Ultraviolet uncertainty challenges biosignatures on TRAPPIST-1 e ✦
Context
The TRAPPIST-1 system, only 12.5 parsecs away, includes seven planets, four of which lie in the habitable zone. The planet TRAPPIST-1 e is one of the primary targets for the search for extraterrestrial life. However, its star — an ultra-cool red dwarf — emits intense and variable UV radiation capable of dramatically altering atmospheric chemistry. For planning observations with JWST and future telescopes, it is crucial to understand how the uncertainty in the star's UV spectrum affects the interpretation of potential biosignatures obtained through transit spectroscopy and photometry.
Methods
The researchers applied a one-dimensional photochemical code, including detailed photolysis cross-sections based on the laws of electrodynamics, for five different reconstructions of TRAPPIST-1’s UV spectrum. The atmosphere was modeled with pressures of 1 bar and 0.1 bar of CO2 at a surface temperature of 288.4 K. Abiotic (weak sinks) and biotic (strong sinks for CO, H2, O2) boundary conditions were considered with CH4 fluxes ranging from 10^8 to 10^11 molecules/cm²/s. For each case, steady-state concentrations were calculated, after which synthetic transmission spectra were generated using spectral synthesis in the NIRSpec and MIRI bands of the James Webb Space Telescope.
Results
The results revealed that, depending on the UV spectrum, the concentration of methane varies by three orders of magnitude, and oxygen ranges from traces to several percent. In the abiotic scenario with a high FUV/NUV ratio (Mega-MUSCLES SEM model), the abundance of O2 near the surface reached 5.3%, and a noticeable ozone layer formed in the atmosphere, detectable in transmission spectra — a field pioneered by William Borucki. Simultaneously, CH4 was present, creating a false-positive biosignature. In biotic cases, active uptake of CO by microorganisms suppressed CO concentration and contributed to the accumulation of O2 (up to 390 ppm) even without oxygenic photosynthesis. Radicals OH, formed from the photolysis of H2O, and the reaction CO + OH → CO2 + H played a key role in regulating the redox balance. Abiotic buildup of O2 was enhanced when this pathway was suppressed due to weak UV radiation in the NUV region.
Implications
The findings mean that the joint detection of CH4 and O3 in the atmosphere of an exoplanet around a red dwarf is not unambiguous evidence of life. High levels of O2 also do not necessarily indicate oxygenic photosynthesis. Carbon monoxide acts as an anti-biosignature: its high concentration points to abiotic processes, while a low one suggests biological uptake. Therefore, reliable interpretation of biosignatures requires a comprehensive analysis of the atmosphere in the context of the stellar UV environment.
Future development
Future research should aim at obtaining more accurate empirical UV spectra of cool stars using JWST and future missions such as the Origins Space Telescope. There is also a need for three-dimensional climate-photochemical models that account for cloud cover and tidal locking on planets like TRAPPIST-1 e. This will reduce uncertainties when planning searches for life.
Impact
The results will impact astrobiology, the study of exoplanet atmospheres, and the strategy for future observations using photometry and high-resolution spectroscopy.
Next steps
The immediate steps include campaigns of transit observations of TRAPPIST-1 e with JWST to search for spectral features of CO and O3, as well as laboratory measurements of photolysis cross-sections at temperatures typical of planetary atmospheres around M dwarfs.
Key open problems
The work is directly connected to the fundamental problem of searching for extraterrestrial life: how to distinguish biogenic gases from abiotic ones under conditions of incomplete knowledge about the star. This echoes long-standing questions about the evolution of Earth's atmosphere, including the faint young Sun paradox, and underscores the need for an interdisciplinary approach in astrobiology.
🎯 TRAPPIST-1 is so dim that a year on planet e lasts only 6.1 Earth days, and its surface is likely always facing the star with one side. Meanwhile, the system is closer to us than many known exoplanets — just 12.5 parsecs away.
Key numbers
- maximum O2 concentration in the abiotic scenario (Mega-MUSCLES SEM): 5.3%
- O2 concentration in the biotic scenario (Revised PHOENIX): 390 ppm
- range of CH4 concentration variation: three orders of magnitude
- range of FUV/NUV ratio for different SEDs: two orders of magnitude
- distance to TRAPPIST-1: 12.5 parsecs