Astronomers have revealed new details about the make-up and age of a visiting comet that was born around a distant star. They conclude that the composition of 3I/Atlas is strikingly different from any object found in our solar system.
A trio of recently published studies shed light on the origins of this exotic comet. 3I/Atlas appears to have been born in a cold environment, possibly around 12 billion years ago.
The comet is an interstellar object (ISO), meaning an asteroid or comet that originated outside the solar system. It is the third such object found, after 1I/ʻOumuamua and 2I/Borisov. It was discovered almost exactly one year ago, travelling inbound on a trajectory that has taken it through the inner solar system and out the other side.
These distant origins make ISOs incredibly interesting to astronomers because they are physical pieces of other planetary systems, delivered to us on galactic tides so that they can be studied from the comfort of our own solar system.
As a comet, 3I/Atlas contained frozen ices that “sublimated” – turned directly from a solid to a gas. These escaped (outgassed) from the comet as it was heated by the Sun, creating a visually spectacular coma (bright atmosphere surrounding the object) and tail.
A comet has no internal light source of its own, instead dust in its coma reflects sunlight and its volatiles (chemicals that readily vaporise or sublimate) display fluorescence.
Eso/L. Calçada
But this is no mere light show: each fluorescing molecule leaves a spectral fingerprint on the light reaching our telescopes. These fingerprints allow us to identify the chemicals contained within the comet.
They are revealed by splitting the light into its constituent wavelengths with a technique called spectroscopy, allowing astronomers to deduce the comet’s chemical composition.
Unique chemical cocktail
This revealed in 3I/Atlas a cocktail of water, carbon dioxide and monoxide, methane, cyanides, sulphides, and even free-floating iron and nickel atoms. These ingredients are actually quite expected, as each of them are regularly detected in our own domestic solar system comets. However, their relative abundances are different in 3I/Atlas: high levels of carbon dioxide (CO₂) and low levels of ammonia (NH₃) mark it as an outsider.
Additionally, molecules made up from atoms with different isotopes (distinct forms of the same chemical element) also have subtly different spectral signatures. With a bright comet such as 3I/Atlas, and with our largest and most sensitive telescopes, these signatures can be distinguished, allowing astronomers measure the comet’s isotope ratios.
One of the new studies, published in Nature, uses the spectral signatures of water and carbon dioxide measured with the James Webb Space Telescope to calculate 3I/Atlas’s ratio of two different isotopes of carbon, ¹²C and ¹³C, and its ratio of deuterium (a heavy form of hydrogen) to standard hydrogen – known as the D/H ratio.
Nasa, Esa, CSA, STScI, M.Cordiner (Catholic University of America, GSFC)
These are very exciting results, because the isotope ratios present in an ISO such as 3I/Atlas should match the ratios in the protoplanetary disk in which it formed. This allows us to make detailed inferences about the conditions 3I/Atlas formed in and the star it must have formed around.
3I/Atlas’s water was found to have a D/H ratio of around 1%, significantly higher than all observed solar system comets. These high levels of deuterium are only found in very cold environments, with temperatures of less than 30 Kelvin (-243°C).
In these conditions, normal hydrogen atoms get replaced by heavier deuterium atoms in the water ice that coats tiny dust grains. Over time, these icy dust grains stick together to form comets.
Ancient traveller
3I/Atlas’ ¹²C/¹³C ratio was equally extreme, far above all solar system values. The ratio of these isotopes function like a cosmic clock. In the beginning, the first generation of stars produced carbon with a high ratio of ¹²C/¹³C, but subsequent cycles of star formation and death have lowered it. For 3I/Atlas to have formed with such a high value of ¹²C/¹³C, it must have formed in the very early history of the Milky Way, around 12 billion years ago.
Studies from shortly after its discovery suggested 3I/Atlas was likely to be at least 7 billion years old based on its velocity, meaning the ancient status of 3I/Atlas is now supported by multiple independent lines of evidence.
Though the night sky beyond the limits of our solar system can feel static and unchanging, both the universe and our galaxy do evolve on billion-year timescales.
Nasa
When 3I/Atlas formed, the universe was just a fraction of its present age, and the Milky Way in which it formed was still in the process of assembling itself through violent collisions and mergers with other galaxies.
If the star that 3I/Atlas formed around was the same mass as our Sun, it is likely to have already reached the end of its life, outlived by the ISOs it released shortly after its birth.
Over the next ten years, cutting-edge discovery telescopes such as Nasa’s NEO Surveyor and the Vera C. Rubin Observatory in Chile will increase the number of known ISOs by an order of magnitude. This will provide astronomers with a fossil record of the evolution of planetary systems across galactic history.
