Enigmatic ‘Dark Comets’ Come in Two Distinct Types, Study Finds

They look like asteroids, but act like comets.

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In 2017, an unexpected visitor with an oddly elongated shape zipped through the solar system. The interstellar object, later named ‘Oumuamua, was initially classified as an asteroid—until measurements revealed it was accelerating away from the Sun, like a comet. This hybrid behavior led scientists to identify it as a dark comet: an object that looks like an asteroid but acts like a comet. Now, new research has doubled the number of known dark comets and grouped them into two distinct populations. A paper 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 reveals 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 for having delivered the building blocks of life to Earth. The first dark comet was spotted in 2016 when scientists noticed that the trajectory of near-Earth asteroid 2003 RM was a little wonky for your typical space rock. “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,” Davide Farnocchia, a researcher at NASA’s Jet Propulsion Laboratory, and co-author of the new study, said in a statement. “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. So, for a short while, we had this one weird celestial object that we couldn’t fully figure out.” The following year, ‘Oumuamua came onto the scene, and scientists immediately made the connection between the two objects. “The fact that the first object we discovered from interstellar space exhibited similar behaviors to 2003 RM made 2003 RM even more intriguing,” Farnocchia added. By 2023, six more celestial objects had been identified as having both asteroid and comet-like qualities, prompting researchers to officially grant them the title of dark comets. With the discovery of seven more dark comets, the researchers behind the new study felt it was time to begin probing the different characteristics of these objects. “We had a big enough number of dark comets that we could begin asking if there was anything that would differentiate them,” Darryl Seligman, a postdoctoral fellow in the department of Physics at Michigan State University, and lead author of the new paper, said in a statement. By analyzing their reflectivity and orbits, the researchers identified two distinct populations of dark comets within the solar system. The outer dark comets, which are similar to Jupiter-family comets, have highly eccentric, or elliptical, orbits and are on the larger side. The second group, inner dark comets, travel within the inner solar system in nearly circular orbits, and are on the smaller side. Researchers are barely scratching the surface of these strange objects. A recent study suggested that nearly 60% of all near-Earth objects are dark comets that may have been a part of much larger bodies in the main asteroid belt, a region of the solar system that lies between Mars and Jupiter, before being expelled and broken-up into smaller fragments that now travel 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 for having delivered the materials to Earth that were necessary for the development of life,”  Seligman said. “The more we can learn about them, the better we can understand their role in our planet’s origin.”

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