An interstellar comet that crossed into our solar system nearly two years ago is revealing more of its secrets.
Researchers have determined that the comet, known as 2I/Borisov, is more pristine and unaltered than any of the comets that have been observed in our solar system. The dust around the comet is also intriguingly different than those around other comets.
Combined, these two findings add evidence to the comet’s origin outside of our solar system. The results were published Tuesday in two different studies in the journals Nature Astronomy and Nature Communications.
It’s only the second observed interstellar object to appear in our solar system, after ‘Oumuamua which was first detected in 2017. While the exact nature of ‘Oumuamua continues to be debated, it’s more asteroid-like in its features, whereas 2I/Borisov has consistently displayed the activity associated with comets.
The comet was discovered by amateur astronomer Gennady Borisov, and astronomers confirmed that it originated from outside of our solar system.
However, previous observations of the interstellar comet suggested it was more like those found in our solar system, and little was known about the comet’s nucleus. A cometary nucleus is the main, solid component of a comet made up of rock, dust and frozen gases.
New observations of the comet, made using the European Southern Observatory’s Very Large Telescope in Chile, helped astronomers measure the polarization of light in the comet’s dust grains.
The science is similar to how polarized sunglasses filter out glare and brightness. In this case, astronomers observed how sunlight was polarized, or filtered, by the comet’s dust to learn about the physical properties of the comet.
Using this technique – known as polarimetry, which is also used to study comets in our solar system – allowed for a comparison between 2I/Borisov and other known comets.
Polarimetry was first used more than 200 years ago to observe the “Great Comet of 1819” by French astronomer François Arago. Now it’s being used to study and observe the first known comet that originated outside of our solar system, said Stefano Bagnulo, lead study author of the Nature Communications study and astronomer at the Armagh Observatory and Planetarium in Northern Ireland.
The polarization for comet 2I/Borisov was much higher than that of comets in our solar system, with the exception of comet C/1995 O1 – otherwise known as Hale-Bopp. Comet Hale-Bopp was highly visible to the naked eye in the late 1990s and was considered to be largely pristine before passing by our sun 1997. That meant Hale-Bopp was largely unaltered from the cloud of gas and dust the comet originally formed from, which is the same stuff our very solar system also originated from 4.5 billion years ago.
What makes the interstellar comet unique is that its polarized light is uniform, which makes it more pristine. Astronomers believe that this suggests 2I/Borisov has remained undisturbed since it formed – until it flew by our sun in 2019.
“Comet 2I/Borisov most likely never passed close to the Sun or any other star, and may represent the first truly pristine comet that has ever been observed,” the authors wrote in their study.
That means 2I/Borisov carries unchanged information about the gas and dust that originally formed the comet.
“It seems that 2I/Borisov originated in an environment not too different from our early solar system,” Bagnulo said. “Therefore, rather than telling something about comets in general, comet 2I/Borisov is telling us that other solar systems may be not so different from our own solar system.”
Astronomers still want to understand more about the nature of the dust being ejected by the comet. Bagnulo noted that scientific operations at the European Southern Observatory were suspended due to Covid in April and May 2020, which may have provided more of that information.
A future mission called the Comet Interceptor, which will be launched by the European Space Agency at the end of this decade, will investigate a pristine comet. The mission’s target will be decided after the launch since it has yet to be discovered.
“Comets that never passed close to the sun are particularly interesting because their material is (presumably) uncontaminated by solar radiation and wind, and as such, they carry information about the environment of our early solar system,” Bagnulo said. “There is even a small chance that the target of that mission is another interstellar comet, if such an exotic object is discovered at the right time.”
In the meantime, large survey telescopes like the Vera Rubin Observatory coming online next year in Chile “will enormously improve our capability to discover new visitors when they come,” he said.
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Another solar system
Comet 2I/Borisov’s origin story can be told by its dust grains.
When astronomers used the European Southern Observatory’s Very Large Telescope and the Atacama Large Millimeter/submillimeter Array of telescopes in Chile, they were able to piece together information about the comet’s past.
“Our observations suggest that materials in the comet are not uniformly distributed. Instead, the comet consists of components that have different compositions and formed at different places,” said Bin Yang, lead author of the Nature Astronomy study and an astronomer at European Southern Observatory in Chile.
The coma, or dust envelope that surrounds the comet’s nucleus, actually contains compact pebbles, or millimeter-sized grains. These grains were also previously detected in Comet Hale-Bopp.
The comet’s water and carbon monoxide content also appeared to change in a very noticeable way when it approached our sun.
Combined, this information suggests that the comet is made of a potpourri of materials from different parts of its original planetary system.
Like in our own solar system early on, the presence of giant planets and their gravity may have caused this mix, stirring up material throughout the system.
“While the most common planets in other exoplanetary systems seem to be super-Earths and mini-Neptunes, our study suggests the presence of giant planets in the home system of 2I/Borisov,” the authors wrote in their study.
Yang anticipates that future telescopes, both ground and space-based, will allow scientists to detect more interstellar objects as they pass through our corner of the universe.
“Imagine how lucky we were that a comet from a system light-years away simply took a trip to our doorstep by chance,” Yang said.