We only know 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 press as a strangely shaped traveler who could be anything from a slice of 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 it may never have interacted with a star before. If so, said Stefano Bagnulo of the Armagh Observatory and Planetarium in Northern Ireland, UK:
2I / Borisov could represent the first truly pristine comet ever observed
Bagnulo led a new study on Comet 2I / Borisov, published March 30, 2021 in the peer-reviewed journal Nature Communications
He and his team believe that 2I / Borisov had never come close to a star before it flew past our sun in 2019. If so, this object may still contain 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 fuzzy object in the center. The stars appear as stripes because the telescope is aimed at 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 used 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 comet dust, researchers can learn more about the physics and chemistry of comets.
This analysis showed that 2I / Borisov was different from almost all of the other comets studied except Hale-Bopp. Astronomers believe Comet Hale-Bopp only passed the Sun once before returning beautifully in the late 1990s. That earlier reentry is believed to have occurred thousands of years ago, in 2215 B.C., when Hale-Bopp may have had a near-collision with Jupiter, changing its orbit, and bringing it past the sun for the first time. Due to the few interactions with the sun, Hale-Bopp is also considered a pristine comet, with a composition very similar to that of the cloud of gas and dust that formed our solar system 4.5 billion years ago.
The polarization data shows that 2I / Borisov is even more pristine than Hale-Bopp. 2I / Borisov shows untouched signatures of the gas and dust cloud of its formation. The evidence also shows that both comets originated under similar conditions, only in different solar systems. As Alberto Cellino from Astrophysical Observatory of Torino said:
The fact that the two comets are remarkably similar suggests that the environment in which 2I / Borisov originated is not that different in composition from the environment in the early solar system.
Comet Hale-Bopp, seen here on April 4, 1997, was one of the most pristine comets known, meaning it had few interactions with its star, in this case our sun. Comet 2I / Borisov is also pristine, with its flyby near the Sun in 2019, believed to be the first to interact with a star. Image via E. Kolmhofer / H. Raab / Johannes-Kepler-Observatory / Wikimedia.
A second team of scientists used telescopic data to analyze 2I / Borisov’s dust particles and published their findings on March 30, 2021 in the peer-reviewed journal Nature astronomy
They found that the comet’s coma – the covering of dust around a comet’s main body – contains compact pebbles. These pebbles are grains about 1 millimeter in size or larger. The team also noted that the comet’s relative amounts of carbon monoxide and water 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 continue [Bin Yang, an astronomer at ESO in Chile] and her team suggests that matter in 2I / Borisov’s planetary house was mixed from close to its star to further away, perhaps due to the existence of gigantic planets, whose strong gravitational pull stirs material into the system.
Astronomers believe that a similar process took place early in the life of our solar system.
When amateur astronomer Gennady Borisov discovered Comet 2I / Borisov in August 2019, it became only the second known object to visit our Solar System from interstellar space. The first discovered was ‘Oumuamua in October 2017’. Oumuamua was moving at a rapid 58,900 miles (94,800 km) per hour until its sweep past the sun accelerated it to 196,200 miles (315,800 km) per hour.
These zippy interstellar visitors are now both leaving the solar system and back on their way to interstellar space.
This diagram compares the paths of the 2 known interstellar objects that have 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, dubbed Comet 2I / Borisov by Earth astronomers, may be one of the most pristine objects ever seen, according to a 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
