Webb Telescope spots interstellar comet 3I/ATLAS with unusual carbon dioxide abundance

Webb Telescope Unveils Interstellar Visitor 3I/ATLAS: A Comet with Surprising Chemistry

The James Webb Space Telescope (JWST) has, for the first time, provided us with a detailed infrared glimpse of the enigmatic comet 3I/ATLAS, a rare visitor from beyond our solar system. This cosmic traveler briefly graced our celestial neighborhood on August 6th, captured by Webb's sensitive infrared cameras and its Near-Infrared Spectrograph (NIRSpec). The discovery of 3I/ATLAS itself dates back to July 1, 2025, thanks to the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey. This makes it only the third known interstellar object to have ventured into our solar system, following in the footsteps of the asteroid 1I/'Oumuamua in 2017 and comet 2I/Borisov in 2019.

Unraveling the Comet's Secrets

Before Webb's groundbreaking observations, the Hubble Space Telescope and the SPHEREx mission had already begun studying 3I/ATLAS. The primary objective was to meticulously characterize its properties: size, physical composition, and crucially, its chemical makeup. Astronomers involved in the JWST observations highlight that studying such comets from other stellar systems offers an unparalleled window into the conditions present during their formation. This ancient stardust can then be compared to the environment of our own solar system approximately 4.6 billion years ago, during the birth of planets, comets, and asteroids.

A Gaseous Spectacle and Unexpected Clues

As comets approach the Sun, they warm up, causing their frozen constituents to sublimate—transforming directly from solid to gas. This process creates the iconic cometary tail and a surrounding cloud of gas and dust known as a coma. Webb's spectrograph was instrumental in detecting several key compounds within 3I/ATLAS's coma, including carbon dioxide, water, water ice, carbon monoxide, and carbonyl sulfide. However, one finding stood out as particularly surprising: the unprecedented ratio of carbon dioxide to water observed in its composition.

Implications for Formation Environments

This unusual carbon dioxide-to-water ratio offers tantalizing clues about where and how 3I/ATLAS might have formed. One compelling hypothesis suggests that the comet's interior is exceptionally rich in carbon dioxide. This could imply that the ices within 3I/ATLAS experienced significantly more radiation exposure than those found in comets originating within our solar system. Another intriguing possibility, not ruled out by researchers, is that 3I/ATLAS coalesced at a specific location within its home system's protoplanetary disk, known as the 'snow line' for carbon dioxide. This is the zone around a young star where temperatures plummet low enough for carbon dioxide to transition from a gas to a solid ice.

A Glimpse into Ancient Cosmic History

Adding to the intrigue, astronomers also noted a relatively low abundance of water vapor. This could indicate the presence of something within 3I/ATLAS that impedes heat from reaching its icy nucleus, thereby slowing the sublimation of water ice compared to other frozen gases like carbon dioxide and carbon monoxide. Preliminary analyses of 3I/ATLAS's trajectory through our solar system suggest a remarkable age, potentially around 7 billion years, making it potentially the oldest comet ever observed. This would mean it predates our own solar system by a staggering 3 billion years!

Originating from the Galactic Deep Past

The steep trajectory of 3I/ATLAS is a strong indicator that it hails from the Milky Way's 'thick disk.' This region of our galaxy is populated by stars that are considerably older than those in the 'thin disk,' where our Sun was born. The findings from these groundbreaking JWST observations have been detailed in a preprint published on Zenodo.org, offering a precious glimpse into the ancient epochs of star system formation.