The team used ultraviolet spectroscopy from the Hubble Space Telescope to analyze WD 1647+375, a white dwarf star showing unusual traces of volatile elements. While most white dwarfs display atmospheric contamination by rocky debris, this star revealed carbon, nitrogen, sulphur, and oxygen - a mix characteristic of frozen bodies similar to comets or dwarf planets.
Lead author Snehalata Sahu of Warwick explained, "It is not unusual for white dwarfs to show signatures of calcium, iron and other metal from the material they are accreting. This material comes from planets and asteroids that come too close to the star and are shredded and accreted. Analysing the chemical make-up of this material gives us a window into how planetesimals outside the Solar System are composed."
Sahu described white dwarfs as "cosmic crime scenes," where infalling debris leaves chemical fingerprints in the stellar atmosphere. Typically those fingerprints reveal rocky remains, but volatile-rich debris has only been confirmed in a few cases.
This time, one chemical stood out: nitrogen. The debris being consumed contained around 5% nitrogen by mass - the highest level ever recorded in such material. Combined with unusually high oxygen, the evidence pointed to a frozen, water-dominated body, about 64% ice by composition.
The star has been steadily absorbing this material for at least 13 years at a rate of 200,000 kilograms per second - equivalent to a blue whale every second. Calculations suggest the disrupted body was at least 3 km wide, and possibly as large as 50 km across with a mass of up to a quintillion kilograms.
Professor Boris T. Gansicke, co-author at Warwick, said, "The volatile-rich nature of WD 1647+375 makes it like Kuiper-belt objects in our solar system... We think that the planetesimal being absorbed by the star is most likely a fragment of a dwarf planet like Pluto. This is based on its nitrogen-rich composition, the high predicted mass and the high ice-to-rock ratio of 2.5, which is more than typical KBOs and likely originates from the crust or mantle of a Pluto-like planet."
This is the first definitive observation of a hydrogen-atmosphere white dwarf accreting an icy body. Whether the fragment originated in the star's native planetary system or was captured as an interstellar wanderer remains unknown. Either scenario confirms that volatile-rich objects - potential water carriers - exist well beyond the Solar System.
The study also demonstrates the unique capability of ultraviolet spectroscopy to detect volatile elements such as nitrogen, oxygen, carbon and sulphur, reinforcing its role in future searches for life-building compounds around distant stars.
Research Report:Discovery of an icy and nitrogen-rich extra-solar planetesimal
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