Last year, our Solar System received an interstellar visitor: an object designated 2I/Borisov. By pointing the Atacama Large Millimeter / submillimeter Array (ALMA) antennas toward the comet on December 15 and 16, 2019, astronomers were able, for the first time, to observe the chemical composition of an object from another planetary system. The findings of this observation were published online on April 20, 2020, in the journal Nature Astronomy. ALMA has the ability to scan electromagnetic radiation which helps astronomers better understand the chemical composition of objects both large and small across the cosmos.

What scientists found, however, was unexpected.

A new analysis of the comet has discovered that the interstellar object probably formed in a distant, cold part of its own solar system, far from its host star.

As astronomers pointed their telescopes to the sky looking at the interstellar visitors, they noticed that the object contained an unusually high amount of carbon monoxide. In fact, they say they’ve discovered more of the chemical than in any other comet near the sun. The new study suggests that the existence of the chemical is 26 times higher than the average solar system comet.

This changes a lot about what we think of interstellar comets. When astronomers observe comets in our solar system, they usually find how water is the most abundant molecules in the come (the atmosphere that engulfs the core of the comet.)

However, observations of 2I/Borisov revealed that the interstellar visitor contained as much as 1.7 times as much carbon monoxide as it did water.

Comets are of great interest to astronomers because they spend most of their time very far from the stars, in very cold environments. Unlike planets, their internal composition does not change much from the time of their formation. Therefore, they can contain valuable information about the processes that occurred during their birth in the protoplanetary disks. “This is the first time we have studied the interior of a comet that originated from outside our Solar System, and it is considerably different from most comets we have observed so far,” says astrochemist Martin Cordiner.

“ALMA has contributed to transforming our way of understanding the nature of the material that makes up the comets of our own Solar System and, now, of this particular object from a neighboring system. It is only thanks to ALMA’s unprecedented sensitivity in the submillimeter spectrum that we have been able to determine the characteristics of the gas released by such unique objects,” explains the co-author of the new study Anthony Remijan of the United States National Radio Astronomical Observatory in Charlottesville, Virginia.

2I/Borisov may have been a comet that was expelled at one point from its home star system, probably when it interacted with a star or planet. As it was kicked towards interstellar space, the cold environment through which it traveled preserved it until it reached our solar system. This means that essentially, we are now looking at the chemical composition of an object that formed in a distant star system alien to ours, billions of years ago.

2I / Borisov is only the second interstellar object detected to date speeding through our solar system. The first, 1I/’Oumuamua, was discovered in October 2017, was already on its way out of our solar system when we spotted it. This made it somewhat difficult for astronomers to determine if it was a comet, an asteroid or something else.

In the case of 2I/Borisov, the presence of an active plume of dust and gas confirmed it as the first interstellar comet. As long as no other interstellar comets are observed, the unusual composition of 2I / Borisov will continue to offer more questions than answers: is it normal in interstellar comets? Will we see more interstellar comets with peculiar chemical compositions in the coming years? What will they teach us about how planets form in other solar systems?

Comet 2I/Borisov—including its coma and tail—measures fourteen times the size of our planet. According to Pieter van Dokkum from Yale University: “It’s humbling to realize how small Earth is next to this visitor from another solar system.” Astronomers have revealed that the comet’s tail is around 160,900 kilometers (100,000 miles) long.