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The first dark comet — a celestial object that looks like an asteroid but moves through space like a comet — was reported less than two years ago. Six more were soon found. In a new paper, researchers announced the discovery of seven more, doubling the number of known dark comets, and found that they fall into two distinct populations: larger ones in the outer Solar System and smaller ones in the inner Solar System, with various other features that distinguish them. their.
The conclusions were published on December 9, in Proceedings of the National Academy of Sciences.
Scientists first guessed the existence of dark comets when in March 2016, a study noticed that the trajectory of the "asteroid" 2003 RM deviated slightly from the expected orbit. This deviation cannot be explained by typical asteroid accelerations, such as the small acceleration known as the Yarkovsky effect.
"When you see this kind of perturbation on a celestial object, it usually means it's a comet, with volatile material ejected from its surface that gives it a little push," said study co-author David Farnocchia of NASA's Jet Propulsion Laboratory in Southern California. “But try as we might, we couldn't find any sign of the comet's tail. It looked like any other asteroid—just a point of light. So for a short time we had one strange celestial object that we couldn't help but fully figure out."
Strange celestial objects
Farnocchia and the astronomical community didn't have to wait long to get another piece of the puzzle. The following year, in 2017, a NASA-sponsored telescope discovered the first-ever documented celestial object that originated outside our solar system. Not only did 1I/2017 U1 ('Oumuamua) appear as a single point of light like an asteroid, its trajectory changed so that it appeared to be ejecting volatile material from its surface like a comet.
"Oumuamua surprised in a number of ways," Farnocchia said. "The fact that the first object we detected in interstellar space exhibited behavior similar to 2003 RM made 2003 RM even more intriguing."
By 2023, researchers have identified seven objects in the Solar System that looked like asteroids but acted like comets. This was enough for the astronomical community to give them their own category of celestial objects: "dark comets". Now that seven more such objects have been found, researchers can begin a new set of questions.
"We had a large enough number of dark comets that we could start asking if there was something that could distinguish them," said Darryl Seligman, a postdoctoral fellow in the Department of Physics at Michigan State University, East Lansing, and lead author of the new paper. "By analyzing the reflectivity" or albedo "and orbits, we found that our solar system contains two different types of dark comets."
Two types of dark comets
The study authors found that one type, which they call outer dark comets, has similar characteristics to Jupiter-family comets: They have highly eccentric (or elliptical) orbits and are on the larger side (hundreds of meters or more across).
The second group, the inner dark comets, are found in the inner part of the Solar System (which includes Mercury, Venus, Earth, and Mars), move in nearly circular orbits, and are on the smaller side (tens of meters or less).
Like many other astronomical discoveries, Seligman and Farnocchia's research not only expands our knowledge of dark comets, but also raises several additional questions: Where did dark comets come from? What causes their abnormal acceleration? Can they contain ice?
"Dark comets are a new potential source for delivering to Earth the materials necessary for life to develop," Seligman said. "The more we can learn about them, the better we can understand their role in the formation of our planet."
