We know of only two interstellar visitors – that is, visitors from other galaxies – to our solar system. They are 1I / ‘Oumuamua and 2I / Borisov. ‘Oumuamua gets a lot of pressure as an alien traveler who can be anything from an exoplanet to an alien spacecraft. The lesser-known 2I / Borisov is more clearly a comet that may have originated near a red dwarf star. Its chemical signature suggests that it may have never interacted with a star. If so, Stefano Bagnulo of the Armagh Observatory and Planetarium in Northern Ireland, UK, said:
2I / Borisov may be the first true pristine comet ever seen
Bagnulo leads a new study on Comet 2I / Borisov, published on March 30, 2021, in the peer-reviewed journal Nature communication.
He and his team believe that 2I / Borisov has never passed near any star before flying past our sun in 2019. If so, this object can still hold clues to the gas and dust cloud in which it formed.
Comet 2I / Borisov passed near our sun in late 2019. In this photo from the Very Large Telescope in northern Chile, the comet is the vague object in the center. The stars appear as stripes because the telescope was trained on the moving comet, not the stars. The rainbow colors of the stars are the result of combining observations at different wavelengths into a composite image. Image via ESO / O. Hainaut.
A team of scientists led by Bagnulo uses a technique called polarimetry to measure polarized light from 2I / Borisov. Light is polarized when it passes through filters, such as the lenses of sunglasses or comet material. By studying the properties of sunlight polarized by the comet’s dust, researchers can learn the physics and chemistry of comets.
This analysis showed that 2I / Borisov differs from almost all other comets studied except Hale-Bopp. Astronomers believe comet Hale-Bopp passed the sun only once before its amazing return in the late 1990s. It is believed that that earlier return took place thousands of years ago, in 2215 BC, when Hale-Bopp may have had an almost collision with Jupiter and changed its orbit, bringing it past the sun for the first time. Because of the little interaction with the sun, Hale-Bopp is also considered a pristine comet, with a composition very similar to the cloud of gas and dust that formed our solar system 4.5 billion years ago.
The polarization data show that 2I / Borisov is still pristine as Hale-Bopp. 2I / Borisov shows unadorned signatures of the cloud of gas and dust from its formation. The evidence also shows that both comets formed under similar conditions, only in different solar systems. As Alberto Cellino of the Astrophysical Observatory of Torino put it:
The fact that the two comets are strikingly similar indicates that the environment in which 2I / Borisov originated is not so different in the composition of the environment in the early solar system.
Comet Hale-Bopp, seen here on April 4, 1997, was another of the most pristine comets known, meaning it had little interaction with its star, in this case our sun. Comet 2I / Borisov is also pristine, and in 2019 it will probably be the first interaction with any star. Image via E. Kolmhofer / H. Raab / Johannes-Kepler Observatory / Wikimedia.
A second team of scientists used telescopic data to analyze the dust grains of 2I / Borisov and published their findings on March 30, 2021 in the peer-reviewed journal. Natural Astronomy.
They discovered that the comet’s coma – which is the dust jacket that contains a comet’s main body – contained compact pebbles. These pebbles are grains about 1 millimeter (0.04 inch) in size or larger. The team also noted that the relative amounts of carbon monoxide and water in the comet changed drastically as it approached the sun. According to these scientists, these changes indicate:
… that the comet is made up of materials that have formed at different places in its planetary system.
The observations by [Bin Yang, an astronomer at ESO in Chile] and her team suggests that matter in 2I / Borisov’s planet house from near its star is mixed to get further, perhaps due to the existence of giant planets, the strong gravity of which stirs material in the system.
Astronomers believe that a similar process took place early in the life of our solar system.
When amateur astronomer Gennady Borisov discovered the comet 2I / Borisov in August 2019, it was only the second known object to visit our solar system from interstellar space. The first discovery was ‘Oumuamua in October 2017.’ Oumuamua moved rapidly 94,900 miles per hour until it swung against the sun to 315,800 km per hour.
These agile interstellar visitors are both now out of the solar system on their way back to interstellar space.
This diagram compares the paths of the two known interstellar objects that entered our solar system, 1I / ‘Oumuamua (red, discovered in 2017) and 2I / Borisov (yellow, discovered in 2019). Image via Wikimedia / Tony873004.
In short, the second known interstellar object, named by terrestrial astronomers Comet 2I / Borisov, is one of the most pristine objects ever seen, according to one new study.
Source: Unusual polarimetric properties for interstellar comet 2I / Borisov
Source: Compact pebbles and the evolution of volatiles in the interstellar comet 2I / Borisov
Via ESO
