London, Dec 3 : Astronomers have discovered a new comet which raises the possibility that it did not originate in our solar system, but instead escaped from another star.
According to a report in New Scientist, David Schleicher of the Lowell Observatory in Flagstaff, Arizona, US, found the comet.
Schleicher 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 percent of the normal level.
Along with some other comets, it is also low on the molecules carbon2 and carbon3.
Schleicher has suggested three possible explanations for the origin of the comet, among which 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?" said 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 the most 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," he added.
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.
Machholz 1 will be close to the Sun again in 2012, when astronomers can study its bright coma of gases again.