Often, just when we think we understand something, something else comes along that makes us stop, rethink, and expand our minds to grasp this new and exciting information. Take space rocks for example. We thought we had them all categorized neatly. Comets are ice-filled pieces of rock that sublime when heated and shoot long tails of gas into space. Asteroids are less icy pieces of rock and metal floating around as less icy pieces of rock and metal.

Sounds simple enough, right? Then scientists had to go and discover it. darkness comets; comets that resemble 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 comets, doubling that number to 14. These new data show that even not all dark comets are the same: There are at least two different types.

And the discovery of many types of dark comets could tell us more about how Earth became habitable.

“We report the discovery of seven dark comets that indicate 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 to Earth materials necessary for the development of life, such as volatiles and organic matter.”

One of the effects of comet outgassing is that it changes the motion of the comet. In fact, there are a number of mechanisms that can accelerate a space rock. The rock has an orbital acceleration that can change as it approaches the Sun. There is also the Yarkovsky influence; Rotation change caused by light and temperature contrasts. Since dark comets do not have tails that we can detect, the acceleration is much greater than that of a normal asteroid.

“When you see a disturbance like this in a celestial object, it usually means it’s a comet with volatile material coming off its surface pushing it a little bit,” says astronomer Davide Farnocchia of NASA’s Jet Propulsion Laboratory.

Although we cannot see these dark comets emitting gas, their acceleration cannot be explained by gravity or Yarkovsky. We now have enough dark comets to try to draw some statistical conclusions.

“We had enough dark comets that we started asking whether there was something that could distinguish them,” Seligman says. “By analyzing the reflection and orbits, we found that our solar system contains two different types of dark comets.”

One species lives in the inner solar system, orbiting 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 type is a little more chaotic. Their orbits are highly elliptical; It extends almost to Jupiter and is closer to the Sun than Mercury. They are also larger than the inner dark comets, reaching hundreds of meters in size.

There’s still a lot we don’t know about these mysterious rocks. Do they contain ice, or do they emit something else that causes them to accelerate strangely? Why are there two different populations? So how many are there?

A recently published paper found that because we rely on accurate orbital simulations to calculate whether an asteroid poses a threat, there may be many more dark comets in the inner Solar System than we know, and this could have implications for Earth’s defenses. Since abnormal acceleration can change the orbit of a space rock, we need to take this information into account to determine whether the rock is potentially hazardous.

But the discovery of these strange rocks is very new, and there is much more we don’t know about them than we do. Finding out what they are, where they come from, and how many they are will undoubtedly be the subject of future studies.

Especially because it has consequences for our own existence.

“Dark comets are a new potential source for delivering to Earth the materials necessary for the development of life,” says Seligman. “The more we learn about them, the better we can understand their role in the formation of our planet.” The study was published on: Proceedings of the National Academy of Sciences.