More evidence that the interstellar comet 3I/ATLAS is much older than our solar system has come to light, along with clues that it formed on the outskirts of the protoplanetary disk belonging to its parent star long ago.
Earlier this year, researchers led by Martin Cordiner of NASA's Goddard Space Flight Center revealed that data from the James Webb Space Telescope (JWST) suggested that 3I/ATLAS is between 10 and 12 billion years old, based on the ratios of its carbon and deuterium isotopes. This would make it more than twice the age of our 4.6-billion-year-old solar system. Now, new results from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the European Southern Observatory's Very Large Telescope support the JWST observations of carbon isotopes, and also introduce measurements of nitrogen isotopes that arrive at very interesting conclusions.
Isotopes are versions of atomic elements with different numbers of neutrons. For example, carbon-12 contains six protons and six neutrons, while carbon-13 contains six protons and seven neutrons. Meanwhile nitrogen-14 has seven protons and neutrons each, while nitrogen-15 has seven protons and eight neutrons.
These isotopes can form through subtly different processes, at different times and in different locations in the galaxy. The ratio of these isotopes in the gases released by comet 3I/ATLAS into its coma and tail as it neared the sun and grew warmer can therefore tell us much about its origin and history.
Consequently, interstellar objects such as 3I/ATLAS "are sort of fossils from a planetary formation process that happened very far away, but we get the chance to study from much closer," said astronomer Cyrielle Opitom of the University of Edinburgh in a statement.
Opitom led the team who got the chance to observe 3I/ATLAS with the VLT. They found that the ratio of carbon-12 to carbon-13 is higher than is found in comets in our solar system or indeed in the local interstellar medium. Carbon-13 is produced in greater abundances than carbon-12 over time, typically in red giant stars, so for there to be far more carbon-12 than carbon-13 tells us that 3I/ATLAS was born long ago before carbon-13 had a chance to build in abundance across the galaxy. This finding supports the JWST carbon isotope measurements.
Additionally Opitom's team, which was co-led by Jean Manfroid and Damien Hutsemékers of the University of Liège in Belgium, measured a ratio of nitrogen-14 to nitrogen-15 in 3I/ATLAS that is more than twice as large as the value measured in comets native to our solar system. In fact, the ratio is typical of that found on the outer edge of planet-forming discs around young stars, implying 3I/ATLAS formed a long way out from its parent star, perhaps in the equivalent of its Kuiper belt.
"Unlike comets from our solar system, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios," said team-member Aravind Krishnakumar, who is also of the University of Liège.
The results give us clues to how 3I/ATLAS found itself wandering the space lanes alone for billions of years. Models indicate that migrating giant planets can kick small bodies into interstellar space, but the location of 3I/ATLAS's birth far from that planetary action means that it is quite possible instead that it was snatched from its parent star by the gravity of a passing star and was subsequently hurled into deep space.
The JWST had also previously shown that 3I/ATLAS is rich in carbon monoxide and carbon dioxide relative to water, and also contains unexpectedly high abundances of nickel and iron and a very high abundance of methanol relative to hydrogen cyanide – all of which tells us that 3I/ATLAS formed in an environment with conditions and chemistry notably alien to our own solar system.
Unfortunately similar measurements were not possible with the other two known interstellar objects – 1I/'Oumuamua was not seen to outgas, while 2I/Borisov was too faint. However, 3I/ATLAS is a tantalizing indication that studies of more interstellar objects caught entering our solar system will be able to teach us about planet-forming conditions across both space and time in our Milky Way galaxy.
"3I/ATLAS is a really exciting opportunity to probe the composition of another planetary system, one that formed long before our Sun and solar system even existed," concluded Rosemary Dorsey, an astronomer from the University of Helsinki in Finland.
The findings were published on July 6 in the journal Nature Astronomy.
