Exobiologists and astrobiologists have searched the cosmos for the presence of life on alien worlds for decades. In recent years, the number of potentially habitable planets discovered by telescopes both on Earth and in space, combined with the increasing work of citizen scientists, has increased exponentially, giving experts an unprecedented number of potential targets in the search for alien life.1
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Several theoretical studies have also proven crucial for this scientific mission as they have expanded the potential range of habitable environments. Life might thrive in environments radically different from anything on Earth, many of which are still poorly understood. The focus has expanded to non-Earth-like rocky planets and potential biosignatures than diverge from the dominant biogenic gases found on Earth.1
Introduction to Biosignatures in Astrobiology
A biosignature is defined as a physical or chemical process that can indicates the presence of living organisms in a region of space such as an exoplanet. This can be detected at a distance via methods such as telescopes and spectroscopy. Since the late 20th century, the understanding of the conditions necessary for supporting at least primitive life forms has drastically increased.2
Traditional biosignatures include oxygen, water vapor, and methane. However, the surprising discovery of organisms thriving in extreme Earth environments, such as deep-sea volcanic vents once thought uninhabitable, has expanded the search to include more unconventional biosignatures. Alien life may exist in vastly different conditions not found to support terrestrial organisms.2
Still, while many potential biosignatures may stem from biological processes, challenges like detection limits and false positives highlight the need to identify alternative or complementary biosignatures. Many biosignatures may come from abiotic and geophysical sources, making detecting the tell-tale signatures of life incredibly complex.
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What are Methyl Halides?
On Earth, methyl halides are typically produced by natural processes, although they can also have industrial origins, such as biotic processes from organisms (including fungi, plants, and bacteria). These organic compounds are composed of a methyl group bonded to hydrogen, and include methyl bromide and methyl chloride.
Methyl chloride, for instance, is sparsely found in nature and produced by salt marsh plants and wood-rotting fungi. It is produced by methyl chloride transferase, an enzyme present in these organisms. 3 As mentioned above, methyl halides such as methyl chloride can be produced by industrial processes, but biotic and abiotic sources can be differentiated by traceable radio isotopes.
Why Methyl Halides Matter in the Search for Life
Methyl halides have already been detected in space. Both the Rosetta spacecraft and telescopes on Earth have detected the presence of methyl chloride outside the Earth’s atmosphere, making these compounds targets in the search for biosignatures.3
These compounds are interesting to astrobiologists due to their volatility, ability to accumulate in planetary atmospheres, and are detectable using spectroscopy. Crucially, they are unlike traditional biosignatures such as methane and water vapor as they are less likely to be produced in large quantities by geological processes alone. Furthermore, their presence is a potential indicator of biological activity, especially if found alongside other biosignatures.
Detection Potential and Technological Relevance
The James Webb Space Telescope (JWST) is one mission that can potentially detect methyl halide biosignatures on exoplanets. A recent paper was published that stated that JWST’s advanced sensors could detect these organic compounds in the atmospheres of Hycean planets with subsurface liquid water oceans.4
The JWST’s onboard sensors can detect the spectral signatures of methyl halides at levels comparable to their biological production on Earth. In some cases, detection has been possible with as few as 5 to 14 planetary transits.
Several atmospheric models are being developed and explored to aid in the search for methyl halide biosignatures and the possibility of life on other planets. The authors of the study acknowledge that this is a complex challenge, made tougher by the limited data available. Current atmospheric models, even those suggesting potentially habitable conditions, struggle to accurately fit the observations.
Robust atmospheric models are therefore needed, but as understanding of extraterrestrial planets and biosignatures improves, scientists should be able to better differentiate between biotic and abiotic scenarios. Several techniques have been proposed in recent years.4
Challenges and Controversies
Several key challenges persist with the search for methyl halide biosignatures that could indicate extraterrestrial life. Scientists may already have found planets that support life: detecting this with a high degree of confidence is a different matter entirely.
First, there's the problem of false positives - signals that can arise from natural, non-biological sources like volcanic activity or photochemical reactions. Second, there's a need for more analogs that could point to life in extreme environments, especially those that differ significantly from the Earth-like conditions we usually associate with habitability.
Moreover, whilst undeniably sophisticated, the technology used to detect these enigmatic biosignatures is still in its relative infancy. These is a need for more highly-sensitive instruments able to detect and differentiate biosignatures across the vast distances of space. Additionally, multi-signal verification is needed to improve the search for these biosignatures and the presence of life in the Universe.
Future Research Directions
To detect these compounds in biosignatures, a number of directions will need to be explored by astrobiologists in the coming years. More laboratory simulations and Earth analog studies are needed to refine our understanding of methyl halide production. Furthermore, expanding libraries of biosignatures to include more “non-traditional” compounds will prove highly useful.
Further Reading and More Information
Cowing, K (2025) Habitability and Biosignatures [online] astrobiology.com. Available at: https://astrobiology.com/2025/04/habitability-and-biosignatures.html (Accessed on 10 May 2025)
Hawk, R (2024) What is a Biosignature? [online] allthescience.org. Available at: https://www.allthescience.org/what-is-a-biosignature.htm (Accessed on 10 May 2025)
ACS (2021) Chloromethane [online] acs.org. Available at: https://www.acs.org/molecule-of-the-week/archive/c/chloromethane.html (Accessed on 10 May 2025)
Leung, M et al. (2025) Examining the Potential for Methyl Halide Accumulation and Detectability in Possible Hycean-type Atmospheres The Astrophysical Journal Letters 982(1) [online] IOP. Available at: https://iopscience.iop.org/article/10.3847/2041-8213/adb558 (Accessed on 10 May 2025)
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