A new paper published in the Proceedings of the National Academy of Sciences reports the detections of seven dark comets in two distinct populations, bringing the total of known dark comets to 14.
Dark comets are a newly defined type of near-Earth object that resemble asteroids but behave like comets, exhibiting dynamic behaviors such as trajectory changes that suggest cometary activity, likely due to outgassing. They do not exhibit traditional cometary features like tails or comae, making them appear asteroid-like in observations. However, their unexpected accelerations imply they may be firing jets of gas, as comets do.
The first dark comet was reported less than two years ago, and soon after, another six were found. By 2023, researchers had identified seven solar system objects that looked like asteroids but acted like comets, leading the astronomical community to categorize them as dark comets. With the discovery of seven more, researchers now have a big enough number of dark comets to begin asking if there was anything that would differentiate them.
Darryl Seligman, lead author of the new paper and a postdoctoral fellow at Michigan State University, explained that analyzing these objects' reflectivity and orbits has provided insights into their differing properties, leading to the identification of two types of dark comets in our solar system, according to The Economic Times.
The study's authors found that one kind of dark comet, called outer dark comets, have 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, inner dark comets, reside in the inner solar system—which includes Mercury, Venus, Earth, and Mars—travel in nearly circular orbits, and are on the smaller side, tens of meters or less.
"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," said Davide Farnocchia, coauthor from NASA's Jet Propulsion Laboratory, as reported by Forbes. Farnocchia further explained the challenges in identifying these objects. "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," he is quoted by NASA.
Scientists first suspected the existence of dark comets in a March 2016 study when they noted that the trajectory of asteroid 2003 RM had moved slightly from its expected orbit, which could not be explained by typical asteroid dynamics. 2003 RM appeared to move slightly from its orbit in a way thought only possible by comets propelled by jets of gas, leading to its classification as a dark comet due to the lack of visible cometary features
The discovery of 2003 RM, dubbed "The Asteroid that Wanted to be a Comet," was followed a year later by the discovery of 1I/2017 U1 (Oumuamua), which exhibited similar behaviors. Oumuamua was the interstellar object that also appeared as a single point of light, like an asteroid, but had a trajectory like a comet due to suspected outgassing of volatile material.
"Oumuamua was surprising in several ways. The fact that the first object we discovered from interstellar space exhibited similar behaviors to 2003 RM made 2003 RM even more intriguing," Farnocchia said, as reported by Forbes.
While Oumuamua is not a dark comet itself, its motion through the solar system has helped researchers better understand the nature of the 14 dark comets discovered so far.
Seligman emphasized that understanding dark comets could shed light on the origins of life on our planet. "Dark comets are a new potential source for having delivered the materials to Earth that were necessary for the development of life," he said, according to Forbes.
As researchers continue to study dark comets, they hope to unravel more mysteries about their nature and significance in the context of planetary formation and the development of life on Earth. "The more we can learn about them, the better we can understand their role in our planet's origin," Seligman added, according to Business News.
In April 2024, the James Webb Space Telescope detected water vapor in comet 238P/Read, confirming the presence of ice in bodies of the main belt. This discovery, although not directly related to dark comets, reinforces the importance of studying small objects to understand the dynamics and evolution of the solar system. NASA has highlighted that these advances contribute to prioritizing future missions aimed at more deeply analyzing the smaller bodies of the solar system.
This article was written in collaboration with generative AI company Alchemiq