Recent observations using the James Webb Space Telescope’s MIRI instrument have provided new, tighter limits on the presence of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere of the exoplanet K2-18b, improving understanding of its atmospheric composition and potential biosignatures. The results of the research, led by astronomers from Cambridge university, are reported in The Astrophysical Journal Letters.
K2-18b is an exoplanet orbiting the red dwarf K2-18, located 124 light-years away from Earth in the Leo constellation and being in the habitable zone of its star, making it a prime candidate for studies related to habitability and potential biosignatures. On Earth, DMS and DMDS are generated by living organisms, particularly by microorganisms like marine phytoplankton. Although there could be an unidentified chemical mechanism responsible for the presence of these compounds in the atmosphere of K2-18b, the findings represent compelling indication that life might be present on a planet beyond our solar system.
Previous studies of K2-18b detected methane and carbon dioxide in its atmosphere: this marked the first instance of carbon-based molecules being found in the atmosphere of an exoplanet situated within the habitable zone. These findings aligned with expectations for a ‘Hycean’ planet, characterized as a potentially habitable world enveloped by a hydrogen-rich atmosphere and covered by oceans. The abundance of methane and carbon dioxide, and shortage of ammonia, supported the hypothesis that there could be a water ocean underneath a hydrogen-rich atmosphere in K2-18b. These initial Webb observations also provided a possible detection of a molecule called dimethyl sulfide (DMS).
The recent study utilized the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) to analyze the light spectrum from K2-18 b during transits. As K2-18 b transits, the JWST observes a decrease in the brightness of the star, with a small portion of that starlight filtering through the planet’s atmosphere before reaching Earth. The absorption of specific wavelengths of starlight by gases in the atmosphere creates distinct signatures in the stellar spectrum, which scientists can analyze to identify the various gases present in the exoplanet’s atmosphere.
The observations have achieved a statistical significance of ‘three-sigma,’ indicating there is a 0.3% likelihood that these results are due to random chance. For these findings to be recognized as a definitive scientific discovery, they would need to surpass the “five-sigma” threshold, which corresponds to a probability of less than 0.00006% that the results occurred by chance. The researchers say that follow-up observations with JWST may help them reach the all-important five-sigma significance.
Despite the great contribution of this discovery to understanding the structure of exoplanets and determining strategies for future missions to search for extraterrestrial life, it’s vital to gain more data before claiming that life has been found on another planet. Scientists admit other ways of formation of the detected gases on the planet, for example, chemical reactions of an unknown nature. speaking about the formation of dimethyl sulfide molecules, there are other ways of forming this organic substance, implying suitable conditions for the chemical reaction: temperature, pressure, the presence of free sulfur atoms or methane molecules, active volcanic activity.