Comets are often found in the asteroid belt between Mars and Jupiter: Here’s why

What do you expect to find in the asteroid belt between Mars and Jupiter?

Unsurprisingly, asteroids — millions of bits of rocky debris. But recently, astronomers have also found some
oddball objects that appear to be hiding in the rubble: Comets.


Hunting Down Misfits
As reported recently in a study, a survey dedicated to hunting these misfits might have spied another icy
individual blasting its own matter into space.

Scientists identified the suspected comet with the wide field camera of the Isaac Newton Telescope on the
Canary Island of La Palma. During three observation runs from 2018 through 2020, they watched 534
different asteroids, looking for signs of a comet’s coma — its ephemeral gassy shell — or tail made by the
dust in the coma being pushed by the Sun’s radiation.

Conventionally, comets are made of a solid core of various ices and dust. As a comet approaches the
Sun, its most volatile ices vaporise, creating a coma and two types of tails.

A Graveyard Of Debris
As in other comets, the ices of a main belt comet vaporise and create a coma as they screech past
the Sun. But bizarrely, they orbit in the asteroid belt, a graveyard of debris that don’t coalesce into planets.

The first main belt comet was discovered in 1996, but “you can always explain one weirdo”, Colin Snodgrass, an astronomer at the University of Edinburgh and a coauthor of the study said, suggesting
that the belt could have captured an interloping comet. However, eight others have since been detected.

Researchers from the study, hoping to spot more mavericks, found just one new candidate: 2001 NL19. “This
is the only one that seems to have a little something,” said Léa Ferellec, a postgraduate student of astronomy
at the University of Edinburgh, describing the object’s faint tail-like feature. It could have been born of
ice vaporisation.

Regardless of how 2001 NL19 is classified, the number of confirmed main belt comets suggests that “these things are native to the asteroid belt”, Snodgrass said. Perhaps main belt comets 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.

One thing is clear: The existence of these asteroid-comet screwballs complicates the urge to put natural phenomena into neat little boxes.