For generations astronomers have wondered whether all comets originated here in our solar system, or if any of them may have come from other stars.
Comets are sometimes seen to get ejected from our solar system with velocities great enough to fling them clean out of the Sun's gravitational domain, forever to roam loose in the dark interstellar spaces of the Milky Way. Moreover, this must have occurred to far more comets near the time of our solar system's birth. The same should be happening among comets around other stars that have planets. (Planets are what perturb comets' orbits and do the flinging.) Most stars are believed to have planets and cometary systems of their own.
One obvious sign of an interstellar comet in our midst would be an arrival speed greater than the Sun's escape velocity. The average motion of other stars with respect to our solar system is a few dozen kilometers per second, and comets ejected from other stars would carry these velocities too. Such a large excess speed would be plain as day, but no observed comet has ever been moving anywhere near so fast.
However, most comets do not have their original orbits. They've been perturbed into new ones by the slight tugs of the planets during earlier swings through the solar system. This is especially true for short-period comets that return every few years. Their orbits have been perturbed radically, mostly by the planet Jupiter.
One comet with a radically modified orbit is Periodic Comet Machholz 1 (also known as 96P/Machholz 1), discovered by amateur comet hunter Don Machholz in 1986 using 29×130 binoculars. It has an orbital period of 5.3 years and a high inclination with respect to the plane of the solar system. A new study finds that its chemical makeup is unique among 150 comets analyzed to date. It must have had a very unusual origin and might even have formed in another solar system where things were quite different than here.
However, dynamical studies have found that the chance of the solar system capturing and holding a passing interstellar comet is very slim.
Lowell Observatory has put out a press release about the new finding. The comet's gaseous exhalations turn out to be extremely deficient in carbon compounds, unlike any other. From the press release:
"Astronomers identified two other classes [of comets] in the 1990s. . . . While Machholz 1 also has strongly depleted C2 and C3 carbon species, what makes it anomalous is that the molecule cyanogen, CN, is extremely depleted. In Machholz 1, CN is missing by about a factor of 72 from the average of other comets, i.e., [it has] only a little above one percent of normal. . . .
"One possible explanation is that Machholz 1 did not originate in our solar system, but instead escaped from another star. In this scenario, the other star's proto-planetary disk might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances. . . ."
A second possible explanation is that Machholz 1 formed farther from the Sun than most comets, in a colder or more extreme environment.
A third possibility is that Machholz 1 has been, paradoxically, altered by extreme heat. In fact, Machholz does swing unusually close to the Sun (0.12 a.u.) every five years, as in the image at the top of this page. However, the only other comet to show a depletion of CN did not reach such high temperatures. This implies that CN depletion does not require extreme heat.
"If additional carbon-bearing species are also strongly depleted," says study author David Schleicher, "then the case for its origin outside of our Solar System would be strengthened." The next opportunity to check for this will be when Machholz 1 returns near the Sun in 2012.
Although the press release touted the interstellar theory, and news stories picked this up, there are reasons to be skeptical. An analysis back in 1986 found that order to shed a typical interstellar velocity of 20 km per second and get captured by our planetary system, a comet would have to make such a close pass by Jupiter that (based on the observed upper limit on the number of interstellar comets) such an event will happen less than once in 60 million years.
On the other hand, the rare interstellar comet than happens to have a very low speed with respect to the Sun (much less than 1 km/sec) would be captured more easily: with only a little more trouble than the capture of a comet falling in from the Oort Cloud, something that happens often.
Comet expert Paul Weissman (Jet Propulsion Laboratory) calls an interstellar origin for Machholz1 "the least likely solution" for its weird chemical composition. "Capture of an interstellar comet into the solar system is a very unlikely event. Extreme thermal processing, or a primordial difference in the composition of Oort Cloud comets, are more likely explanations. In the past the explanation for the carbon depletions is that those comets formed farther out from the Sun." The high inclination of Machholz 1's orbit, 59°, suggests that it came from the Oort Cloud rather than the Kuiper Belt, which is the source of most short-period comets.
Here's Schleicher's research paper on Machholz 1, which appears in the November Astronomical Journal.