As in other comets, the ices of a main belt comet vaporize and create a coma as they screech past the sun. But bizarrely, they orbit in the asteroid belt, a graveyard of debris that did not coalesce into planets.
The first main belt comet was discovered in 1996, but “you can always explain one weirdo,” Dr. Snodgrass said, suggesting that the belt could have captured an interloping comet. However, eight others have since been detected. Around 20 other belt-bound objects seen shedding mass — possibly because of comet-like periodic ice vaporization, wild spinning or recent impacts by asteroids — are considered candidates to be comets.
Researchers from the study, hoping to spot more mavericks in the main belt, found just one new candidate: 2001 NL19.
“This is the only one that seems to have a little something,” Ms. Ferellec said, describing the object’s faint tail-like feature streaking away from the sun. It could have been born of ice vaporization, making the object cometary. More observations will be necessary as it re-approaches the sun, when a coma or tail is most likely to appear.
Regardless of how 2001 NL19 is classified, the number of confirmed main belt comets suggests that “these things are native to the asteroid belt,” Dr. Snodgrass said. Their genesis remains hazy, though some ideas have been put forward.
Perhaps main belt comets, like their more distant, conventional counterparts, formed far from the sun during the chaotic early days of the solar system, but instead of remaining remote they were jostled by the gravity of other objects and placed into what is now the asteroid belt. After billions of years, any surviving primordial ice would be buried deep below their rocky surfaces. If they are hit by another asteroid, some of this ice will be excavated, exposing it to scorching starlight.
Uncertainties aside, one thing is clear: The existence of these asteroid-comet screwballs complicates the urge to put natural phenomena into neat little boxes.
“I always say, ‘Everything is a comet,’” said Kacper Wierzchos, an astronomer at the Lunar and Planetary Laboratory at the University of Arizona who was not involved with the study. “If you brought my couch close enough to the sun, it would start melting and having a coma.”