"For a short while, we had this one weird celestial object that we couldn’t fully figure out.”
Davide Farnocchia of NASA’s Jet Propulsion Laboratory is one of a team of astronomers who've been tracking down so-called 'dark comets'.
These strange objects look like asteroids but behave like comets, and the first of their kind was discovered as recently as 2016.
Image of asteroid Dinkinesh captured by NASA's Lucy mission on 1 November 2023. The image revealed that Dinkinesh is not one asteroid, but two. Credit: NASA/Goddard/SwRI/Johns Hopkins APL/NOAO
Image of asteroid Dinkinesh captured by NASA's Lucy mission on 1 November 2023. Credit: NASA/Goddard/SwRI/Johns Hopkins APL/NOAO
Then astronomers found six more.
Now Farnocchia and his colleagues have discovered another seven dark comets, doubling the known number.
What's more, the increasing tally has enabled astronomers to categorise them in two types: large ones that live in the outer Solar System, and smaller ones in the inner Solar System.
Artist’s impression of `Oumuamua, the most famous example of a 'dark comet'. Credit: ESO/M. Kornmesser
Discovering dark comets
In March 2016, astronomers noted the trajectory of an asteroid known as 2003 RM had moved from its expected orbit.
"When you see that kind of perturbation on a celestial object, it usually means it’s a comet, with volatile material outgassing from its surface giving it a little thrust," says Farnocchia.
"But try as we might, we couldn’t find any signs of a comet’s tail. It looked like any other asteroid — just a pinpoint of light."
Then in 2017, the famous interstellar visitor known as 'Oumuamua was discovered.
'Oumuamua appeared as a single point of light, like an asteroid, but its trajectory changed as though it were outgassing, like a comet.
Outgassing is a typical trait of comets, as seen here on Comet 67P. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Outgassing is a typical trait of comets, as seen here on Comet 67P. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
"Oumuamua was surprising in several ways," says Farnocchia.
"The fact that the first object we discovered from interstellar space exhibited similar behaviours to 2003 RM made 2003 RM even more intriguing."
By 2023, seven such objects had been discovered, and the name 'dark comets' was born.
"We had a big enough number of dark comets that we could begin asking if there was anything that would differentiate them," says Darryl Seligman of Michigan State University, lead author of the paper.
"By analysing the reflectivity and the orbits, we found that our Solar System contains two different types."
Two types
The team behind the study find there are larger types known as 'outer dark comets' and smaller types known as 'inner dark comets'.
Outer dark comets are similar to Jupiter-family comets, in that they have eccentric (or elliptical) orbits and measure hundreds of metres or more across.
Inner dark comets are in the inner Solar System and share the same space as Mercury, Venus, Earth and Mars.
They are much smaller: just tens of metres or less.
Dark comets are objects that look like asteroids but behave like comets. Credit: Jonathan Knowles / Getty Images
Dark comets are objects that look like asteroids but behave like comets. Credit: Jonathan Knowles / Getty Images
So where do dark comets come from and how do they form? Do they contain ice?
Many questions remain, and it seems astronomers are only just beginning to unravel the secrets of these mysterious objects.
"Dark comets are a new potential source for having delivered the materials to Earth that were necessary for the development of life," said Seligman.
"The more we can learn about them, the better we can understand their role in our planet’s origin."
The study was published on Monday 9 December 2024 in the Proceedings of the National Academy of Sciences.