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While flirting with the Sun, this comet encountered its first star

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New observations with the Very Large Telescope of the European Southern Observatory indicate that interstellar comet 2I / Borisov is one of the most “pristine” ever observed. Before passing near the Sun, she had probably never met a star.

On August 30, 2019, amateur astronomer Gennady Borisov spots a comet that apparently shouldn’t be there. He then informed the Minor Planet Center, of the International Astronomical Union, which launched follow-up analyzes. This work confirms the unusual trajectory of the object. Great news : this comet does not come from us.

This new “visitor” to space is 2I / Borisov, named in honor of the one who spotted him first. To date, it is the second passing interstellar object in our system ever cataloged, after Oumuamua.

More good news: while Oumuamua, which could be a piece of planet, was leaving our system, this comet arrives. Astronomers then seize this unexpected opportunity to analyze the object in more detail.

A very different composition

Very quickly, the first observations suggest that 2I / Borisov displays a composition quite close to those of comets in our Solar System. In reality, she is very different.

According to two new studies (here and here) based on observations from Hubble and ALMA (Atacama’s large millimeter / submillimeter antenna array), the coma of this comet (the atmosphere surrounding its nucleus as it approaches the sun) is distinguished by a large amount of carbon monoxide. However, in our system, the coma of comets is mostly water.

These observations therefore suggest that the comet must have formed quite far from its star, in a very cold region (below -250 ° C). The object would then have been kicked out of its system, but it is not yet clear how (and we will probably never know).

At the end of a long journey of several millions, even billions of years, 2I / Borisov then presented itself to our Sun. And obviously, it was a real baptism of fire.

2I / Borisov. Credits: Pieter van Dokkum, Cheng-Han Hsieh, Shany Danieli and Gregory Laughlin

Stellar first flirt

As part of a new study, the results of which are published in Nature communication, a team from the Armagh Observatory and Planetarium, in Northern Ireland, relied on the FORS2 instrument of the Very Large Telescope (VLT), in northern Chile, to study the polarimetry of the comet (measurement the polarization of its light).

The team found that 2I / Borisov had polarimetric properties distinct from those of comets in the Solar System, with the exception of Hale-Bopp, one of the most observed of the 20th century. This comet had generated a lot of public interest during its passage through perihelion on April 1, 1997. Astronomers were surprised to find that this comet was one of the most pristine ever seen.

Prior to her most recent stint, it is believed that Hale-Bopp had only rubbed shoulders with the Sun once. The comet had thus hardly been affected by the wind and the radiation of our star. And, in fact, its composition was very similar to that of the cloud of gas and dust from which it formed about 4.5 billion years ago.

Comparing the data between the two comets, the team suggests that 2I / Borisov is even more pristine than Hale-Bopp. In other words, this comet was not not rubbed against any star before its passage near the Sun in 2019.

Bagnulo hopes astronomers will have another, even better, opportunity to study other interstellar objects before the end of the decade. The European Space Agency (ESA) is indeed continuing the development of its unprecedented mission to “intercept” a comet entering the Solar System. Its launch is planned in 2028.


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