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 carbon2 and carbon3."

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 carbon2 and carbon3 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.

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."