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So often, just when we think we understand something, another thing comes along that makes us stop, reconsider, and expand our minds to take in this new and exciting information. Take, for example, space rocks. We thought we had them all neatly categorized. Comets are chunks of rock filled with ice that sublimate when heated, throwing long tails of gases into space. Asteroids are less icy chunks of rock and metal that just hang around being less icy chunks of rock and metal.
Sounds simple enough, right? Then scientists had to go and discover dark comets; comets that look like asteroids but behave like comets. The identity of these objects was revealed in 2023: first there was one dark comet, then six more. Now, an international team of astronomers has discovered seven more, doubling that number to 14. This new data shows that not even all dark comets are the same: there are at least two different kinds.
And, in turn, the discovery of many types of dark comets can tell us more about how Earth became habitable.
"We report the discovery of seven dark comets that demonstrate two distinct populations based on their orbits and sizes," wrote a team of researchers led by Michigan State University astrophysicist Darryl Seligman. "These objects represent a class of Solar System objects that may have delivered materials necessary for the development of life, such as volatiles and organic matter, to Earth."
One of the effects of cometary outgassing is that it changes the motion of the comet. There are actually a number of mechanisms that can accelerate a space rock. There is an orbital acceleration that can change as the stone approaches the Sun. There is also the Yarkovsky effect - a change in rotation caused by contrasts of light and temperature. Because dark comets have no tails that we can detect, the acceleration is much greater than that of a normal asteroid.
"When you see a disturbance like this on a celestial object, it usually means it's a comet, with volatile material coming off its surface, giving it a little push," says astronomer Davide Farnocchia of NASA's Jet Propulsion Laboratory.
Although we cannot see that these dark comets emit gases, their acceleration cannot be explained by gravity or Yarkovsky. We now have enough dark comets to try to draw some statistical conclusions.
"We had a large enough number of dark comets that we could start asking if there was anything that could distinguish them," Seligman says. "By analyzing the reflectivity and orbits, we found that our solar system contains two different types of dark comets."
One species lives in the inner part of the solar system, inside the orbit of Mars. They tend to be on the smaller side, around a few tens of meters, and their orbits around the Sun are fairly circular and well-defined.
The other kind is somewhat more chaotic. Their orbits are highly elliptical, extending almost to Jupiter and closer to the Sun than Mercury. They are also larger than inner dark comets, reaching sizes up to hundreds of meters across.
There's still a lot we don't know about these mysterious rocks. Do they contain ice, or do they give off something else that causes them to accelerate strangely? Why are there two different populations? And how many are there?
A recent paper has found that there may be many more dark comets in the inner Solar System than we know, with implications for Earth's defenses as we rely on accurate orbit simulations to calculate whether an asteroid poses a threat. Because anomalous acceleration can change the orbit of a space rock, we need to take this information into account to determine if the rock was potentially dangerous.
But the discovery of these strange rocks is very new, and the extent of what we do not know about them is much greater than what we do know. Finding out what they are, where they come from, and how numerous they are will undoubtedly be the subject of future work.
Especially since they have consequences for our own existence.
"Dark comets are a new potential source of delivery to Earth of materials necessary for the development of life," says Seligman. "The more we can learn about them, the better we can understand their role in the formation of our planet." The study was published in Proceedings of the National Academy of Sciences.
