K2-18 b, a notable exoplanet in its star’s habitable zone, has been the subject of intense scrutiny as researchers seek to determine whether it could harbour life. Previous observations using the James Webb Space Telescope (JWST) detected tentative signs of dimethyl sulfide (DMS), a potential biosignature gas. However, these findings were complicated by challenges such as flux offsets between JWST’s NIRISS and NIRSpec instruments.
To address these uncertainties and independently assess the presence of DMS and other biosignature gases, this study employs the JWST MIRI spectrograph to obtain a mid-infrared transmission spectrum of K2-18 b. This approach provides a complementary spectral range (5–12 μm) that encompasses strong features of DMS and other key molecules, offering a valuable opportunity to confirm or refute prior findings and advance our understanding of the planet’s atmospheric composition and potential habitability.
Recent observations using the James Webb Space Telescope‘s Mid-Infrared Instrument (MIRI) have provided critical insights into the atmosphere of the exoplanet K2-18b, particularly regarding detecting dimethyl sulfide (DMS). Previous studies utilising the Near Infrared Imager and Slitless Spectrograph (NIRISS) and the Near Infrared Spectrograph (NIRSpec) suggested a tentative presence of DMS. Still, they were complicated by flux offsets, likely due to calibration discrepancies between instruments. Flux offsets refer to inconsistencies in brightness measurements across different instruments, which can introduce errors in data interpretation.
The MIRI instrument has addressed these challenges by offering complementary spectral coverage where DMS exhibits strong absorption features. This allows for cross-verification of earlier findings and enhances confidence in detecting DMS. The retrieval approach employed involves atmospheric modelling to identify molecular signatures and constrain their abundances, comparing expected levels from both abiotic processes and biotic sources, such as microbial activity analogous to Earth’s marine algae.
DMS is notable as a potential biosignature because it is produced by living organisms on Earth. However, its production can also occur through non-biological processes, which complicates its interpretation as an exclusive indicator of life. The study underscores the importance of independent confirmations to rule out false positives caused by instrumental artefacts, ensuring that detected signals are genuine.
By integrating data from multiple instruments—NIRISS, NIRSpec, and MIRI—researchers aim to characterise K2-18b’s atmosphere comprehensively. This multi-instrument approach strengthens the reliability of findings and highlights the value of cross-verification in exoplanet research. Observing faint exoplanet atmospheres presents significant challenges, necessitating rigorous data processing to mitigate uncertainties such as flux offsets.
Observations have reached the ‘three-sigma’ level of statistical significance, meaning there is a 0.3% probability that they occurred by chance. The observations would have to cross the five-sigma threshold to get the accepted classification for scientific discovery, meaning there would be below a 0.00006% probability they occurred by chance.
The researchers say between 16 and 24 hours of follow-up observation with JWST may help them reach the all-important five-sigma significance. Their results are reported in The Astrophysical Journal Letters.
In conclusion, this study contributes to understanding K2-18b’s atmospheric composition and potential habitability. While the detection of DMS is intriguing, further research is needed to confirm its biotic origin. Nonetheless, this work represents a promising step towards identifying biosignatures on exoplanets, potentially indicating the presence of microbial life beyond our solar system.
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