A team of researchers has uncovered seven new dark comets in the solar system. In addition to doubling the number of known dark comets, the researchers have identified two distinct types based on differences in their orbit and size.

Published in the Proceedings of the National Academy of Sciences, the findings reveal seven new dark comets and two types: outer dark comets and inner dark comets. Outer dark comets are large objects with eccentric orbits that may originate in the outer solar system. Inner dark comets are smaller objects with nearly circular orbits that travel in the inner solar system, closer to Earth, and may originate in the asteroid belt.

"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," said Michigan State University researcher Darryl Seligman, who led the study and is a recipient of a U.S. National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship, supported in part by a gift from Charles Simonyi to NSF.

"The discovery of more and different types of dark comets whizzing through the solar system is a prime example of why NSF continues to support the innovative research of postdoctoral scientists," says Louise Edwards, program director of the NSF Astronomy and Astrophysics Postdoctoral Fellowships program.

Dark comets — which look like asteroids but have comet-like motion — are a relatively recent discovery. In 2003, scientists noticed that the trajectory of "asteroid" 2003 RM had moved slightly from its expected orbit. That movement could not be explained by accelerations typical for asteroids. Later, the interstellar object ‘Oumuamua passed through the solar system and perplexed astronomers in 2017. Like typical asteroids and comets, it was moving primarily 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. However, ‘Oumuamua was missing the signature bright dust tail of a comet and thus could not be classified easily as an asteroid or a comet.

The newly published research not only expands understanding of such near-Earth objects but also prompts potential future research opportunities, such as investigating exactly where dark comets originate from or determining if dark comets could have delivered material to Earth long ago.

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