Free Astronomy Magazine November-December 2025

25 NOVEMBER-DECEMBER 2025 ASTRO PUBLISHING sion of the Kuiper Belt, an icy ring of debris that encircles our Solar System. The results were published in the Monthly Notices of the Royal Astronomical Society . An interna- tional team of astronomers were able to determine this carnage by analysing the chemical composi- tion of the doomed object as its pieces fell onto the white dwarf. In particular, they detected “vola- tiles” (substances with low boiling points) including carbon, sulphur, nitrogen, and a high oxygen con- tent that suggests the strong pres- ence of water. “We were sur- prised,” said Snehalata Sahu of the University of Warwick in the United Kingdom. Sahu led the data analysis of a Hubble survey of white dwarfs. “We did not expect to find water or other icy content. This is because the comets and Kuiper Belt-like objects are thrown out of their planetary systems early, as their stars evolve into white dwarfs. But here, we are de- tecting this very volatile-rich ma- terial. This is surprising for as- tronomers studying white dwarfs as well as exoplanets, planets out- side our Solar System.” Using Hub- ble’s Cosmic Origins Spectrograph, the team found that the fragments were composed of nearly two thirds water ice. The fact that they detected so much ice meant that the pieces were part of a very mas- sive object that formed far out in the star system’s icy Kuiper Belt analogue. Using Hubble data, sci- entists calculated that the object was bigger than typical comets and may be a fragment of an exo- Pluto. They also detected a large fraction of nitrogen − the highest ever detected in white dwarf de- bris systems. “We know that Pluto’s surface is covered with nitrogen ices,” said Sahu. “We think that the white dwarf accreted fragments of the crust and mantle of a dwarf planet.” Accretion of these vola- tile-rich objects by white dwarfs is very difficult to detect in visible light. These volatile elements can only be detected with Hubble’s unique ultraviolet light sensitivity. In optical light, the white dwarf would appear ordinary. About 260 light-years away, the white dwarf is a relatively close cosmic neighbor. In the past, when it was a Sun-like star, it would have been expected to host planets and an analogue to our Kuiper Belt. Bil- lions of years from now, when our Sun burns out and collapses to a white dwarf, Kuiper Belt objects might be attracted by the immense gravity of the stellar remnant. “These planetesimals will then be disrupted and accreted,” said Sahu. “If an alien observer looks into our Solar System in the far future, they might see the same kind of remains we see today around this white dwarf.” The team hopes to use the NASA/ESA/CSA James Webb Space Telescope to detect molecular fea- tures of volatiles such as water vapour and carbonates by observing this white dwarf in infrared light. By further studying white dwarfs, scientists can better understand the frequency and composition of these volatile-rich accretion events. Sahu is also following the recent discovery of the interstellar comet 3I/ATLAS. She is eager to learn its chemical composition, especially its fraction of water. “These types of studies will help us learn more about planet for- mation. They can also help us under- stand how water is delivered to rocky planets,” said Sahu. Boris Gän- sicke, of the University of Warwick and a visitor at Spain’s Instituto de Astrofisica de Canarias, was the prin- cipal investigator of the Hubble pro- gram that led to this discovery. “We observed over 500 white dwarfs with Hubble. We’ve already learned so much about the building blocks and fragments of planets, but I’ve been absolutely thrilled that we now identified a system that resem- bles the objects in the frigid outer edges of our solar system,” said Gänsicke. “Measuring the composi- tion of an exo-Pluto is an important contribution toward our under- standing of the formation and evo- lution of these bodies.” !