon is depleted across the board? That sounds to me as if it came from somewhere else and is not just an oddball from our solar system.”

Astronomers expect some comets to be hurled out of their stellar systems by the gravity of giant planets. In theory, they should be wandering around out there, and sometimes come flying though our system – although no comet has yet been found moving fast enough to be identified as an extrasolar interloper.

If Machholz 1 did fly in from interstellar space, a chance encounter with Jupiter could have made it a captive of our solar system.

This would be an extraordinary discovery, but much more evidence is needed before the alien origin is any more than a hypothesis. Fortunately, Machholz 1 will be close to the Sun again in 2012, when astronomers can study its bright coma of gases again.

David Jewitt of the University of Hawaii is happier with the data than with Schleicher’s conclusions.

“The question is how to make the jump from measuring chemical depletions to a conclusion about where the comet originated,” he told New Scientist. “We don’t know enough to do that with any certainty. Still, the observational result is a good one and Dave’s speculation about the source of the comet is very . . . stimulating.”

Journal reference: Astronomical Journal, vol 136, p 2204