When the scientists peeked inside the halo of gas that formed around comet 2I/Borisov as it came closer to the Sun, they detected something peculiar: 2I/Borisov was releasing gas with a greater concentration of carbon monoxide (CO) than anyone had detected in any comet at a similar distance from the Sun.

When amateur astronomer Gennady Borisov discovered an interstellar comet zipping through our solar system on Aug. 30, 2019, astronomers promptly turned their telescopes towards it hoping to catch a glimpse of this rare and ephemeral event. After all, no one had ever set eyes on a confirmed comet from a foreign star system, and it was clear from its projected trajectory that the alien visitor, named 2I/Borisov, would soon disappear from the sky forever.

Before it dimmed from view, a team of international scientists led by Martin Cordiner and Stefanie Milam at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, probed it with the world’s most powerful radio telescope. The comet was near its closest approach to Earth at about 180 million miles, or nearly 300 million kilometers, away.

When the scientists peeked inside the halo of gas that formed around the comet as it came closer to the Sun and its ices began to vaporize, they detected something peculiar. 2I/Borisov was releasing gas with a greater concentration of carbon monoxide (CO) than anyone had detected in any comet at a similar distance from the Sun (within less than 186 million miles, or 300 million kilometers). 2I/Borisov’s CO concentration was estimated to be between nine and 26 times higher than that of the average solar system comet.

Until more interstellar comets are observed, this result raises more questions than it answers. If it was unaltered during its journey, could observations of 2I/Borisov be our first glimpses of the chemistry that shaped another planetary system? Or, could this comet’s composition hint that there’s a wider diversity of CO among solar system comets than scientists are aware of?

“Whatever the answer is, 2I/Borisov opens up a whole new can of worms for cometary science,” says Milam, a planetary scientist and co-author of the April 20 paper on the subject in Nature Astronomy.

Milam, Cordiner, and their colleagues observed the comet using a radio telescope observatory in northern Chile. Known as the Atacama Large Millimeter/submillimeter Array (ALMA), the telescope is made of 66 dishes that work together to observe the sky in great detail. Radio telescopes are ideal for looking at cold, low-energy gas in objects like comets.

On December 15 and 16, 2019, ALMA pointed at a location in the sky where 2I/Borisov was expected to be. This was based on an orbit trajectory that was precise to within 1/5,000th of a degree. The telescope collected a spectrum of energy emitted by the comet’s gases which helped materialize an amorphous image of it and information about its chemical makeup.

The researchers started by looking for the most easily detectable cometary gas, hydrogen cyanide. Detecting it was not too much of a surprise, considering that other scientists using optical telescopes had found the closely related cyanide gas in 2I/Borisov.

Carbon monoxide was a different story, however. Unlike hydrogen cyanide, emissions from CO are much weaker and harder to detect. The researchers almost didn’t bother looking for the gas it in their dataset.

But when the data came back from Chile, the team’s low expectations were shattered. The signal for CO — a lot of it, relative to the other gases — was irrefutable. “As soon as the spectrum came in, we knew it was a significant result,” Milam says.

Carbon monoxide is one of the most abundant molecules in space. Scientists expect to see it inside all comets, which are balls of frozen gases, rock and dust that form far away from the heat of the Sun. Yet, there’s a huge variation in the concentration of CO in comets and no one quite knows why. Some of this might be related to where in the solar system a comet was born; some has to do with how often a comet’s orbit brings it closer to the Sun and leads it to release more fragile ices.

So far, nothing explains why 2I/Borisov is “off the charts” of the carbon monoxide continuum, as Cordiner puts is. “It is dramatically different from other comets we’ve seen before,” says Cordiner, an astrochemist who led the Borisov observations.

The length of nine football fields, or 3,200 feet (975 meters), 2I/Borisov is a relatively small comet. Its path through the solar system made it clear to scientists that the comet came from outside our system and was not bound to it by the Sun’s gravity. 2I/Borisov was traveling too fast for that, at an extraordinary speed of about 74,000 miles (120,000 kilometers) per hour, in a hyperbolic trajectory that would bring it around the Sun and then fling it back out into the depths of space.

Carbon monoxide is the most volatile of all comet ices, or the quickest to vaporize in warmer temperatures. Thus, the comet likely formed very far away from its star in one of the coldest environments known, based on how much CO it had preserved. If it had formed around our Sun, 2I/Borisov would have been born in the vicinity of the Kuiper Belt. This is a donut-shaped region of icy bodies beyond the orbit of Neptune, where temperatures during the formation of the solar system could have reached -420 degrees Fahrenheit (-250 degrees Celsius) according to models. Scientists suspect that gravitational disturbances from young, jostling planets may have thrown 2I/Borisov out of its home star system.

“Then, after a cold, lonely voyage, 2I/Borisov made its close encounter with our solar system and started outgassing and showing us what it’s got inside,” Cordiner says.

He and his colleagues offer a few possible explanations for 2I/Borisov’s exotic CO levels, though none explains why this comet would’ve emerged this way while solar system comets didn’t. One idea is that 2I/Borisov could be a fragment of a dwarf planet that had a lot of carbon monoxide near its surface. “If that object collided with another, then the carbon monoxide-rich fragments could be released into space,” said Cordiner.

But 2I/Borisov may have simply formed as a comet with a high concentration of carbon monoxide. Or, perhaps it has a thick, thermally insulating outer layer that’s keeping gases like hydrogen cyanide and water frozen, while the more volatile carbon monoxide outgasses and appears more abundant relative to the other gases that make up comets.

2I/Borisov is not the first comet to stump scientists with its inexplicably high concentration of CO. Discovered in 2016, a comet known as C/2016 R2 (PanSTARRS), was even more striking, with CO levels estimated to be dozens of times higher than Borisov’s.

Scientists determined that R2/PanSTARRS came from the Oort Cloud, a spherical shell of comets and other objects enveloping the solar system. But today’s finding raises the possibility that this mysterious comet may have originated elsewhere. Could R2/PanSTARRS and other presumed solar system comets with unusual chemistries have come from other star systems?

“It’s a provocative idea,” Milam says. “The question that will help comet observers sort this out is whether every interstellar object is going to be unusually high in CO? Will they all have this unique chemistry, or is there going to be a distribution of CO like we see in our own solar system?”

The only other known interstellar object astronomers have ever observed was 'Oumuamua. When it was discovered in October 2017 — the first interstellar object ever identified — 'Oumuamua gripped scientists and the public. But this space rock was already on its way out of the solar system, so it didn’t reveal chemical details, nor settle questions about whether it was a comet, asteroid, or something else.

With increasingly advanced technologies and more large telescopes coming online in coming years, scientists expect to see more Borisovs and 'Oumuamuas. Though these aliens will remain a relatively rare sight, Cordiner warns.

“Our solar system is so tiny compared to the distances between star systems. For an interstellar comet to come in and hit the bullseye like Borisov did is incredible,” he says.

ALMA is a partnership of ESO (representing its Member States), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (South Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

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