On "Dark Comets"

Ethan Wong

August 23, 2024

On October 19th, 2017, the first sighting of an interstellar body was discovered in our Solar System passing by the Sun. The object was named ‘Oumuamua, stated to be a comet, and soon became a mysterious wonder in both the scientific community and social media due to its strange behavior and “appearance.”

At the time, ‘Oumuamua was said to be shaped like an elongated cigar, and artists’ depictions of a thin and long cylindrical body caused the internet to go crazy as the illustrations highly resembled an alien spacecraft from Arthur C. Clark’s “Rendezvous with Rama.”  However, it wasn’t just the shape of ‘Oumuamua that convinced people to believe it was being controlled artificially; instead, it was mainly the abnormal behaviors of the interstellar object. 

When a traditional comet passes by the Sun, its water ice and occasional frozen gases sublime from the star’s heat and create a coma–a frontal cloud accompanied with dust carried by the sublimation of ice. Their tails are formed by expelled gas and dust. But ‘Oumuamua didn’t have a dust coma, and was not recorded expelling any gas, dust or water vapor like a normal comet tail would. ‘Oumamua additionally experienced an odd non gravitational acceleration as it exited the Solar System (was not caused by Sun/slingshot method). 

After the sighting in 2017, Avi Loeb, the head of astronomy at Harvard, proposed that ‘Oumuamua must have been an alien spacecraft sent to Earth. Loeb became persistent on this idea, making claims that it was artificial, and that its acceleration had been powered by a solar sail. Darryl Seligman reasoned ‘Oumuamua’s mysterious acceleration on science, and the two battled it out in a series of research efforts to prove themselves right. In 2023, a paper was released by Jennifer Bergner and Seligman that proposed an explanation of ‘Oumuamua’s strange behaviors using scientific reasoning. They stated that hydrogen gas had been providing the comet with an additional push which had caused its acceleration out of the Solar System. They noted that as the ‘Oumuamua traveled through interstellar space before arriving, it had interacted with cosmic rays (subatomic particles created through astronomical bodies or events that travel through space and interact with other astronomical objects) which had broken down some of the comet’s water ice, leading to trapped hydrogen gas in the ice. As the comet passed by the Sun, the heat merely rearranged the ice of ‘Oumuamua which let the hydrogen gas out, accelerating the comet. However, this meant that the ice hadn’t sublimed like typical comets, which explained the lack of a dust coma. Despite this research claim, Loeb continued to advocate for an alien-influenced ‘Oumuamua. However, we can never truly confirm if either scientist was right as ‘Oumuamua will likely never reappear in our Solar System.

After this publication, Seligman wondered if other comets existed in the Solar System that had unexplainable gravitational accelerations. Seven astronomical objects were found, later named “dark comets” in the Solar System that fit ‘Oumuamua’s characteristics: 1998 KY26, 2005 VL1, 2016 NJ33, 2010 VL65, 2016 RH120, 2003 RM and 2010 RF12. 

These “dark comets” described astronomical hybrids that exhibited the behaviors of comets (with the addition of non gravitational acceleration and lack of dust cloud), but also had similar compositions and appearances of asteroids. However, unlike ‘Oumuamua, they originated in the Solar System and were not accelerating due to the release of hydrogen gas. One study on the comet 1998 KY26 can support the rejection of outgassing (frozen/entrapped gas release similar to ‘Oumuamua) in these comets’ nongravitational acceleration. Using a Very Large Telescope in Chile, researchers compiled images of the comet taken by the telescope in 2020, and found that KY26 lacked a tail and dust cloud. While this finding doesn’t disprove outgassing, it discouraged the likeliness of this method being the root cause of the nongravitational accelerations. But even though“dark comets” have been detected to retain their masses (countering outgassing possibility), the research paper “Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids” still investigated the possibility of outgassing. They found that “dark comets” like KY26 were capable of outgassing with tiny mass loss (0.1%); however, the timing didn’t add up between the comets’ existences and their outgassing rates, meaning that while outgassing could potentially be the cause, there would need to be a reason for the time discrepancy, such as an “on and off switch” or additional contributors to acceleration. As of today, scientists are still unsure on the mechanism that maneuver these “dark comets.”

Seligman is also currently part of a research team at the University of Michigan, led by Aster Taylor, to learn more about these “dark comets.” The team was able to map out orbits of the seven near-Earth objects on a 100,000 year time-frame, finding that they all traced back to the asteroid belt. 2003 RM was the one exception, yet its orbit only extended a little further (to edge of asteroid belt/ near Jupiter). Given that many of these “dark comets” potentially possess ice, similar to what helped power the acceleration of ‘Oumuamua, Taylor stated via Forbes that “We don't know if these dark comets delivered water to Earth. We can’t say that. But we can say that there is still debate over how exactly the Earth's water got here.” So maybe “dark comets” are a bigger idea than we think.

Currently, however, there is little known about “dark comets” and their actual abundance in our Solar System. Thanks to ‘Oumuamua though, there is increasing attention toward the smaller mysteries in our Solar System that might’ve been overlooked in the past.