Artist's impression of Comet 2I/Borisov. Credit: ESO/M.Kormesser

An Undisturbed Ancient Relic of Our Galaxy is Flying Through Our Solar System

Comet 2I/Borisov appears to be more unique than astronomers previously thought.


Astronomers have found that the first-ever interstellar comet Borisov was also the first observable relic of its planetary system, which never approached the stars – its matter remained intact since its formation in the protoplanetary disk.

Comet 2I / Borisov was discovered on August 30, 2019, by astronomer Gennady Borisov and became the second interstellar object in the history of sky observations (the first was asteroid 1I / Oumuamua).


Rapprochement with the Sun took place It approached the Sun as close as possible in December 2019, and in the spring of last year, the core collapsed. The comet is currently leaving the solar system and returning to interstellar space.

The study of such objects allows us to understand the composition of comets from other planetary systems and check how different the matter of the circumstellar disks, from which small bodies are formed, from the matter from our system.

In particular, the first observations of Borisov’s comet showed its similarity with comets in the solar system, but then significant differences in composition were found, for example, the abundance of carbon monoxide.

Image of Comet 2I / Borisov and background stars taken at the end of 2019 with the VLT telescope. Credit: Olivier Hainaut / ESO
Image of Comet 2I / Borisov and background stars taken at the end of 2019 with the VLT telescope. Credit: Olivier Hainaut / ESO

Comet Borisov is more unique than astronomer previously thought

Two groups of astronomers, led by Stefano Bagnulo from the Armagh Observatory in Northern Ireland and Bin Yang from the European Southern Observatory, published the results of their analysis of polarimetric observations of Comet Borisov in December 2019 and January 2020 using the FORS2 receiver on the VLT in Chile and millimeter-wave data from the ALMA radio telescope system.


It turned out that the coma consists of compact “pebbles” with a radius of more than one millimeter, which suggests that the dust particles in the protoplanetary disk, where the comet was formed, condensed as a result of mutual collisions.

The rate of dust formation by the nucleus was estimated at 200 kilograms per second, so between the moments of opening and passing the perihelion point, the comet lost 2 × 10 9 kilograms of dust. At the same time, dust in a coma is more than three times more than gas, and ice grains are practically absent.

CO formation rate was 3.3 × 10 26 molecules per second, while the ratio of CO / H2O molecules in a coma changed sharply before and after perihelion, which indicates the inhomogeneity of the comet’s nucleus, with components formed in different places outside the snow line that could mix with each other.

Scientists have come to the conclusion that they observe a similarity in the behavior of comet Borisov and comet Hale-Bopp. This suggests that in whatever astrophysical environment an interstellar comet appeared, such a medium had properties that led to the formation of a body similar to bodies formed in the outer regions of the solar system.

At the same time, unlike comet Hale-Bopp and many other comets, which could have approached the Sun more than once, 2I / Borisov had never flown near any other star before meeting our star and may represent the first primordial comet, which, when -or observed.


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Bagnulo, S., Cellino, A., Kolokolova, L., Nežič, R., Santana-Ros, T., Borisov, G., . . . Devogèle, M. (2021, March 30). Unusual polarimetric properties for interstellar comet 2i/borisov.
ESO. (n.d.). First interstellar comet may be the most PRISTINE ever found.
Patel, N. (2021, March 31). Interstellar visitor 2i/borisov is the most pristine COMET humans have studied.
Strickland, A. (2021, March 30). Pristine interstellar comet came from a system containing giant planets.
Yang, B., Li, A., Cordiner, M., Chang, C., Hainaut, O., Williams, J., . . . Villard, E. (2021, March 30). Compact pebbles and the evolution of volatiles in the interstellar comet 2i/borisov.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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