The bizarre chemical make-up of a comet suggests the blob of ice is an interloper, possibly flung into our solar system from beyond, astronomers now say, adding that the wacky comet is forcing them to create a new category for such objects.

The comet, called Machholz 1, was discovered in 1986 by Donald Machholz of Loma Prieta, Calif. Since then, the icy denizen has made a few appearances, including one in 2007.
""A large fraction of comets in our own solar system have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped,"" said David Schleicher, a planetary astronomer at Lowell Observatory in Arizona. ""Some of these will have crossed paths with the sun, and Machholz 1 could be an interstellar interloper.""
The oddball, described in the November issue of the Astronomical Journal, could shed light on the formation and evolution of comets. These blobs of dust and frozen gases may be the oldest, most primitive bodies in the solar system. In fact, they hold the earliest record of material from the nebula that formed the sun and planets.
Schleicher measured the amounts of certain carbon and other compounds in the coma or head of Machholz 1 in 2007. He compared the composition with information from 150 other comets, finding that Machholz 1 had an odd make-up. Particularly, the comet contains much less of a carbon-nitrogen molecule called cyanogen, by a factor of about 72, compared with the average found in other comets.
The comet also contained much less of two molecules called C2 and C3 (which have two and three atoms of carbon in their structures, respectively) than the average comet.
Schleicher puts forth three scenarios that could be responsible for the odd chemistry, although no one explanation stands out as right on. ""I'm not real thrilled with any of the three
[scenarios],"" Schleicher told SPACE.com.
One possible explanation is that Machholz 1 did not originate in the solar system, but instead escaped from another star's gravity.
In this scenario, the other star's proto-planetary disk (the disk of debris from which planets are thought to form) might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances.
Several comets previously in our solar system have gotten the boot when their orbits came into contact with that of Jupiter, whose gravity kicked them out.
Another possible explanation for Machholz 1's anomalous composition is that it formed inside the solar system but even farther from the sun in a colder or more extreme environment than other comets.
A third possibility is that Machholz 1 originated as a so-called carbon-chain depleted comet, but that this chemistry was subsequently altered by extreme heat and so cyanogen was also depleted.
While no other comet has exhibited changes in chemistry due to subsequent heating by the sun, Machholz 1's orbit takes the comet to well inside Mercury's orbit every five years. (Other comets get even closer to the sun, but not as often).
(Source: Space.com)