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Astronomers have discovered seven new examples of a cosmic phenomenon called dark comets that could help scientists determine whether the celestial bodies helped transport vital elements such as water to Earth early in the planet’s history.
The discovery doubles the known number of these mysterious celestial objects in our solar system, which resemble asteroids but hurtle through space like comets, albeit without the “tails” comets are known for.
The new research, published Dec. 9 in the journal Proceedings of the National Academy of Sciences, also shows that there are two distinct populations of these objects.
The discovery of additional dark comets has helped astronomers realize that these objects are not as rare as previously thought.
“One of the most important reasons we study small bodies like asteroids and comets is that they tell us how material is transported in the solar system,” said lead study author Darryl Seligman, a postdoctoral researcher in physics and astronomy at Michigan State University, East Lansing, in a statement.
“Dark comets are a new class of near-Earth objects that may contain water. They are therefore a new potential source for supplying Earth with materials necessary for the development of life. The more we can learn about them, the better we can understand their role in the formation of our planet.”
The revelation of an interstellar visitor
Astronomers studying an asteroid called 2003 RM two decades ago found that the object had moved slightly away from its predicted orbit.
The movement could not be explained by accelerations typical of asteroids, such as the Yarkovsky effect, in which space rocks absorb solar heat and then release it as infrared radiation, which can give the asteroid a small boost.
Instead, RM 2003 moved more like a comet. The sun causes material like the ice inside comets to turn directly into a gas through a process called sublimation. This evaporation gives the comets thrust. The thrust can be seen in the tail that trails behind comets and is made of vaporizing material.
“But no matter how hard we tried, we couldn’t find any signs of a comet’s tail. It looked like any other asteroid – just a tiny dot of light. So for a short time we had this one strange celestial object that we couldn’t fully understand,” study co-author Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement. “This is a mystery that challenges the way we have always classified objects as either asteroids or comets.”
Then in 2017, astronomers discovered ‘Oumuamua. It was the first observed object in our solar system to originate outside of our solar system.
Scientists briefly observed ‘Oumuamua with telescopes before the interstellar visitor completed its rapid journey through our solar system, giving rise to a variety of hypotheses about its true nature, including the possibility that it may have been an alien probe.
Telescopic observations showed that ‘Oumuamua looked like a single point of light, similar to an asteroid, but its trajectory changed as if it was shedding material, similar to a comet, making it difficult to determine whether it was either object.
“‘Oumuamua was surprising in several ways,” Farnocchia said. “The fact that the first object we discovered in interstellar space exhibited similar behavior to 2003 RM made 2003 RM even more intriguing.”
Farnocchia and Seligman each published research in 2023 describing a total of seven objects discovered in our solar system with unusual aspects similar to Oumuamua, blurring the line between asteroids and comets. Scientists called this new class of objects dark comets.
With the discovery of seven more dark comets, researchers have now been able to see differences between their populations.
“We had such a large number of dark comets that we could start asking if there was anything that could distinguish them,” Seligman said. “By analyzing reflectivity and orbits, we found that our solar system contains two different types of dark comets.”
Inner dark comets are found in the inner solar system, which includes the planets Earth, Venus, Mars and Mercury, and move in nearly circular orbits around the sun. The celestial objects are tiny, reaching only tens of meters in diameter, and could come from the main asteroid belt located between the orbits of Mars and Jupiter.
Outer dark comets, on the other hand, which may come from the outer regions of our solar system near Jupiter and beyond, have distinctly oval orbits and can span hundreds of meters or more.
Researching the origin of water
Now that astronomers have found populations of dark comets, they want to find out whether the objects contain ice, what causes their acceleration and where they come from.
A better understanding of dark comets could shed light on whether these objects contributed to Earth’s early evolution by colliding with it during the planet’s formation.
“What many people may not think about on a regular basis is that the solar system is a chaotic place,” Seligman said. “We don’t know where things come from, but with the 14 dark comets we now know orbiting our solar system, there are opportunities in the next few years for us to collect more data and hopefully find answers to that Creation of our own planet.”
Previous research has shown that up to 60% of near-Earth objects could be dark comets.
While asteroids are ice-free because they orbit closer to the sun, comets are icy bodies that resemble dirty ice cubes with more distant orbits, said Aster Taylor, co-author of the new study and lead author of a July study on dark comets published online in the journal Icarus. Taylor is a doctoral student in astronomy at the University of Michigan.
But dark comets, which are near-Earth objects, could be found in the main asteroid belt between the orbits of Mars and Jupiter and could contain ice. And if ice is common on small bodies like dark comets in the near-Earth environment, they could have been responsible for bringing water to Earth.
Astronomers are also trying to figure out why dark comets are so small and spin so quickly, Taylor said.
“It is quite possible that the activity on the objects in the inner main belt was triggered by some kind of physical change, such as the destruction of the object, as suggested by the small size and fast rotation speed of the inner dark comets,” Taylor said. “However, these are all hypotheses, and while they are consistent with all of our data, more work is needed to confirm or falsify these ideas.”
Fortunately, the Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft is expected to encounter one of the dark comets, 1998 KY26, in 2031 as part of its extended mission, which could reveal further unique properties of these not-so-unusual celestial objects. Taylor said.
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