First known interstellar comet is most pristine ever discovered
The first comet known to visit our Sun after being born around an alien star may also be the most pristine comet ever discovered, according to the results of a new study. The research sheds light on the distant solar system in which the wandering body, known now as 2I/Borisov, formed, before being cast out into the depths of interstellar space.
Comets are essentially ancient chunks of discarded material left over from a solar system’s planetary formation phase. They are highly prized targets for astronomers looking to understand the early history of our solar system, as they offer an opportunity to analyze the ancient material from which everything in our little corner of the cosmic neighborhood coalesced.
But just because they are relatively well preserved does not mean that they are perfectly preserved. Over vast swathes of time, stellar wind and radiation takes its toll on wandering comets that pass close to a star, triggering changes in their outer layers.
On August 10, 2019, an astronomer named Gennady Borisov discovered a new comet-like wandering body, that, upon further analysis, appeared to have originated from outside of our solar system. The comet’s extrasolar nature was later confirmed by the International Astronomical Union, which conferred upon it the name 2I/Borisov.
This unexpected visitor was only the second object, after rocky oddball ‘Oumuamua in 2017, known to have visited our Sun after being born in the orbit of another star.
This made it a tantalizing target for astronomers who saw in 2I/Borisov an incredibly rare opportunity to glean insights into the primordial cloud of dust and gas from which it was born.
According to a new study, this wandering hunk of space debris may represent the most pristine comet ever discovered, which may in turn indicate that it has never passed close to a stellar body.
The authors of the new study observed 2I/Borisov via a technique called polarimetry, using the FORS2 instrument located on ESO’s Very Large Telescope in Chile.
In this context, polarimetry allowed the astronomers to measure the polarization of light from the Sun as it passed through the interstellar visitor’s dusty coma.
2I/Borisov was found to be photometrically distinct from the vast majority of solar system comets, and boasted higher levels of light polarization compared to its terrestrial counterparts.
Only one solar system wanderer was found to have similar polarimetric properties to 2I/Borisov – the famous Hale-Bopp comet. In 1997 Hale-Bopp passed close to the Sun, and for a time became a spectacular sight in the night sky.
Astronomers believe that Hale-Bopp had only passed the Sun once in its long history before 1997. However, the polarimetric data from 2I/Borisov reveals that, unlike Hale-Bopp, it plays host to a polarimetrically homogeneous coma.
According to the authors of the study, this relatively uniform coma of dust and gas indicates that 2I/Borisov is even more pristine than Hale-Bopp. This led the team to suggest that the alien comet may never have passed close to a stellar body.
The otherwise similar polarimetric properties to Hale-Bopp also indicate 2I/Borisov coalesced from a primordial cloud similar to that from which our solar system formed.
A separate study published in the journal Nature Astronomy has found further evidence 2I/Borisov's home star system is similar to our own.
This second paper focuses on the levels of carbon monoxide and water particles present in the comet’s coma, and the way that the amounts of these particles varied as it approached the Sun. It also took into account the presence of grains roughly a millimeter across or larger.
According to the team behind the Nature Astronomy paper, these characteristics suggest that 2I/Borisov formed from materials that were created in different parts of a distant solar system.
These materials were subsequently mixed due to the gravitational influence of large planets, and from this combination of familiar yet alien materials 2I/Borisov was born. This echoes events that scientists believe took place in our home system, wherein huge planets prompted a similar mixing process.
The papers have been published in the scientific journals Nature Communications and Nature Astronomy.
Source: European Southern Observatory
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