Dark comets are enigmatic celestial objects that resemble asteroids but exhibit the dynamic behavior of comets.
Astronomers have doubled the known population of dark comets—enigmatic celestial objects that resemble asteroids but exhibit the dynamic behavior of comets. A newly published study highlights the discovery of seven additional dark comets, bringing the total count to 14. Even more fascinating, these objects appear to fall into two distinct categories based on their size, orbit, and location within the solar system.
What Are Dark Comets?
The term “dark comet” describes a unique type of celestial body that lacks the visible tail typically associated with comets. Despite this, their trajectories suggest they experience subtle accelerations caused by volatile substances outgassing from their surfaces. This unusual behavior first came to light in 2016, when researchers observed that the orbit of “asteroid” 2003 RM deviated slightly in a way uncharacteristic of normal asteroids.
“When you see that kind of perturbation on a celestial object, it usually means it’s a comet,” explained Davide Farnocchia of NASA’s Jet Propulsion Laboratory. “But we couldn’t detect any signs of a comet’s tail, leaving us with a strange object that didn’t fully fit into either category.”
This peculiar case became even more compelling the following year with the discovery of ‘Oumuamua, the first documented interstellar object to visit our solar system. Like 2003 RM, ‘Oumuamua’s behavior combined features of both asteroids and comets, further suggesting the existence of a broader category of such hybrid objects.
Two Distinct Populations of Dark Comets
The latest study, led by Darryl Seligman of Michigan State University, has shed light on the diversity within this celestial category. By analyzing the albedo (reflectivity) and orbital characteristics of these objects, the researchers identified two distinct groups:
- Outer Dark Comets
These larger objects, spanning hundreds of meters or more, reside in the outer reaches of the solar system. Their orbits are highly elliptical, resembling those of Jupiter-family comets, which often originate in the Kuiper Belt. - Inner Dark Comets
Smaller in size, typically tens of meters across, these comets travel in nearly circular orbits within the inner solar system. Their proximity to the Sun and planets like Earth raises intriguing questions about their composition and behavior.
Unanswered Questions and the Role of Dark Comets in Earth’s History
While the discovery of more dark comets has expanded our understanding, it has also opened the door to new mysteries. For instance, researchers are investigating whether these objects might contain ice and what mechanisms drive their anomalous accelerations. Additionally, the role of dark comets in delivering essential materials to Earth—potentially influencing the origins of life—is a tantalizing area of inquiry.
“Dark comets are a new potential source for delivering the materials necessary for life on Earth,” said Seligman. “The more we study them, the more we can uncover about their role in shaping our planet’s history.”
