3I/ATLAS: An interstellar comet has water unlike any other 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 and much older than the Sun.

Astronomers have been watching 3I/ATLAS since it entered our solar system last year — and it’s strange. It appears to contain far more carbon dioxide and water than almost any other comet we’ve seen, and early estimates put it at 8 billion years old—almost twice the age of the Sun.

Now, Martin Cordiner at NASA’s Goddard Space Flight Center in Maryland and his colleagues 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’ve seen before.

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

“It’s really special,” he says Paul Hartogh at the Max Planck Institute for Solar System Research in Germany. “This ratio of deuterium to hydrogen in water is extremely unusual and nobody would expect it.”

Such high levels of deuterium are typically only seen in the coldest regions of the Milky Way, he says Ewin van Dishoeck at the Leiden Observatory in the Netherlands. “That means it’s probably in the very outer part of the disk around whatever star it’s been orbiting, and that also makes it easier to kick 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 usually created after stars explode in a supernova. The low levels of carbon-13, also found in young star-forming clouds, point to the formation of 3I/ATLAS at a time in the galaxy’s history when there were not as many polluting supernovae. That suggests the comet must have formed around a star system about 10 billion to 12 billion years old, more than twice the age of the Sun, Cordiner says.

However, Dishoeck says the precision we have for the carbon levels means we can’t be sure of its age.

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