So often when we think we understand something, something else comes along that forces us to pause, reconsider, and expand our minds to encompass these new and fascinating pieces of information.
Take space rocks, for example. We thought we had them all neatly categorized. Comets are boulders filled with ice that sublimate as temperatures warm, sending long tails of gases into space. Asteroids are less icy chunks of rock and metal just hanging around as less icy chunks of rock and metal.
Sounds pretty simple, right? Well, then scientists had to start exploring dark comets; comets that look like an asteroid but behave like a comet.
The identity of these objects was revealed in 2023, with first one dark comet and then six more. Now an international team of astronomers has unmasked another seven, doubling the number and bringing the known total to fourteen.
This new data shows that not even all dark comets are the same: there are at least two different types.
And in turn, the discovery of multiple types of dark comets could tell us more about how Earth was made habitable for life.
“We report detections of seven dark comets that show that there are two distinct populations based on their orbits and sizes,” writes a team of researchers led by astrophysicist Darryl Seligman of Michigan State University.
“These objects represent a class of objects in the solar system that may have provided material to Earth necessary for the development of life, such as volatiles and organic compounds.”
One effect of cometary outgassing is that it changes the motion of the comet. Actually, there are a number of mechanisms that can accelerate a space rock. There is an orbital acceleration that can change as the rock’s proximity to the sun changes. There is also the Yarkovsky effect: a change in rotation caused by contrasts in light and temperature.
Because dark comets don’t have tails that we can detect, the acceleration is why there’s more going on than the normal asteroid.
“When you see a disturbance like this on a celestial body, it usually means it’s a comet, with volatile material coming out of the surface and giving it a bit of momentum,” says astronomer Davide Farnocchia of NASA’s Jet Propulsion Laboratory.
Although we can’t see what these dark comets are ejecting, their acceleration cannot be explained by gravity or Yarkovsky.
What we do have now is a sufficient number of dark comets to make some statistical inferences.
“We had such a large number of dark comets that we wondered if there was anything that could distinguish them,” says Seligman. “By analyzing the reflectivity and orbits, we discovered that our solar system contains two different types of dark comets.”
One species inhabits the inner solar system, within the orbit of Mars. They are usually on the smaller side, about a few tens of meters lower, and their orbits around the sun are quite circular and neat.
Outer (left) and inner (right) orbits of dark comets. (Seligman et al., PNAS2024)
The other kind is a bit more chaotic. Their orbits are highly elliptical, traveling almost as far as Jupiter, and closer to the Sun than Mercury. They are also larger than the inner dark comets, measuring up to hundreds of meters in diameter.
There’s still a lot we don’t know about these enigmatic rocks. Do they contain ice, or do they release something else that makes them speed up in strange ways? Why are there two different population groups? And how many are there?
A recent paper found that there could be many more dark comets in the inner solar system than we know, which has implications for Earth’s defenses as we rely on accurate orbit modeling to calculate whether an asteroid poses a threat. Because abnormal acceleration can change the trajectory of a space rock, we need to consider that information when determining whether a rock is potentially dangerous.
We’ve just doubled the number of known ‘dark’ comets in the solar system
An illustration of the interstellar object ‘Oumuamua, whose strange acceleration may be comparable to that of dark comets. (NASA)
But the discovery of these strange rocks is very new, and the extent of what we don’t know about them far exceeds what we do know. Finding out what they are, where they come from and how many there are will undoubtedly be the subject of future work.
Especially because they have consequences for our own existence.
“Dark comets are a new potential source for delivering the materials to Earth that were necessary for the development of life,” says Seligman. “The more we can learn about them, the better we can understand their role in the creation of our planet.”
The research has been published in the Proceedings of the National Academy of Sciences.
