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Hubble sees white dwarf eating piece of Pluto-like object: a new study reports the accretion of an icy extrasolar planetesimal on to WD 1647+375

In our nearby stellar neighbourhood, a burned-out star is snacking on a fragment of a Pluto-like object. With its unique ultraviolet capability, only the NASA/ESA Hubble Space Telescope could identify that this meal is taking place.

The stellar remnant is a white dwarf about half the mass of our Sun, but that is densely packed into a body about the size of Earth. Scientists think the dwarf’s immense gravity pulled in and tore apart an icy Pluto analogue from the system’s own version of the Kuiper Belt, an icy ring of debris that encircles our Solar System. The findings were reported on 18 September 2025 in the Monthly Notices of the Royal Astronomical Society.

An international team of astronomers were able to determine this carnage by analysing the chemical composition of the doomed object as its pieces fell onto the white dwarf. In particular, they detected “volatiles” (substances with low boiling points) including carbon, sulphur, nitrogen, and a high oxygen content that suggests the strong presence of water.

“We were surprised,” 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 detecting this very volatile-rich material. This is surprising for astronomers studying white dwarfs as well as exoplanets, planets outside our Solar System.”

Only with Hubble

Using Hubble’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 massive object that formed far out in the star system’s icy Kuiper Belt analogue. Using Hubble data, scientists 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 debris 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 volatile-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.

Like seeing our Sun in the future

Billions of years from now, when our Sun burns out and collapses to a white dwarf, Kuiper Belt objects will be pulled in by the stellar remnant’s immense gravity.

“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 features 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 formation. They can also help us understand how water is delivered to rocky planets,” said Sahu.

Boris Gänsicke, of the University of Warwick and a visitor at Spain’s Instituto de Astrofisica de Canarias, was the principal investigator of the Hubble program 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 resembles the objects in the frigid outer edges of our solar system,” said Gänsicke. “Measuring the composition of an exo-Pluto is an important contribution toward our understanding of the formation and evolution of these bodies.”

An illustration showing a glowing white object in the upper left corner. This object is encircled by hundreds of thin, concentric, pale-yellow rings on an angle from bottom left to top right. The rings are palest closest to the central, glowing white object. A curving trail of gray, rock-like fragments marches across the right side, through the thin rings and joins the rings at far right. The eight largest fragments of varying sizes appear in the foreground. These objects have white, comet-like tails streaking away from the glowing white object in the rings’ center. The curving trail of fragments bends toward the glowing white object. At the bottom left corner is the label Artist’s Concept. — Thanks to the Hubble Space Telescope, a new study reports the accretion of an icy extrasolar planetesimal on to white dwarf WD 1647+375. This artist’s concept shows a white dwarf surrounded by a large debris disc. Debris from pieces of a captured, Pluto-like object is falling onto the white dwarf.
Credit: T. Pyle (Caltech, NASA’s Jet Propulsion Laboratory)

Bibliographic information:

Snehalata Sahu, Boris T Gänsicke, Jamie T Williams, Detlev G Koester, Jay Farihi, Steven J Desch, Nicola Pietro Gentile Fusillo, Dimitri Veras, Sean N Raymond, Maria Teresa Belmonte, Discovery of an icy and nitrogen-rich extrasolar planetesimal, Monthly Notices of the Royal Astronomical Society, Volume 543, Issue 1, October 2025, Pages 223–232, https://doi.org/10.1093/mnras/staf1424

Press release from ESA Hubble

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WD 0525+526: Hubble uncovers rare white dwarf merger remnant

An international team of astronomers using the NASA/ESA Hubble Space Telescope have discovered a stellar rarity: an ultra-massive white dwarf that formed when a white dwarf merged with another star, rather than through the evolution of a single star. This discovery, which was made possible by Hubble’s sensitive ultraviolet observations, suggests that these rare white dwarfs may be more common than previously suspected.

A white dwarf is the end state for a star that is not massive enough to explode as a core-collapse supernova. The transition to a white dwarf begins when a star exhausts the supply of hydrogen in its core. The changes in and around the star’s core cause the star to expel its outer layers in a massive stellar sigh, revealing the star’s dense, Earth-sized core, which evolves into a white dwarf. The cores of white dwarfs are mostly composed of either carbon and oxygen or oxygen and neon, depending on the mass of the progenitor star. The Sun will become a white dwarf in about 5 billion years.

White dwarfs can theoretically have masses up to about 1.4 times the mass of the Sun, but white dwarfs that are more massive than the Sun are rare. These objects, which astronomers call ultra-massive white dwarfs, can form either through the evolution of a single massive star or through the merger of a white dwarf with another star.

Recently, astronomers used Hubble’s Cosmic Origins Spectrograph to investigate one such ultra-massive white dwarf, WD 0525+526. WD 0525+526 is just 128 light-years away and is 20% more massive than the Sun.

In visible light, the spectrum of WD 0525+526’s atmosphere resembled that of a typical white dwarf. However, Hubble’s ultraviolet spectrum revealed something unusual: evidence of carbon in the white dwarf’s atmosphere.

White dwarfs that form through the evolution of a single star have atmospheres composed of hydrogen and helium. These thick atmospheres blanket the carbon–oxygen or oxygen–neon surface of the white dwarf, usually preventing these elements from appearing in its spectrum.

When carbon appears in the spectrum of a white dwarf, it can signal a more violent origin than the typical single-star scenario: the collision of two white dwarfs, or of a white dwarf and a subgiant star. Such a collision can burn away the hydrogen and helium atmospheres of the colliding stars, leaving behind a scant layer of hydrogen and helium around the merger remnant that allows carbon from the white dwarf’s core to float upward, where it can be detected.

“It’s a discovery that underlines that things may be different from what they appear to us at first glance,” said the principal investigator of the Hubble programme, Boris Gaensicke, of the University of Warwick in the United Kingdom. “Until now, this appeared as a normal white dwarf, but Hubble’s ultraviolet eyes revealed that it had a very different history from what we would have guessed. It’s like asking a person you think you know well a different kind of question.”

This discovery marks the first time that a white dwarf born from colliding stars has been identified by its ultraviolet spectrum. Prior to this study, six white dwarf merger products were discovered via carbon lines in their visible-light spectra. All seven of these are part of a larger group that were found to be bluer than expected for their masses and ages from a study with ESA’s Gaia mission in 2019, with the evidence of mergers providing new insights into their formation history.

WD 0525+526 is remarkable even within the small group of white dwarfs known to be the product of merging stars. With a temperature of almost 21 000 kelvins and a mass of 1.2 solar masses, WD 0525+526 is hotter and more massive than the other white dwarfs in this group.

WD 0525+526’s extreme temperature posed something of a mystery for the team. For cooler white dwarfs, such as the six previously discovered merger products, a process called convection can mix carbon into the thin hydrogen–helium atmosphere. WD 0525+526 is too hot for convection to take place, however. Instead, the team determined that a more subtle process called semi-convection brings a small amount of carbon up into WD 0525+526’s atmosphere. WD 0525+526 has the smallest amount of atmospheric carbon of any white dwarf known to result from a merger, about 100 000 times less than other merger remnants.

The high temperature and low carbon abundance mean that identifying this white dwarf as the product of a merger would have been impossible without Hubble’s sensitivity to ultraviolet light; spectral lines from elements heavier than helium, like carbon, become fainter at visible wavelengths for hotter white dwarfs, but these spectral signals remain bright in the ultraviolet, where Hubble is uniquely positioned to spot them.

“Hubble’s Cosmic Origins Spectrograph is the only instrument that can obtain the superb quality ultraviolet spectroscopy that was required to detect the carbon in the atmosphere of this white dwarf,” said study lead Snehalata Sahu from the University of Warwick.

Because WD 0525+526’s unusual origin was revealed only once astronomers glimpsed its ultraviolet spectrum, it’s likely that other seemingly ‘normal’ white dwarfs are actually the result of cosmic collisions — a possibility that the team is excited to explore in the future.

“We would like to extend our research on this topic by exploring how common carbon white dwarfs are, and how many stellar mergers are hiding among the normal white dwarf family,” said study co-lead Antoine Bedrad from the University of Warwick. “That will be an important contribution to our understanding of white dwarf binaries, and the pathways to supernova explosions.”

This illustration depicts a white dwarf star siphoning material off of a red giant star. The white dwarf has a long tail behind it and an orange shockwave in front of it as it flies through the extended atmosphere of the red giant, shown as a red glow. A broad stream of red gas is being pulled up from the red giant’s surface to the white dwarf. — This image is an illustration of a merging white dwarf remnant.
An international team of astronomers using the NASA/ESA Hubble Space Telescope have discovered a stellar rarity: an ultra-massive white dwarf that formed when a white dwarf merged with another star, rather than through the evolution of a single star. This discovery, which was made possible by Hubble’s sensitive ultraviolet observations, suggests that these rare white dwarfs may be more common than previously suspected.
Credit: NASA, ESA, R. Crawford (STScI)

Bibliographic information:

Sahu, S., Bédard, A., Gänsicke, B.T. et al. A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon, Nat Astron (2025), DOI: https://doi.org/10.1038/s41550-025-02590-y

Press release from ESA Hubble.

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