When a comet from interstellar space was seen passing through the solar system in late August last year, astronomers drew attention to its composition. 2I / Borisov was an unusually rare opportunity to study the formation of comets in other stars.
In these initial measurements, 2I / Borisov appeared to look like comets from the outer solar system. But a new analysis showed that our visitor is much more alien than we thought.
In the cloud of gas that the comet began to emit as it approached the Sun, the team of astronomers discovered more carbon monoxide than they had ever seen in a comet within a radius of 300 million kilometers from the Sun.
“This is the first time we look inside a comet from outside our solar system,” said astrochemist Martin Cordiner of the Catholic University of America, “and it is in stark contrast to most other comets we've seen before.”
2I / Borisov was not the first known interstellar object in the solar system; it was Oumuamua, but that is not yet clear. 2I / Borisov is the first known comet.
Comets spend most of their time in the cold outer reaches of the planetary system; at least that's the case in the solar system.
As a consequence, comets are like time capsules, changing very little since their formation. Therefore, it is believed that these tiny chunks of rock and ice can tell us a lot about the composition and processes of early planetary systems.
We don't know much about how this happens in other planetary systems, but we can't wait to find out – not least because comets falling to early Earth could have spawned life, carrying water and other ingredients necessary for the process.
Thus, when 2I / Borisov flew away from the Sun after perihelion on December 8, astronomers carefully studied his coma, the cloud of gas that surrounds the comet as it approaches the Sun, and the ice inside is sublimated by the heat.
The comet's coma contained hydrogen cyanide; this is fairly typical of comets in the solar system. But the CO levels made scientists wonder.
Although most comets in the solar system contain wildly different amounts of CO, 2I / Borisov has much more of it than we usually see – 9–26 times more than the average comet in the solar system.
Astronomers aren't sure why comets in the solar system have such different proportions of CO, but they think it might have something to do with the temperature at which the comet was formed. The lower the temperature, the higher the CO concentration. This means that 2I / Borisov formed somewhere very cold.
'The comet must have formed from a material very rich in CO ice, which is only present at the lowest temperatures found in space, below -420 degrees Fahrenheit (-250 degrees Celsius),' said planetary scientist Stephanie Milam of NASA.
“2I / Borisov gave us a first look at the chemistry that shaped another planetary system,” Milam said. “But only when we can compare the object to other interstellar comets will we know if the comet is a special case or if most interstellar objects have unusually high CO levels.”
The study was published in the journal Nature Astronomy.
Sources: Photo: (NRAO / AUI / NSF, S. Dagnello)
