In 2017, an unexpected visitor with a strange elongated shape flew through the solar system. The interstellar object, later named Oumuamua, was initially classified as an asteroid until measurements showed that it was accelerating away from the Sun like a comet. This hybrid behavior allowed scientists to identify it as a dark comet: an object that looks like an asteroid but behaves like a comet. New research has doubled the number of known dark comets and grouped them into two distinct populations.
An article published this week in the Proceedings of the National Academy of Sciences details the discovery of seven dark comets, bringing the total number of known dark comets to 14. The study also shows that these mysterious objects fall into two distinct groups based on their orbit and size. With this latest discovery, scientists are learning more about dark comets and their potential to deliver the building blocks of life to Earth.
The first dark comet was spotted in 2016, when scientists noticed that the trajectory of the near-Earth asteroid 2003 RM was a bit wobbly, as for a normal space body. “When you see perturbations like this on a celestial object, it usually means it’s a comet, with volatiles emitting from its surface and giving it a little bit of thrust,” says Davide Farnocchia, a researcher at NASA’s Jet Propulsion Laboratory and co-author of the new study. “But no matter how hard we tried, we couldn’t find any sign of the comet’s tail. It looked like any other asteroid – just a point light. So, for a while, we had this one strange celestial object that we couldn’t quite figure out.”
The following year, Oumuamua came on the scene, and scientists immediately made the connection between the two objects. “The fact that the first object we discovered from interstellar space had similar behavior to 2003 RM made 2003 RM even more intriguing,” Farnocchia added.
By 2023, six more celestial objects were discovered that had both asteroid and comet properties, prompting researchers to officially name them dark comets. After the discovery of seven more dark comets, the authors of the new study decided that it was time to start studying the various characteristics of these objects. “We had a large enough number of dark comets to start looking to see if there was anything that could distinguish them,” said Darryl Seligman, a research associate in the Department of Physics at Michigan State University and lead author of the new paper, in a statement.
By analyzing their reflectivity and orbits, the researchers identified two distinct populations of dark comets in the solar system. The outer dark comets, similar to those of the Jupiter family, have very eccentric, or elliptical, orbits and are on the larger side. The second group, the inner dark comets, move in the inner part of the solar system in almost circular orbits and are on the smaller side.
Researchers are barely scratching the surface of these strange objects. A recent study found that nearly 60% of all near-Earth objects are dark comets that may have been part of much larger bodies in the main asteroid belt, the region of the solar system that lies between Mars and Jupiter, before being ejected and broken up into smaller fragments that now fly closer to Earth.
Dark comets may have played a crucial role in the origin of life on Earth. “Dark comets are a new potential source that could have delivered the materials necessary for the development of life to Earth,” Seligman said. “The more we learn about them, the better we will be able to understand their role in the origin of our planet.”
