NASA Scientists Discover “Dark Comets” Come in Two Populations.

On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first-ever detection of an interstellar object, named 1I/2017 U1 ‘Oumuamua (the Hawaiin word for “scout”). This object created no shortage of confusion since it appeared as an asteroid but behaved like a comet (based on the way it accelerated out of the Solar System). Since then, scientists have noticed a lot of other objects that behave the same way, known as “dark comets.”

These objects are defined as “small bodies with no detected coma that have significant nongravitational accelerations explainable by outgassing of volatiles,” much like ‘Oumuamua. In a recent NASA-supported study, a team of researchers identified seven more of these objects in the Solar System, doubling the number of known dark comets. Even more important, the researchers were able to discern two distinct populations. They consist of larger objects that reside in the outer Solar System and smaller ones in the inner Solar System.

The study was led by Darryl Z. Seligman, an NSF Astronomy and Astrophysics Postdoctoral Fellow from the Carl Sagan Institute at Cornell University and Michigan State University. He was joined by researchers from the European Space Agency’s Near-Earth Object Coordination Centre (NEOCC), the European Southern Observatory (ESO), the Planetary Science Institute (PSI), Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado Boulder, NASA’s Jet Propulsion Laboratory. Their findings were published on December 9th in the Proceedings of the National Academy of Sciences (PNAS).

Scientists got their hint that dark comets exist in 2016 when they found that the “asteroid” 2003 RM had deviated slightly from its expected orbit. This behavior could not be explained by the Yarkovsky effect, where asteroids absorb solar energy and re-radiate it into space as heat. Said study co-author Davide Farnocchia of NASA JPL said in a NASA press release:

“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. 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 next piece of the puzzle came in 2017 with the detection of the first interstellar object (‘Oumuamua). While it appeared as a single point of light to telescopes and had no coma, its trajectory changed as if it were outgassing volatile material from its surface. “‘Oumuamua was surprising in several ways,” said Farnocchia. “The fact that the first object we discovered from interstellar space exhibited similar behaviors to 2003 RM made 2003 RM even more intriguing.”

By 2023, seven dark comets had been identified, leading the astronomical community to designate them as a distinct category of celestial objects. With this latest study, the authors identified seven more of these objects in the Solar System and noticed some interesting traits among them. “We had a big enough number of dark comets that we could begin asking if there was anything that would differentiate them,” said Seligman. “By analyzing the reflectivity,” or albedo, “and the orbits, we found that our solar system contains two different types of dark comets.”

One group, which the team calls “outer dark comets,” is similar to the “families” of asteroids that orbit Jupiter. In addition to being larger, measuring hundreds of meters or more across, the first group has highly elliptical orbits. The second group, “inner dark comets,” are smaller (tens of meters or less) and travel in nearly circular orbits within the orbits of Mercury, Venus, Earth, and Mars. In addition to expanding astronomer’s knowledge of dark comets, the team’s research raises several additional questions regarding their origin, behavior, and composition.

Of particular interest is whether these objects could contain water ice, which would have implications for our understanding of how water (and possibly life) was distributed throughout the Solar System billions of years ago. “Dark comets are a new potential source for having delivered the materials to Earth that were necessary for the development of life,” said Seligman. “The more we can learn about them, the better we can understand their role in our planet’s origin.”

Further Reading: NASA, PNAS