This week, Astronomy.com reported some intriguing comet research at Lowell Observatory in Arizona. A Lowell scientist, Dave Schleicher, studies the chemistry of comets. He and his colleagues recently found that Comet 96P/Machholz 1 has a weird chemistry. Machholz is extremely low in a chemical called cyanogen compared to other comets. The Lowell researchers think it may be a totally new class of comets, possibly cooked up billions of years ago in a very different way than their comet comrades in the early solar system.
Or — and this is pretty speculative — Machholz could be a refugee from another star system, kicked out by a gravitational glitch onto an unfathomably long journey to our corner of space, whereupon the Sun’s gravity snatched it from the void.
The press officer for Lowell, Steele Wotkyns, called me to see if I wanted to talk to the scientist, Dave Schleicher, about the work. This is a routine interaction. Scientific institutions want coverage of their work and we want to pass along interesting new findings to readers.
“Interesting” is the key word. What’s interesting to a reader? Making that call is part experience, part gut instinct. I ask myself what is inherently cool about the discovery. In this case, it came down to two issues: chemistry and origin.
Chemistry can be a tough sell to a broad audience. So I thought I’d ask Schleicher what this oddball cyanogen-poor comet would smell like. Most people relate to the universal basics of smell, touch, taste, sound, and sight.
Schleicher seemed a little dubious about my motives for asking this silly question, but he was a good sport about it. He says if you thawed out a chunk of comet in the lab and sniffed it, it might smell of ammonia. It also contains lots of pure carbon compounds, but these might not smell like anything in particular.
As for cyanogen, it was a poison gas used in World War I. “Presumably it had an identifiable odor,” Schleicher told me in an e-mail, “such that soldiers knew when to put on their gas masks.” Indeed, cyanogen chloride is a toxic, foul-smelling nerve gas that causes rapid paralysis and death. So don’t play with a comet, should you ever come across one.
OK, we got the smell thing out of the way. Now for the serious stuff: Could this comet be, like Clark Kent (a.k.a. Superman), a strange visitor from another world?
If so, the travel time to our solar system would be stupendous. For the comet to be captured by our solar system, it can’t come into the neighborhood too fast. Otherwise it might just fling itself back out into interstellar space.
Assuming a relatively slow inbound speed — something roughly the velocity of a commuter jet — Machholz would take something like 10 million years to reach us from the nearest star, Alpha Centauri. “More likely,” Schleicher said, “if Machholz 1 did come from another stellar system, it has been on its journey for hundreds of millions or even more than a billion years.”
It would be tough to prove extrasolar origin for a comet, Schleicher notes, because such an interstellar refugee would enter the inner solar system at about the same speed as any other comet originating from the Oort Cloud — a cloud of comets extending from 1,000 to 50,000 times the Sun-Earth distance.
However, if a comet came into the solar system at a relatively high velocity and with a hyperbolic orbit, astronomers might argue that it came from another star. But no one has seen such a comet — yet.
Interestingly, Schleicher said, about one comet probably escapes our solar system each year. “After a billion years or so, that's a lot of comets that we have lost, and there is no reason to believe that the same thing isn't happening to comets in other stellar systems.”
Schleicher’s project has studied about 150 comets so far, probing them for spectroscopic clues to their chemical composition. Chemistry tells us how and where they formed and could reveal important new details about the early solar system. If astronomers ever do discover a comet from another star, Schleicher said, statistical analysis of lots of comets will probably provide the key evidence.
Another approach, Schleicher says, is to journey to the comet in question and bring a sample of it home. Detailed analysis of its chemistry could potentially identify it as from another star system.