How the ‘Devil Comet,’ almost as big as Everest, brought scientists closer to solving Earth’s biggest mystery

Scientists have found compelling evidence that Earth's water may have originated from comets, based on a study of Comet 12P/Pons-Brooks, known as the "devil comet." The research, published in Nature Astronomy in August 2025, shows that the water vapor detected in this Halley-type comet has an almost identical chemical composition to Earth's oceans, offering the strongest support yet for this theory.

Comet’s Close Approach During Solar Eclipse
Comet 12P/Pons-Brooks, roughly the size of Mount Everest, made its first close pass to Earth since 1954 during the April 2025 total solar eclipse. The comet follows a 71-year orbit and belongs to a group of ancient Halley-type comets containing material from the solar system's formation.


Advanced Observations Confirm Water’s Origin
Using the Atacama Large Millimeter/submillimeter Array in Chile and NASA's Infrared Telescope Facility in Hawaii, researchers measured the deuterium-to-hydrogen ratio in the comet’s water vapor—a chemical signature distinguishing water on different celestial bodies. Scientists mapped both regular water (H₂O) and heavy water (HDO) in the comet's coma, confirming the vapor originated from deep within its nucleus.

By mapping both H₂O and HDO, we can tell if these gases are coming from the frozen ices within the solid body of the nucleus, rather than forming from chemistry or other processes in the gas coma, explained NASA’s Stefanie Milam, a co-author of the study.

Strongest Evidence Yet for Cometary Water Delivery
This is the strongest evidence to date supporting the idea that comets could have helped make our planet habitable, said NASA astrophysicist Martin Cordiner, who led the research.

This discovery addresses a long-standing question about Earth's origins. Scientists believe Earth formed as a largely dry planet, unable to retain water due to high early temperatures. The prevailing theory suggested that water arrived later via impacts from icy bodies like comets or asteroids.

Why This Finding Matters
Previous studies had failed to show a close enough match between cometary and terrestrial water. However, 12P/Pons-Brooks provided one of the closest matches ever recorded, strengthening the case that Halley-type comets played a key role in shaping Earth's environment.

Implications for Future Missions
The findings could influence upcoming missions, including the European Space Agency’s Comet Interceptor and NASA’s proposed CAESAR project, aimed at returning comet samples to Earth. Researchers suggest these ancient comets might have delivered not only water but also organic compounds, creating conditions for life. Studying comet classes with matching deuterium-to-hydrogen ratios could offer further insights into Earth’s origins and the solar system’s formation.