3I/ATLAS: Interstellar comet has water unlike any in our solar system

The interstellar comet 3I/ATLAS contains water and carbon molecules at levels never before seen in our solar system. This suggests that it formed around an alien star radically different from and much older than the sun.

Astronomers have been tracking 3I/ATLAS since it entered our solar system last year – and it is weird. It appears to be packed with far more carbon dioxide and water than almost any other comet we have seen, and early estimates put its age at 8 billion years – almost twice as old as the sun.

Now, Martin Cordiner at NASA’s Goddard Space Flight Center in Maryland and his colleagues have found that its levels of deuterium – a form of hydrogen with an extra neutron – are at least 10 times higher than in any comet we have seen before.

Deuterium naturally exists in small amounts in Earth’s oceans, but the levels in 3I/ATLAS are more than 40 times higher. “3I/ATLAS continues to astonish us with what it reveals about the similarities and differences of its host system compared with our own solar system,” says Cordiner. He and the team used the James Webb Space Telescope to make the observations.

“It’s really exceptional,” says Paul Hartogh at the Max Planck Institute for Solar System Research in Germany. “This deuterium-to-hydrogen ratio in water is extremely unusual, and nobody would have expected this.”

Such high levels of deuterium are typically seen in only the coldest regions of the Milky Way, says Ewine van Dishoeck at Leiden Observatory in the Netherlands. “That means it’s probably in the very outer part of the disc around whatever star it was circling, and that makes it also easier to kick it out,” says Dishoeck.

Cordiner and his colleagues also found relatively low levels of carbon-13 – a form of carbon with an extra neutron that is typically produced after stars have exploded in a supernova. Low levels of carbon-13, which have also been found in young star-forming clouds, point to 3I/ATLAS forming at a time in the galaxy’s history when there weren’t as many polluting supernovae. This suggests the comet must have been formed around a star system around 10 billion to 12 billion years old, more than twice as old as the sun, says Cordiner.

However, Dishoeck says that the precision we have for the carbon levels means we can’t be certain about its age.