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| This artist’s concept depicts interstellar object 1I/2017 U1 (‘Oumuamua), whose motion through the solar system has helped researchers better understand the 14 dark comets discovered so far. |
A new paper published on Monday, December 9, in the Proceedings of the National Academy of Sciences has revealed the discovery of seven more “dark comets,” doubling the number of these mysterious celestial objects. These objects, which resemble asteroids but behave like comets, have now been categorized into two distinct populations based on their size, location in the solar system, and other unique traits.
The first dark comet was identified less than two years ago, with another six discovered shortly thereafter. Researchers first suspected the existence of dark comets in March 2016 when the trajectory of asteroid 2003 RM deviated slightly from its expected orbit. Unlike typical asteroids, which exhibit very predictable movements, this deviation suggested a comet-like outgassing phenomenon. However, despite the abnormal movement, no visible tail — a hallmark of comets — could be detected.
“We had this one weird celestial object that we couldn’t fully figure out,” said Davide Farnocchia, co-author of the study and a scientist at NASA’s Jet Propulsion Laboratory. “It looked like an asteroid, but its behavior pointed to something else.”
Further investigation into 1I/2017 U1 (‘Oumuamua), a celestial object that originated outside the solar system and displayed similar traits, piqued scientists’ interest. In 2023, seven more objects were identified, confirming the existence of a new category of celestial bodies: dark comets.
Lead author Darryl Seligman, a postdoctoral fellow at Michigan State University, analyzed the characteristics of these objects, uncovering two different types of dark comets. The first type, called outer dark comets, have highly elliptical orbits and are relatively large, similar to Jupiter-family comets. The second, inner dark comets, are smaller and travel in nearly circular orbits in the inner solar system, closer to planets like Mercury, Venus, Earth, and Mars.
The findings raise numerous new questions for astronomers. Where did dark comets originate? What causes their strange acceleration? And could they contain water ice or other essential materials that could help explain the origins 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.”
As the study of dark comets progresses, astronomers hope to uncover more about these enigmatic objects and their potential impact on our understanding of the early solar system and the origins of life.