In October 2019 astronomers first observed the comet that later came to be called ‘2I/Borisov’. A somewhat special comet for being the second known to come from another solar system, being an interstellar comet. But as it has been studied more, more interesting details have been discovered about it, such as its composition or, now, its unaltered shape.

In a new study published in Nature, researchers have taken a closer look at the comet to understand its origin and history. It turns out that it is the most pristine astronomical object discovered to date.

Comets are essentially ancient chunks of material discarded from the planetary formation phase of solar systems. Those that come from our own solar system are of great interest to astronomers as they provide a better understanding of the early stages of Solar System formation.

But what if they come from another solar system? They are even more interesting.

The problem with comets is that they degrade over time, a lot. Over time, the solar wind and radiation consume the comet, causing it to change its composition, shape and characteristics. The main cause? The stars and the solar wind that they emit towards all nearby celestial bodies such as comets.

But the case of 2I/Borisov seems to be different. In addition to being the second interstellar object discovered after Oumuamua, it also appears to be the least altered. That is, according to the new study, this space object may represent the most pristine comet ever discovered. In turn, this may indicate that it has never passed close to a stellar body before our Sun.

The study authors analyzed the polarimetry of 2I/Borisov to measure the polarization of light from the Sun as it passed through the comet’s dusty tail. 2I/Borisov was found to be photometrically distinct from the vast majority of comets in our Solar System, with much higher levels of light polarization.

Previously, in fact, only one other comet has been discovered to approach it at such polarization levels, comet Hale-Bopp. Hale-Bopp passed close to us in 1997 and because of how little disturbed it was it is believed that it had never before in its history passed close to the Sun, hence it had hardly any disturbances. But the case of 2I/Borisov is even more interesting, which could mean that it has never been close to a star before, not even the one where it was formed.

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