Could the chemical debris of a shattered world hold clues to life elsewhere in the universe? Astronomers studying the white dwarf WD 1647+375 have discovered the biggest quantity of nitrogen ever seen in the remains of such a star corpse a definite hallmark of an icy, volatile-rich body like Pluto.
The discovery, spearheaded by University of Warwick’s Snehalata Sahu, employed the Hubble Space Telescope’s Cosmic Origins Spectrograph to perform far-ultraviolet spectroscopy, a technique sensitive to volatile elements like carbon, sulfur, oxygen, and nitrogen. Although atmospheres rich in hydrogen or helium are typical in the majority of white dwarfs, the spectrum of WD 1647+375 revealed the surprising presence of volatiles, with nitrogen contributing about 5% of the accreted mass and oxygen by 84% over rocky-world predictions. These findings suggest a water-abundant composition, with a water-to-rock ratio of around 2.5.
Such volatile-rich signals are not frequent. When stars evolve into white dwarfs, gravitational upheaval has a tendency to eject distant frozen bodies out of their systems. Discovering one being consumed is therefore uncommon. “We think that the white dwarf accreted fragments of the crust and mantle of a dwarf planet,” Sahu said. “We know that Pluto’s surface is covered with nitrogen ices.” The deduced 64% water ice and abundance in nitrogen are akin to the outer crusts of Kuiper Belt objects, the icy remnants external to Neptune thought to have delivered water to our planet in ancient times.
Hubble observations, captured at two epochs, were required to be painstakingly calibrated. The COS instrument resolving power R = 16,000 allowed the researchers to separate photospheric lines from contamination due to the interstellar medium. Airglow from Earth’s atmosphere was stripped using specialized templates to ensure accurate oxygen abundance measurement. These enhancements allowed for confident detection of the volatile elements, overcoming the challenge presented by feeble ultraviolet signals from a 78.5 parsecs distant star.
Observations indicate WD 1647+375 has been accreting this icy trash for a minimum of 13 years at 200,000 kilograms per second approximately the weight of an adult blue whale falling on the star every second. Even with this optimistic timescale, the parent object would have needed to be a minimum of 3 kilometers in diameter, though prolonged accretion periods may imply a diameter of around 50 kilometers and mass on the order of a quintillion kilograms.
The highly volatile-rich nature of the debris makes WD 1647+375 a twin of our own Kuiper Belt, where objects preserve the primitive ices of the early solar nebula. Co-author of the study Boris T. Gänsicke noted that the nitrogen-rich composition and elevated ice content are characteristic of a Pluto-like dwarf planet fragment instead of an ordinary comet. These bodies, with their geologically complex frameworks, can trap volatiles within crustal and mantle reservoirs, not releasing them until they are disturbed.
From an engineering perspective, the detection underscores the worth of ultraviolet capability in exoplanetary science. In optical wavelengths, WD 1647+375 is no different; far-UV spectroscopy alone can reveal nitrogen’s spectral lines. The method, in the core of searches for ingredients of life in future reaches back to ways used to discover atmospheric biosignatures on transiting exoplanets. Transmission spectroscopy of planets and stellar atmosphere “detector,” here serve similar purposes.
The effects extend to planetary system formation. Where the icy fragment was actually created in WD 1647+375’s natal system or captured from the interstellar medium remains a mystery. In either case, the finding strengthens that volatile-rich planetesimals, capable of transporting water and organics, exist beyond our solar system. With improved observational technology, the combination of ultraviolet spectroscopy with infrared capabilities from telescopes like the James Webb Space Telescope may allow us to detect molecular volatiles like water vapor and carbonates, furthering our understanding of how these kinds of bodies contribute to habitability in distant worlds.
