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The interstellar comet 2I / Borisov provides insight into the planetary building blocks of another star system, using new observations from NASA’s Hubble Space Telescope.
Borisov is the first known comet to originate from a different star system than ours. Measurements find that it has an unusual abundance of carbon monoxide largely unlike comets belonging to our solar system. The researchers say its unusual composition points to a likely birthplace of a carbon-rich circumstellar disk around a cold class of red dwarf star. These observations are an excellent opportunity to sample the chemistry of the material in a primordial disk around another star.
Comets are condensed samples of gas, ice, and dust that form by rotating in the disk around a star during the birth of their planets. Studying comets is important because astronomers are still trying to understand the role they play in the accumulation of planets. They can also redistribute organic material among young planets, and they may have brought water to early Earth. These activities are likely to occur in other planetary systems, as Borisov’s composition demonstrates.
“With an interstellar comet passing through our own solar system, it is as if we were getting a sample from a planet orbiting another star that appears in our own backyard,” said John Noonan of the Lunar and Planetary Laboratory at the University of Arizona, Tucson, who is a member of the Hubble research team led by Dennis Bodewits of Auburn University in Alabama.
The team used Hubble’s unique ultraviolet sensitivity to spectroscopically detect carbon monoxide gas escaping from the nucleus of the solid comet of Comet Borisov. Hubble’s cosmic origins spectrograph observed the comet on four different occasions, from December 11, 2019 to January 13, 2020, allowing researchers to see the object’s chemical composition change rapidly, as different ice mixtures, including carbon monoxide, oxygen, and water, sublimated under the heat of the sun.
Hubble astronomers were surprised to find that the coma of the interstellar comet, the gas cloud surrounding the nucleus, contains a large amount of carbon monoxide, at least 50% more abundant than water vapor. This quantity is more than three times greater than the quantity previously measured for any comet entering the internal solar system. The water measurement was carried out by NASA’s Neil Gehrels Swift satellite, the observations of which were made in conjunction with the Hubble study.
Carbon monoxide ice is very volatile. It doesn’t take much sunlight to heat the ice and turn it into gas that escapes from the nucleus of a comet. For carbon monoxide, this activity occurs far away from the Sun, about 11 billion miles away, more than twice the distance from Pluto at its furthest point from the Sun. In contrast, water remains in its form frost to about 200 million miles from the Sun, the approximate distance from the inner edge of the asteroid belt.
However, for Comet Borisov, measurements from Hubble suggest that some carbon monoxide ice was locked inside the comet’s nucleus, revealed only when the Sun’s heat removed the ice sheets from water. “The amount of carbon monoxide did not decrease as expected when the comet receded from the Sun.”
This means that we are looking at the primitive layers of the comet, which really reflect what this object is made of, “Bodewits explained.” Because of the abundance of carbon monoxide ice that survived so close to the Sun, we believe Comet Borisov comes from a much colder location and a very different debris disk around a star than ours. “
At 200 million miles from the Sun, the degassing rates of a comet’s surface water are almost always much higher than those of carbon monoxide, the researchers said. Only about one or two known comets in the solar system have challenged that rule. “What Hubble measured on Comet Borisov is not a property of most comets in the solar system,” Bodewits said. “That is why Comet Borisov stood out for us because we reasoned that Borisov is likely to be a representative of the star system it came from.”
The researchers suggest that the comet may have been ejected from a carbon-rich disk of icy debris around a red dwarf star, the most common type of star in our galaxy, the Milky Way. Red dwarfs are weaker and less massive than the Sun. Their circumstellar disks, therefore, can be much colder than our solar system. “These stars have exactly the low temperatures and luminosities where a comet could form with the type of composition found in Comet Borisov,” Noonan said.
A large planet the size of Jupiter may have driven the comet out of the alien system. The researchers said that many red dwarfs have large planets that orbit in a region far enough from their host star where carbon monoxide exists in its icy form. “If a Jupiter-sized planet migrates inward, it could expel many of these comets,” Bodewits said.
Comet Borisov was seen on August 30, 2019 by kite hunter Gennady Borisov in Crimea. The tramp comet is similar to other comets in the solar system, but astronomers determined its interstellar origins based on its orbital path. Since its discovery, a large number of telescopes, including Hubble, have observed the comet as it traveled through the solar system and passed the Sun. It will eventually leave the solar system and continue its journey through space.
Comet Borisov is the first bona fide interstellar comet to visit the solar system. The first known homeless visitor was an object called 1I / ‘Oumuamua, which was discovered in 2017 when it was moving away from the Sun. Unlike a normal comet,’ Oumuamua did not have a visible coma of gas and dust escape around it, therefore, astronomers could not use spectroscopy to sample its chemical content to characterize it.
Astronomers hope to find more of these wandering comets from outside the solar system with current and future telescopes that scan the entire sky.
related links
Space Telescope Science Institute
Asteroid and Comet Mission News, Science and Technology
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Fragmentation of the comet ATLAS observed in the first images of a multitude of citizen astronomers
Mountain View CA (SPX) April 20, 2020
A group of citizen astronomers scattered around the world has just demonstrated how a network of Unistellar digital eVscopes can work together to deliver the first publicly generated images of Comet ATLAS as it decays.
Discovered in December, Comet ATLAS was expected to become the brightest comet of 2020, visible to the naked eye. However, several days ago, astronomers began to suspect that the comet had split into multiple parts when it began to rapidly darken. In Unistellar … read more
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