'Alien comet' visitor has weird composition

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NRAO/AUI/NSF, S. Dagnello

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Artwork: The interstellar visitor was detected in our Solar System last year

The first known comet to visit us from another star system has an unusual make-up, according to new research.

The interstellar comet 2I/Borisov was detected in our Solar System last year.

This mysterious visitor from the depths of space has provided astronomers with an unprecedented opportunity to study how different it is from comets that have been formed around the Sun.

New data suggests it contains large amounts of carbon monoxide – a possible clue to where it was “born”.

The findings appear in two separate scientific papers published by the journal Nature Astronomy.

In one of the papers, an international team led by Martin Cordiner and Stefanie Milam from the US space agency’s Goddard Space Flight Center in Greenbelt, Maryland, pointed the Atacama Large Millimeter/submillimeter Array (Alma) toward the comet on 15 and 16 December 2019.

Alma consists of 66 radio telescopes on a mountaintop in Chile that observe the sky at sub-millimetre wavelengths.

In the other study, Dennis Bodewits from Auburn University in Alabama and colleagues gathered ultraviolet observations of the interstellar comet using the Hubble Space Telescope and the Neil Gehrels Swift Observatory.

This allowed the researchers to directly observe the chemicals stored inside an object from a distant planetary system.

Comets are made up of gas, ice, and dust; they form swirling in the disc of material that surrounds a star when its planets are being born. They can seed young worlds with the chemical building blocks of biology, and may have brought water to the early Earth.

The teams identified two molecules in the gas ejected by the comet: hydrogen cyanide (HCN) and carbon monoxide (CO).

HCN has been detected previously in 2I/Borisov, and is present at similar amounts to those found in Solar System comets.

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Alma / NRAO / AUI / NSF /

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Two compounds were detected: Hydrogen cyanide (HCN) is common in comets from our Solar System, but carbon monoxide (CO) was found at unusually high concentrations

However, they were surprised to see large amounts of CO. The researchers using Alma for their observations estimated that 2I/Borisov’s CO concentration was between nine and 26 times higher than that of an average Solar System comet.

“This is the first time we’ve ever looked inside a comet from outside our Solar System,” said Dr Cordiner, “and it is dramatically different from most other comets we’ve seen before.”

Carbon monoxide is one of the most common molecules in space and is found inside most comets. But there’s huge variation in the concentration of CO in these icy objects and no-one quite knows why.

Some of this might be related to where in a solar system a comet was formed. It may also be related to how often a comet’s orbit brings it closer to its star and leads it to release its more easily evaporated ices.

Nevertheless, said Dr Cordiner, “if the gases we observed reflect the composition of 2I/Borisov’s birthplace, then it shows that it may have formed in a different way than our own Solar System comets, in an extremely cold, outer region of a distant planetary system.”

He added: “This region can be compared to the cold region of icy bodies beyond Neptune, called the Kuiper Belt.”

Stefanie Milam commented: “The comet must have formed from material very rich in CO ice, which is only present at the lowest temperatures found in space, below -420F (-250C).”

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Eso / M. Kornmesser

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Artwork: ‘Oumuamua, detected in 2017, was the first known visitor from outside the Solar System

The teams can only speculate about the kind of star system where 2I/Borisov originated.

“Most of the proto-planetary discs (the rotating mass of dense gas and dust from which planetary systems later form) observed with Alma are around younger versions of low-mass stars like the Sun,” said Martin Cordiner.

“Many of these discs extend well beyond the region where our own comets are believed to have formed, and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger discs.”

In their paper, Dr Bodewits’ team write: “[2I/Borisov’s] host system must be chemically distinct from our own to form a CO/H2O (carbon monoxide/water) ratio greater than 1, a ratio not observed in our Solar System even among dynamically new comets.”

This team suggests that the comet could have originated around a red dwarf star, the most common type Milky Way. “These stars have exactly the low temperatures and luminosities where a comet could form with the type of composition found in comet Borisov,” said Dr Bodewits.

Based on its high speed (33km/s; 21 miles/s), astronomers suspect that 2I/Borisov was ejected from its host system, probably when it interacted with a passing star or giant planet.

It then spent millions or billions of years on a lonely journey through interstellar space before its discovery on 30 August 2019 by amateur astronomer Gennady Borisov.

Astronomers continue to study the interstellar interloper, and recent observations of the comet’s behaviour suggested it might be breaking up.

Despite the fact that visits from interstellar objects have been predicted for years, 2I/Borisov is only the second such body to have been detected in our Solar System.

The first, known as ʻOumuamua, was discovered in October 2017, at which point it was already on its way out of our cosmic neighbourhood. While initially classified as a comet, it showed no signs of the kinds of outbursts of gas and dust characteristic of these objects (and observed in 2I/Borisov).

A study published earlier this month in Nature Astronomy suggested ʻOumuamua, which has an odd, highly elongated shape like a cigar, could be a shard from a planet ripped apart by its host star’s gravity.

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