UV radiation from host stars complicates search for life on TRAPPIST-1 e

Star system: TRAPPIST-1 | Distance from the Sun: 40.7 light years

The search for life on distant planets often relies on identifying biosignatures — gases like oxygen or methane that are produced by biological processes. However, a new study highlights how the specific ultraviolet (UV) radiation from a host star can significantly influence a planet's atmospheric chemistry, potentially creating false-positive signals that might be mistaken for life. The research focused on the planet TRAPPIST-1 e, which orbits an ultracool M dwarf star known as TRAPPIST-1.

Researchers used a photochemical model to simulate an atmosphere resembling Earth's early Archean epoch on TRAPPIST-1 e. The Archean period is particularly interesting because it represents a time when a planet's atmosphere was shifting from being controlled by geological processes to biological ones. The study tested how different uncertainties in the UV spectrum of the star TRAPPIST-1 affect the predicted levels of gases such as methane, carbon monoxide, oxygen, and ozone.

The simulations revealed that different assumptions about the star's UV output led to massive variations in atmospheric gas concentrations. In some scenarios, an atmosphere without any life could sustain high levels of both methane and ozone, which are typically considered a potential biosignature when found together. Furthermore, the researchers found that even in the absence of oxygenic photosynthesis, oxygen and ozone could accumulate in the atmosphere due to specific surface chemical processes. The study concludes that carbon monoxide may be a more reliable indicator for distinguishing between biotic and abiotic processes. Overall, the results show that characterizing the UV radiation of ultracool M dwarf stars is critical for accurately interpreting the atmospheres of temperate exoplanets.

Key findings
- Ultraviolet radiation from ultracool M dwarf stars like TRAPPIST-1 can profoundly alter the atmospheric composition of orbiting planets.
- Atmospheric gas concentrations, including methane, oxygen, and ozone, vary by orders of magnitude depending on the assumed stellar UV spectrum.
- The combination of methane and ozone, often considered a potential biosignature, can be produced abiotically in certain scenarios, creating false positives.
- Carbon monoxide is identified as a potentially more reliable discriminator to distinguish between biological and non-biological planetary atmospheres.

Scientific study

Ultraviolet-Driven Atmospheric Degeneracies Challenge Conventional Biosignature Frameworks for Terrestrial Planets with Ultracool M Dwarf Hosts: An Archean-Analog TRAPPIST-1 e Case Study (external link)