Ever since interstellar comet 3I/ATLAS whizzed into our cosmic neighborhood in July, astronomers have been racing to uncover its characteristics. Now that the powerful James Webb Space Telescope has taken a good look at this icy interloper, it seems to be weirder than anyone imagined.
A preprint submitted to the Astrophysical Journal Letters for peer review on Monday, August 25, describes the first results from JWST’s survey of 3I/ATLAS.
A team of astronomers observed the comet with the telescope’s Near-Infrared Spectroscopic (NIRSpec) instrument to measure the composition of its coma—the cloud of gas and dust that surrounds its nucleus—and determine what drives its activity. Their surprising findings bring 3I/ATLAS’s origin into clearer focus, helping astronomers retrace the comet’s long journey to our solar system.
3I/ATLAS, detected by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope on July 1, is only the third interstellar object ever discovered. These celestial bodies hail from star systems beyond our own. Studying them offers a glimpse of the conditions and processes that shaped these distant systems. Over the past two months, researchers have already uncovered unprecedented details about this latest cosmic visitor.
JWST spies unusual characteristics
Now, JWST has revealed even more of 3I/ATLAS’s distinctive features. Most comets have comas dominated by water, but this one is chock-full of carbon dioxide, according to the study. In fact, the researchers found that its ratio of carbon dioxide to water is among the highest ever observed in any comet. This may indicate that 3I/ATLAS has a nucleus that’s intrinsically rich in carbon dioxide, suggesting it formed in an environment with higher levels of radiation than our solar system.
Alternatively, the carbon dioxide-dominated coma may indicate that 3I/ATLAS formed near the CO2 ice line within the protoplanetary disk that surrounded its parent star, according to the researchers. This is the distance from a young star where the temperature drops low enough for carbon dioxide gas to freeze into ice. What’s more, the lack of water in the coma points to unusual surface properties—or perhaps an insulating crust—that may prevent heat from penetrating the comet’s icy core.
A comet unlike any other
These new findings suggest the comet formed under conditions far different from those in our corner of the galaxy, adding to a growing list of traits that make it unlike any seen before. Prior to this JWST survey, astronomers found evidence to suggest 3I/ATLAS is the oldest interstellar comet ever discovered—potentially older than our solar system. This, coupled with its trajectory, suggests it originated from a relatively old, low-metallicity star system in the Milky Way’s “thick disk”—the part of the galaxy that contains 10% of its total stellar mass.
Astronomers have put forth a wealth of astonishing new information about 3I/ATLAS since its discovery, but this is only the beginning. Experts expect this comet to remain observable through mid-2026, providing ample research opportunities. The more information scientists gather on this interstellar object, the closer they’ll get to unraveling the secrets of its origin.