Astronomers have detected the chemical signature of a nitrogen-rich, icy planetary body being devoured by a distant white dwarf star. The discovery provides the strongest evidence to date that objects similar to those in our own solar system’s Kuiper Belt—which are credited with delivering water and other essential ingredients for life to Earth—exist in other star systems.
The new observations of the white dwarf, known as WD 1647+375, reveal an abundance of volatile elements, including the highest concentration of nitrogen ever seen in the atmospheric debris of one of these stellar remnants. This finding, published in the Monthly Notices of the Royal Astronomical Society, offers a direct look at the bulk composition of an extrasolar planetesimal, suggesting it is a fragment from a water-rich worldlet that was likely similar to Pluto.
A Cosmic Crime Scene Investigation
White dwarfs are the dense, cooling cores of stars that have exhausted their nuclear fuel. Their powerful gravity pulls in and shreds any nearby planets or asteroids, a process called accretion. The material from these disrupted bodies pollutes the star’s normally clean hydrogen or helium atmosphere, leaving behind a unique chemical fingerprint. Researchers act like cosmic detectives, analyzing this atmospheric pollution to determine the composition of the original object.
To investigate WD 1647+375, located approximately 255 light-years from Earth, an international team led by astronomers from the University of Warwick used the Hubble Space Telescope. They employed the telescope’s Cosmic Origins Spectrograph (COS) to study the star in the far-ultraviolet spectrum. This method allows for the direct detection of elements like nitrogen, whose characteristic absorption lines are difficult to capture in visible light but leave a clear signature in ultraviolet spectroscopy.
An Unusual Chemical Fingerprint
While it is common to find rocky elements like calcium and iron in the atmospheres of polluted white dwarfs, the analysis of WD 1647+375 revealed something markedly different. Its atmosphere was heavily enriched with volatile elements, including carbon, nitrogen, and sulfur, which are typically associated with icy bodies rather than dry, rocky ones.
The two most significant findings were the concentrations of nitrogen and oxygen. The analysis showed a nitrogen mass fraction of about 5.1%, the highest abundance ever recorded in the debris consumed by a white dwarf. Furthermore, there was an 84% excess of oxygen beyond what could be explained by minerals alone. This large oxygen surplus is a strong indicator for the presence of a substantial amount of water, in the form of ice, within the original planetesimal.
Portrait of a Distant Planetesimal
The unique elemental cocktail allowed the research team to paint a detailed picture of the object being consumed. The data suggests the planetesimal had a water-to-rock ratio of approximately 2.45, meaning it was extremely water-rich. Its overall composition is highly analogous to the icy bodies found in the Kuiper Belt, the region beyond Neptune in our own solar system.
Based on the high nitrogen content, scientists believe the object is most likely a fragment of a dwarf planet, similar in nature to Pluto, rather than a comet. While the exact size of the parent body is unknown, estimates suggest the fragment itself could have been up to 50 kilometers across. This makes it one of the largest and most compositionally detailed extrasolar objects of its kind ever studied.
The Star’s Voracious Appetite
The observations indicate that this accretion event is not a recent occurrence. The data shows that material from the icy body has been raining down onto the white dwarf’s surface for at least 13 years. The rate of consumption is immense, calculated to be around 200 million grams per second. This is roughly equivalent to the mass of an adult blue whale being consumed by the star every second. Over the duration of the accretion phase, which could last for 100,000 years or more, the star could consume a significant portion of the original dwarf planet.
Implications for Exoplanetary Systems
This study represents the first unambiguous case of a hydrogen-atmosphere white dwarf accreting material from a volatile-rich, icy body. It provides compelling, direct evidence that the kinds of icy planetesimals thought to be responsible for seeding Earth with water and organic molecules are not unique to our solar system. Their presence in other star systems suggests that the building blocks for habitable planets may be common throughout the galaxy.
One question that remains unanswered is the origin of the planetesimal. While it could be a native part of the white dwarf’s original planetary system, having been knocked from a distant orbit into the star’s path, researchers cannot yet rule out another possibility. It is also conceivable that the object was an interstellar visitor, a rogue body captured from deep space by the white dwarf’s gravity. Future observations may help distinguish between these two scenarios, offering further insight into the architecture and evolution of planetary systems.