A good year ago the interstellar comet 2I / Borisov raced through our solar system. Astronomers took advantage of this opportunity to learn more about its composition from the light reflected from this chunk. Your evaluations now reveal: The comet differs from almost all comets in the solar system except for one. Comet Hale-Bopp, considered particularly original, showed a similar polarization pattern in the 1990s. The researchers conclude that 2I / Borisov must have formed under similar conditions in its home system – and that it is the most pristine comet ever observed.
Our solar system does not move in isolation through space: Objects of extrasolar origin also repeatedly fly through our cosmic home. The first known interstellar visitor is the Oumuamua object, discovered in October 2017. This cigar-shaped, 400-meter-long asteroid or comet sped past the sun and disappeared into space before astronomers could examine it further. This is different with the second interstellar visitor: 2I / Borisov was discovered by the amateur astronomer Gennady Borisov in August 2019 when he was still approaching the inner solar system. The high speed of 150,000 kilometers per hour and the flight path suggest that this chunk was also of extrasolar origin. First observations showed that 2I / Borisov had a core surrounded by a cloud of gas and dust and a tail – both clear features of a comet.
Light polarization reveals cometary properties
To find out more about its composition, researchers led by Stefano Bagnulo from the Armagh Observatory in Ireland targeted the interstellar comet several times in winter 2019/2020 with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. “The arrival of 2I / Borisov from interstellar space represents the first opportunity to study the composition of a comet from another planetary system,” explains co-author Ludmilla Kolokolova of the University of Maryland. “In this way we can check whether the material that originates from this comet differs in any way from our native variation.” The team used the FORS2 instrument at the VLT to examine the polarization of the light reflected by 2I / Borisov. This polarimetry allows conclusions to be drawn about the chemical and physical composition based on the changes in the oscillation plane of the light and the characteristics of the interstellar comet to be compared with those of comets from our solar system.
It turned out: “The polarization of 2I / Borisov is quite different from what we observe with comets in our solar system,” report the astronomers. Specifically, the interstellar comet showed a significantly higher polarization than the native chunks – with one exception: Comet Hale-Bopp. It was easily visible to the naked eye during its passage in the late 1990s and is considered to be one of the most pristine comets known to date. “At the time of its measurement, the polarimetric curve of Hale-Bopp was steeper and higher than that of any other comet seen before,” explain Bagnulo and his colleagues. These features are typical of very original, not yet weathered comets that have been altered by star winds. Astronomers therefore assume that Hale-Bopp first flew past our sun before his last passage.
Even more extreme than Hale-Bopp
The interstellar comet 2I / Borisov shows a similar polarization curve as Hale-Bopp – although the latter with a 20 to 35 kilometer core is far larger than the 2I / Borisov, which is only around 400 meters in size. “The fact that the two comets are remarkably similar suggests that the environment in which 2I / Borisov was formed is not that different in composition from the environment in the early solar system,” says co-author Alberto Cellino from Turin Astrophysical Observatory. Both objects could therefore have been formed on the icy outer edge of their system. The data also suggest that both comets consist mainly of smaller dust and ice particles.
(Video: ESO)
But as the measurement data for 2I / Borisov showed, this interstellar comet has an even more extreme polarization curve than the Hale-Bopp from our solar system. Bagnulo and his colleagues conclude from this that this icy chunk could be even more original and unchanged than the native comet. “2I / Borisov could be the first truly unadulterated comet ever observed,” says Bagnulo. The team is convinced that the comet has never flown close to a star before it passed the Sun in 2019. It still carries the molecules from which it was once created in its extrasolar environment. Now the astronomers hope to have an even better chance of investigating the next passage of an interstellar object. “The European Space Agency, ESA, plans to launch Comet Interceptor in 2029, which will have the ability to reach a next visiting interstellar object if it is detected on an appropriate trajectory,” says Bagnulo.
Source: Stefano Bagnulo (Armagh Observatory & Planetarium, Armagh) et al., Nature Communications, doi: 10.1038 / s41467-021-22000-x