Astronomers have recently identified seven additional instances of a mysterious cosmic phenomenon referred to as dark comets, which may provide valuable insights into whether these celestial bodies played a role in transporting essential elements such as water to Earth during its early formation. This discovery has effectively doubled the previously known count of these enigmatic objects within our solar system. They exhibit characteristics of both asteroids and comets, yet they lack the distinctive tail that is typically associated with comets.

The findings, published on December 9th in the Proceedings of the National Academy of Sciences, have also revealed the existence of two separate groups of these objects. The increased number of identified dark comets has led astronomers to conclude that they are not as rare as was once thought. "Studying small celestial bodies like asteroids and comets is crucial for understanding how materials are distributed throughout the solar system," explained Darryl Seligman, the lead researcher and a postdoctoral fellow in physics and astronomy at Michigan State University, East Lansing. "Dark comets, which may contain water, represent a new potential source for the delivery of materials to Earth that were vital for the emergence of life. The more we learn about them, the better we can comprehend their significance in the origins of our planet."

In the past, astronomers examining an object they believed to be an asteroid named 2003 RM noticed a slight deviation from its expected trajectory. This deviation could not be accounted for by the usual accelerations experienced by asteroids, such as the Yarkovsky effect, where space rocks absorb heat from the sun and then re-emit it as infrared radiation, creating a small amount of thrust. Instead, 2003 RM's movement was more comet-like. The sun's heat causes materials like the ice within comets to sublimate, turning directly into gas and providing the thrust that results in the visible tail streaming behind comets.

"Despite our efforts, we found no evidence of a comet's tail. The object appeared as a mere point of light, much like an asteroid. For a time, we had this peculiar celestial object that we couldn't fully understand," said Davide Farnocchia, a navigation engineer at NASA's Jet Propulsion Laboratory in Pasadena, California. "This poses a challenge to our traditional classification of celestial objects as either asteroids or comets."

The discovery of 'Oumuamua in 2017 marked the first observed object in our solar system that originated from outside of it. Scientists had a brief opportunity to observe 'Oumuamua with telescopes before it swiftly traversed our solar system, sparking numerous theories about its nature, including the possibility that it might have been an extraterrestrial probe. Telescope observations indicated that 'Oumuamua appeared as a single point of light, akin to an asteroid, but its trajectory suggested material shedding, similar to a comet, complicating the determination of its classification. "‘Oumuamua was surprising in many ways," Farnocchia noted. "The fact that the first object we discovered from interstellar space displayed behaviors similar to 2003 RM made 2003 RM even more intriguing."

Both Farnocchia and Seligman published research in 2023 that described a total of seven objects in our solar system with unusual characteristics akin to 'Oumuamua, blurring the lines between asteroids and comets. These objects were termed dark comets. With the identification of seven more dark comets, researchers have been able to distinguish differences between their populations.

"With a sufficient number of dark comets, we could start to investigate if there were any distinguishing features," Seligman said. "By analyzing their reflectivity and orbits, we discovered that our solar system contains two distinct types of dark comets." Inner dark comets are located within the inner solar system, which includes the planets Earth, Venus, Mars, and Mercury, and they orbit the sun in nearly circular paths. These celestial bodies are relatively small, reaching only tens of meters in diameter, and may have originated from the main asteroid belt, situated between the orbits of Mars and Jupiter. In contrast, outer dark comets, potentially originating from the outer regions of our solar system near Jupiter and beyond, have distinctly elliptical orbits and can span hundreds of meters or more.

Now that astronomers have identified populations of dark comets, they aim to determine whether these objects contain ice, what drives their acceleration, and their origins. Gaining a deeper understanding of dark comets could reveal whether these celestial bodies contributed to Earth's early evolution by impacting the planet during its formation. "What many people may not consider regularly is that the solar system is a chaotic place," Seligman remarked. "We are uncertain about the origins of various objects, but with the 14 dark comets we now know are orbiting within our solar system, there are opportunities in the coming years to gather more data and hopefully uncover answers about the formation of our own planet."

Previous research has suggested that up to 60% of near-Earth objects could be dark comets. While asteroids, due to their closer orbits to the sun, are devoid of ice, comets are icy bodies that resemble dirty ice cubes with more distant orbits, according to Aster Taylor, coauthor of the new study and lead author of a study published online in the journal Icarus in July about dark comets.

Taylor is a doctoral student in astronomy at the University of Michigan. However, dark comets that are near-Earth objects could be found in the main asteroid belt, between the orbits of Mars and Jupiter, and they may contain ice. If ice is common on small bodies like dark comets in the near-Earth environment, they may have been responsible for bringing water to Earth. Astronomers are also attempting to determine why dark comets are so small and rotate so rapidly, Taylor said.

"It's quite possible that the activity on the inner main belt objects was triggered by some form of physical change, such as the destruction of the object, which is suggested by the small sizes and rapid rotation rates of the inner dark comets," Taylor said. "However, these are all hypotheses, and while they are consistent with all of our data, further research is needed to confirm or refute these ideas."

Fortunately, the Japan Aerospace Exploration Agency's Hayabusa2 spacecraft is expected to meet with one of the dark comets, 1998 KY26, in 2031 as part of its extended mission, which could reveal more unique characteristics of these not-so-rare celestial objects, Taylor said.