The first interstellar comet may be the most pristine ever found

New observations with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) indicate that rogue comet 2I / Borisov, which is only the second and most recently detected of interstellar visitors to our solar system, is one of the most pristine ever observed. Astronomers suspect that the comet probably never passed near a star, making it an intact relic of the cloud of gas and dust from which it formed.

2I / Borisov was discovered by amateur astronomer Gennady Borisov in August 2019 and was confirmed to come from beyond the solar system a few weeks later. “2I / Borisov could represent the first truly pristine comet ever to be observed,” says Stefano Bagnulo of the Armagh Observatory and Planetarium in Northern Ireland, UK, who led the new study published today in Nature communications. The team estimates that the comet had never passed near a star before flying near the Sun in 2019.

Bagnulo and his colleagues used the FORS2 instrument on ESO’s VLT, located in northern Chile, to study 2I / Borisov in detail using a technique called polarimetry. As this technique is regularly used to study comets and other small bodies in our solar system, it allowed the team to compare the interstellar visitor with our local comets.

The team found that 2I / Borisov had polarimetric properties distinct from those of solar system comets, with the exception of Hale-Bopp. Comet Hale-Bopp garnered a lot of public interest in the late 1990s because it was easily visible to the naked eye and also because it was one of the most pristine comets that astronomers have ever seen. views. Prior to its most recent passage, Hale-Bopp is believed to have passed through our Sun only once and therefore had hardly been affected by solar wind and radiation. This means that it was intact, with a composition very similar to that of the cloud of gas and dust that it – and the rest of the solar system – formed about 4.5 billion years ago.

By analyzing the polarization as well as the color of the comet to gather clues to its composition, the team concluded that 2I / Borisov is in fact even more pristine than Hale-Bopp. This means that it bears untarnished signatures of the cloud of gas and dust from which it formed.

“The fact that the two comets are remarkably similar suggests that the environment in which 2I / Borisov was born is not that different in composition from the environment in the early solar system,” says Alberto Cellino, co-author of the study, from the Turin Astrophysical Observatory, National Institute of Astrophysics (INAF), Italy.

Olivier Hainaut, an astronomer at ESO in Germany who studies comets and other near-Earth objects but was not involved in this new study, agrees. “The main result – that 2I / Borisov is unlike any other comet except Hale-Bopp – is very strong,” he says, adding that “it is very plausible that they formed under very similar ”.

“The arrival of 2I / Borisov from interstellar space represented the first opportunity to study the composition of a comet from another planetary system and to verify whether the material that comes from this comet is somehow different from our native variety, ”says Ludmilla Kolokolova, of the University of Maryland in the United States, who participated in the Nature communications research.

Bagnulo hopes astronomers will have another, even better, opportunity to study a rogue comet in detail before the decade is out. “ESA plans to launch Comet Interceptor in 2029, which will have the ability to reach another visiting interstellar object, if an object on an appropriate trajectory is discovered,” he said, referring to an upcoming mission from the European Space Agency.

An origin story hidden in the dust

Even without a space mission, astronomers can use Earth’s many telescopes to better understand the different properties of rogue comets like 2I / Borisov. “Imagine how lucky we were that a comet from a system light years away simply came to our door by chance,” says Bin Yang, astronomer at ESO in Chile, who also took advantage of the passage. of 2I / Borisov through our Solar. System to study this mysterious comet. The results of his team are published in Nature astronomy.

Yang and his team used data from the Atacama Large Millimeter / submillimeter Array (ALMA), in which ESO is a partner, as well as from ESO’s VLT, to study 2I / Borisov dust grains to to collect clues about the birth of the comet and the conditions in its home system.

They found that the 2I / Borisov coma – a dust envelope surrounding the main body of the comet – contains compact pebbles, grains of about a millimeter or more. In addition, they found that the relative amounts of carbon monoxide and water in the comet changed dramatically as it approached the Sun. The team, which also includes Olivier Hainaut, says this indicates that the comet is made up of materials that formed in different places in its planetary system.

The observations of Yang and his team suggest that the matter in the planetary house of 2I / Borisov was mixed from near his star to farther away, possibly due to the existence of giant planets, whose strong gravity stirs the material in the system. Astronomers believe a similar process occurred early in the life of our solar system.

Although 2I / Borisov was the first rogue comet to pass the Sun, it was not the first interstellar visitor. The first interstellar object to be observed passing through our solar system was “Oumuamua, another object studied with ESO’s VLT in 2017. Originally classified as a comet,” Oumuamua was later reclassified as an asteroid because it lacked coma.