Interstellar comet shows chemical signature unlike anything in our solar system
Astronomers studying a rare visitor from another star have found its composition is strikingly different from every comet and asteroid in our own neighbourhood.
A comet born around a distant star billions of years ago has revealed a chemical makeup that sets it apart from every object astronomers have studied within our solar system.
The object, known as 3I/Atlas, is the third interstellar object ever discovered—physical debris from beyond our solar system that has drifted across space to reach us. Three recently published studies have now shed light on its origins and composition, using observations from some of humanity's most powerful telescopes.
A visitor from afar
Interstellar objects (ISOs) are asteroids or comets that originated outside the solar system. The first, 1I/ʻOumuamua, was detected in 2017. The second, 2I/Borisov, followed in 2019. 3I/Atlas, discovered almost exactly one year before the reporting of these studies, appeared to have been born in a cold environment roughly 12 billion years ago.
What makes these objects so valuable to astronomers is that they are literal samples of distant planetary systems—delivered by the gravitational currents of the galaxy into a region where we can observe them with our best instruments.
Chemical fingerprints in the light
When 3I/Atlas passed through the inner solar system, the Sun's heat caused frozen ices within the comet to sublimate—turning directly from solid to gas. This process released a spectacular coma, the bright atmosphere surrounding the comet, and a luminous tail visible to telescopes.
The released molecules leave signatures in the light they reflect and emit. By splitting this light into its constituent wavelengths using spectroscopy, astronomers can identify the chemical ingredients of a comet with precision.
In 3I/Atlas, researchers found a cocktail of water, carbon dioxide and monoxide, methane, cyanides, sulphides, and even free-floating iron and nickel atoms.
The outsider's edge
Each of these ingredients has been detected in comets within our own solar system before. But their relative abundances tell a different story. 3I/Atlas carries unusually high levels of carbon dioxide and surprisingly low levels of ammonia—a fingerprint that marks it as an alien object.
Astronomers have also measured the isotope ratios of the comet—the proportions of different forms of the same chemical elements. With a bright comet like 3I/Atlas and access to the most sensitive telescopes available, including the James Webb Space Telescope, these subtle spectral signatures can be distinguished and measured, revealing the comet's elemental composition in finer detail.
These findings, published in peer-reviewed studies including one in the journal Nature, represent a rare opportunity to study material forged in a distant planetary system, now available for examination from Earth's vantage point.