Methyl halides, commonly produced by life on Earth, could accumulate in the atmospheres of ocean-rich “Hycean” worlds, offering a novel clue in the search for extraterrestrial biology.

A new study, published in ‘The Astrophysical Journal’, suggests that methyl halides — gases such as methyl chloride (CH₃Cl) and methyl bromide (CH₃Br) — could serve as detectable indicators of life on Hycean exoplanets, a class of hydrogen-rich, ocean-covered worlds recently proposed as promising targets in the hunt for habitable environments.

Hycean planets, first theorized in 2021, are characterized by thick hydrogen atmospheres overlaying vast liquid water oceans. Larger and hotter than Earth but potentially habitable in their subsurface layers, these planets are abundant in the cosmos and easier to observe than rocky Earth-like worlds due to their extended atmospheres. The study highlights their unique potential to host microbial life in ocean environments, similar to extremophiles in Earth’s deep seas.

On Earth, methyl halides are primarily produced by biological processes, such as marine algae and fungi. However, their rapid destruction by oxygen radicals and UV light in Earth’s atmosphere makes them weak biosignatures here. The new research argues that Hycean atmospheres, dominated by hydrogen and with reduced UV exposure, could allow these gases to accumulate to detectable levels.

Using advanced photochemical models, the team simulated conditions on Hycean planets and found that methyl halide concentrations could reach parts-per-billion levels if produced by hypothetical oceanic organisms. These levels fall within the detection limits of the James Webb Space Telescope (JWST). The James Webb Space Telescope is well-suited for this detection due to methyl halides’ strong infrared absorption features. This allows identification within about 13 hours — a timeframe more efficient than detecting other biosignatures like oxygen or methane.

While methyl halide detection alone isn’t proof of life, it would make a Hycean planet a high-priority follow-up target. If validated, methyl halides could expand the range of detectable biosignatures beyond traditional markers like oxygen or methane. This approach aligns with JWST’s ongoing efforts to study exoplanet atmospheres, offering a new strategy to explore diverse worlds previously deemed uninhabitable.