section_path: Discovery and research history
section_name: Discovery and research history
paragraph_text: Discovery and research history
  
  The planet was discovered in 2015 by the Kepler space telescope, and its existence was later confirmed with the Spitzer Space Telescope and through Doppler velocity techniques. Analyses of the transits ruled out that they were caused by unseen companion stars, by multiple planets or systematic errors of the observations. Early estimates of the star's radius had substantial errors, which led to incorrect planet radius estimates and the density of the planet being overestimated. The discovery of the spectroscopic signature of water vapour on K2-18b in 2019 was the first discovery of water vapour on an exoplanet that is not a Hot Jupiter and drew a lot of discussion.
  
  K2-18b has been used as a test case for exoplanet studies. The properties of K2-18b have led to the definition of a " hycean planet ", a type of planet that has both abundant liquid water and a hydrogen envelope. Planets with such compositions were previously thought to be too hot to be habitable; findings at K2-18b instead suggest that they might be cold enough to harbour liquid water oceans conducive to life. The strong greenhouse effect of the hydrogen envelope might allow them to remain habitable even at low instellation rates. K2-18b is probably the best known "hycean planet". Other, non-hycean compositions are possible, both habitable and uninhabitable.
  
  There is some evidence of dimethyl sulfide (DMS) and methyl chloride being present in the atmosphere. The presence of DMS is a potential biosignature, as the bulk of the DMS in Earth's atmosphere is emitted from phytoplankton in marine environments, although further observation to confirm the presence of DMS and rule out a geological or chemical origin for the compound is required. Some scientists have voiced concerns about the statistical significance of the DMS signal at K2-18b, and how its putative detection might be understood in an environment different from Earth's. In 2024, Wogan et al. suggested that the high methane concentrations might either imply the presence of methanogenic life or upwards mixing of gases from the deep interior, if the planet is too hot for life.
  
  A podcast on the Planetary Society 's website in January 2024 featured NASA astrophysicist Knicole Colón describing some of the scientific results from the observations of K2-18b by JWST. Data from JWST's MIRI instrument was expected to be gathered in January 2024 about which Colón said: "MIRI will be able to see additional features, absorption features from these molecules, and validate again, the presence of what we've seen and even the abundance." Colón also talks about the lack of evidence of water in the atmosphere: "The fact that the JWST data basically didn't find strong evidence of water in the atmosphere, that could indicate a couple things". Further, two observations using MIRI which lasted over 8 hours were taken on 25 and 26 April 2024 as part of JWST project 2722.
  
  In April 2025, the same team that claimed the detection of DMS, reported a detection of DMS and/or DMDS with a 3σ significance. The observations were made with JWST MIRI LRS transmission spectroscopy. The publication was accompanied by a press release that claimed "strongest hints yet of biological activity outside the solar system". This was criticised by other scientists. NASA issued a statement saying "detection of a single potential biosignature would not constitute discovery of life" and planetary scientist Sarah Hörst pointed to lab experiments that show DMS can be produced without life. A follow-up study examining a subset of the same data claims there is no strong statistical evidence for any particular gasses (including the alleged detection of DMS), while another argued that the supposed evidence for DMS was a result of incomplete model assumptions, and that other gases, such as propyne, a gas found on Neptune, can provide a better explanation of the observations. However, a later study by the original team found that, considering all available data and exploring 650 possible gases, DMS remained one of only three molecules that best explain all the observations. The other two gases are diethyl sulfide and methacrylonitrile, both of which are more complex than DMS, have no known abiotic sources in planetary atmospheres, and are present in negligible abundances on Earth, from biogenic and anthropogenic sources.
  
  A later study examining the combined NIRISS + NIRSpec + MIRI JWST transmission spectrum found little evidence for DMS and DMDS due to their supposed spectral features in the MIRI spectrum being much larger than those at shorter wavelengths and the fact that they require a larger atmospheric scale height and a much higher temperature, which is inconsistent with NIRISS and NIRSpec observations. This study estimated that observing 25 more transits using the MIRI instrument would be required to rule out a flat line spectrum compared to one with DMS and DMDS.