New observations of the second known outside visitor to our solar system, a comet named Borisov, reveal information about its composition as well as its home system, which seems to contain comets similar to those orbiting the sun and Jupiter-like planets that toss around other objects with their massive gravity.
Presented in two studies from different research groups and published in Nature journals, the findings provide a new level of detail about Borisov in a rare glimpse into interstellar objects, which had not been spotted in Earth's backyard before 2017.
In a Nature Communications paper, scientists studied the polarization of light reflecting off Borisov and found it looked so pristine that it probably never passed close to any star before. Such a comet has previously never been seen. Another paper in Nature Astronomy used both visible-light and radio-wave measurements to find that the comet is made of relatively large pebbles possibly created by an early history of collisions.
Formally named 2I/Borisov, the star-hopping comet was first discovered in August 2019 by a Crimean amateur astronomer and is about 3,200 feet across. It was confirmed to be an interstellar object a month later — though its native star system is currently unknown — and is only the second ever found, after the asteroid-like 'Oumuamua was detected in October 2017.
Borisov made its closest approach to the sun in December 2019 when it passed between the orbits of Mars and Jupiter, about twice as far out from the sun as the Earth. The interstellar comet is on a trajectory to leave the solar system, but scientists took advantage of the close approach with observations that became the backbone of the two new studies.
One paper used observational data from the Very Large Telescope in Chile to study the polarization of light reflecting off Borisov's dust. Polarization refers to the orientation of a light wave's electromagnetic oscillations, which occur at a right angle to the direction it is traveling. Polarized sunglasses, for instance, work by blocking light with a certain direction of polarization.
By comparing the polarization with the angle between the comet, Earth and sun from December 2019 to March 2020, the scientists found that the polarization was unusually smooth.
"That is an indication that the comet is quite pristine and probably has never seen a star before at close distance," said Stefano Bagnulo, one of the paper's lead authors and an astronomer at the Armagh Observatory and Planetarium in Northern Ireland.
That would make the interstellar visitor the first fully pristine comet ever observed, and through the polarized lens, Borisov resembled no other known comet except for Comet Hale-Bopp. The famous comet lit up the skies in 1997 and had passed by the sun possibly only one other time.
Though Borisov appeared even more untarnished, the similarities between it and Hale-Bopp also led the astronomers to suggest that their home star systems are at least somewhat similar.
"What we suggest is that 2I/Borisov is even more pristine than Hale-Bopp and probably originated in a solar system that was not so different from our own solar system," Bagnulo said.
Another group of scientists analyzed the physical and chemical makeup of Borisov, using both visible-light data from the Very Large Telescope data and radio-wave observations from the Atacama Large Millimeter Array, also located in Chile. They observed Borisov's coma, a temporary atmosphere created by the sun's heat and made of melted ice and dust.
Modeling based on these measurements showed that Borisov is largely made of densely packed pebbles between 1 and 10 millimeters long, which are bigger than the dust particles seen on most comets. Its density may have been formed by numerous collisions during its formation that compacted dust into larger clumps, the researchers said.
They also found that the ratio between the comet's water ice and frozen carbon monoxide substantially changed throughout its journey, suggesting that it contains a mix of material from both the inner and outer parts of its home solar system during its formation. Research published in April 2020 found that Borisov contained a higher concentration of carbon monoxide than any other observed comet.
Both of these observations led the astronomers to theorize that the system has a massive planet similar to Jupiter, with a gravitational pull large enough to mix material from different parts of the solar system and even eject some objects out of the system altogether.
"Jupiters throw things around, and regions have this very hot material mixed with cold material," said study author Bin Yang, an astronomer at the European Southern Observatory. "We think probably Borisov's solar system is kind of like our solar system, and there's giant planets [that] formed early on and start to mix things around."
Yang and her team additionally calculated that the comet was losing about 440 pounds of dust per second on its journey near the sun, and that its surface has eroded by at least 6 feet since it was discovered.
The two papers disagreed about the size of particles on Borisov. While Yang's group found that they were pebbles at least a millimeter long, Bagnulo and his coauthors detected dust at least 10,000 times smaller.
Yang noted that the groups used observations made in different wavelengths of light that biased their findings toward differently sized particles. The differing results highlight the importance of using many measurements to better understand the comet, she said.
The search for interstellar objects is expanding with the opening of new observatories on the horizon. The Vera C. Rubin Observatory will begin operations in late 2022, and it is expected to lead to the discovery of about one interstellar object per year, according to Yang. She said the Extremely Large Telescope, planned to open later this decade with a 129-foot lens, will be able to reveal more detail of the celestial visitors. Both are being constructed in Chile.
"It's a very exciting time. We have two interstellar objects, and they behave wildly different, and we think we'll find more of them," Yang said. "So far there's no big surprises, but we think future interstellar objects may be totally different from our system."
The study, "Unusual polarimetric properties for interstellar comet 2I/Borisov," published March 30 in Nature Communications, was authored by Stefano Bagnulo and Apostolos Christou, Armagh Observatory & Planetarium; Alberto Cellino, Observatory of Turin; Ludmilla Kolokova, University of Maryland, College Park; Rok Nežič, Armagh Observatory & Planetarium, University College London and University of London; Toni Santana-Ros, University of Alicante and University of Barcelona; Galin Borisov, Armagh Observatory & Planetarium and Bulgarian Academy of Sciences; and Philippe Bendjoya, University of Côte d'Azur and Arecibo Observatory.
The study, "Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov," published March 30 in Nature Astronomy, was authored by Bin Yang and Olivier Hainaut, European Southern Observatory; Aigen Li, University of Missouri; Martin Cordiner, NASA Goddard Space Flight Center and Catholic University of America; Chin-Shin Chang and Eric Villard, Joint ALMA Observatory; and Jonathan Williams, Karen Meech and Jacqueline Keane, University of Hawaii.