DESI side project reveals the spectra of disintegrated exoplanets

Astronomers using the Dark Energy Spectroscopic Instrument (DESI) have uncovered new insights into the composition of disintegrating exoplanets by analyzing the spectra of white dwarfs polluted by falling planetary debris. The findings, published in Monthly Notices of the Royal Astronomical Society, reveal that the chemical makeup of these debris-laden white dwarfs mirrors that of rocky bodies in our own solar system, providing a rare opportunity to study the building blocks of terrestrial planets beyond our Sun. During its five-year primary mission, DESI was intended to map the large-scale structure of the universe by measuring the redshifts of distant galaxies. However, due to unfavorable observational conditions, the instrument occasionally had idle periods. Instead of idling, researchers turned to nearby celestial objects, including white dwarfs, which are the remnants of stars like our Sun. These compact, dense stars retain traces of material from their former planetary systems, allowing scientists to infer the composition of long-dead worlds. White dwarfs form when stars like our Sun exhaust their nuclear fuel and collapse under gravity, leaving behind a core composed mostly of carbon and oxygen. Over time, leftover planetary bodies, often rocky, icy, or metallic, can be pulled inward by the white dwarf’s gravitational pull. As these objects break apart, their fragments fall onto the star’s surface, enriching its atmosphere with heavier elements such as oxygen, magnesium, silicon, calcium, and iron. These elements serve as fingerprints, revealing the chemical signatures of the planets that once orbited the star. To date, approximately 20% to 50% of observed white dwarfs exhibit signs of metal pollution, indicating the presence of planetary debris. Among these, around 1,750 are known to be actively accreting material, though only a small fraction possess spectra detailed enough to determine their elemental composition. This makes each such object a valuable target for understanding the diversity of planetary systems. The DESI collaboration focused on 12 particularly metal-rich white dwarfs, examining their spectra in greater detail than ever before. Their analysis revealed that the debris accreting onto these stars contains a wide range of heavy elements, many of which are essential for forming rocky planets. For example, oxygen and magnesium are critical components of silicate minerals, while calcium and iron contribute to the formation of metallic cores. These findings align with the composition of Earth and Mars, suggesting that the processes shaping planetary systems may be broadly similar across the galaxy. Among the studied white dwarfs, six exhibited sufficiently clean spectra to allow for more precise compositional analysis. Four showed characteristics consistent with dry, rock-forming materials, while two displayed distinct oxygen signatures, potentially indicating the presence of water-rich planetesimals. Such discoveries offer tantalizing hints about the potential for habitable environments in other solar systems. Paula Izquierdo, lead author of the study and researcher at the University of Warwick, emphasized the importance of these findings. “Understanding whether our solar system is common or rare requires a large sample of exoplanets with well-characterized chemical compositions,” she explained. “By studying white dwarfs, we gain access to a unique archive of planetary material.” While DESI was originally designed to explore the universe’s expansion, its unexpected contributions to planetary science highlight the value of serendipitous discoveries. As the collaboration continues to analyze additional data, further revelations about the origins and evolution of exoplanets may emerge, deepening our understanding of how planets form and evolve across the cosmos.