New Research Doubles Number of Known Dark Comets

Dark comets are small bodies with no detected coma that have significant accelerations explainable by outgassing of volatiles, analogous to the first interstellar object 1I/2017 U1 ‘Oumuamua. These objects represent a potentially widespread class of small bodies that further populate the continuum between asteroids and comets and for which the active nature is inferred from their orbital motion. Michigan State University astronomer Darryl Seligman and colleagues have now detected seven new dark comets and demonstrated that there are two distinct populations of these objects.

In 2003, astronomers noticed the trajectory of the asteroid 2003 RM had moved slightly from its expected orbit. That movement could not be explained by accelerations typical for asteroids.

Then, interstellar object ‘Oumuamua passed through our Solar System and perplexed astronomers in 2017.

Like typical asteroids and comets, it was moving partly due to the gravitational pull of other objects in the Solar System, like the Sun.

However, astronomers also found ‘Oumuamua was accelerating beyond that, likely due to the release of previously trapped gas.

The strength of the added propulsion from such outgassing defied that of typical asteroids and matched a comet’s pace.

The problem? ‘Oumuamua was missing the signature bright dust tail of a comet. Thus, it could not be classified easily as an asteroid or a comet.

In 2023, astronomers identified seven objects within our Solar System that mirrored the same unusual characteristics of ‘Oumuamua.

The objects were categorized as members of an entirely new class of objects — the first seven dark comets.

“One of the most important reasons why we study small bodies, like asteroids and comets, is because they tell us about how material is transported around the Solar System,” Dr. Seligman said.

“Dark comets are a new class of near-Earth objects that may contain water, so they’re a new potential source for delivering materials to Earth which were necessary for the development of life.”

“The more we can learn about them, the better we can understand their role in our planet’s origin.”

Dr. Seligman and co-authors not only uncovered seven new dark comets, but also compared the total known population to identify two distinct types.

The first type are called outer dark comets. These are large objects with eccentric orbits, similar to the Jupiter-family comets, which may originate in the outer Solar System.

The second type are called inner dark comets. These are smaller objects with nearly circular orbits that travel in the inner Solar System, closer to Earth, that may originate in the asteroid belt.

“The interesting thing about these objects is that they look like asteroids, but their motion resembles that of comets,” said Dr. Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory.

“This is a puzzle that challenges the way we have always classified objects either as asteroids or comets.”

“Expanding our knowledge on dark comets helps to contextualize ‘Oumuamua, which was such a serendipitous event that we can no longer observe since it has exited our Solar System,” Dr. Seligman said.

“What many people may not think about on a regular basis is that the Solar System is a chaotic place.”

“We don’t know where things come from, but with the 14 dark comets we now know about that are orbiting within our Solar System, there are windows of opportunity in the next few years for us to gather more data and hopefully uncover answers about the formation of our own planet.”

The discovery is reported in a paper in the Proceedings of the National Academy of Sciences.

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Darryl Z. Seligman et al. 2024. Two distinct populations of dark comets delineated by orbits and sizes. PNAS 121 (51): e2406424121; doi: 10.1073/pnas.2406424121