James Webb analyses the atmosphere of planet K2-18b

Illustration of K2-18b and its sister planet K2-18c. Credit: NASA/ESA/CSA/Joseph Olmsted, STScl

The James Webb Space Telescope (JWST) has observed the atmosphere of exoplanet K2-18b (discovered by Kepler's extended K2 mission in 2015). K2-18b is considered a possible "hycean" planet. Hycean is a combination of hydrogen and ocean, and means that the planet has deep oceans and a thick hydrogen dominated atmosphere.

The planet is 124 Light years away from Earth in the constellation Leo. K2-18b is located in  the habitable zone of the red dwarf star K2-18, which means it could have liquid water on its surface. K2-18b is thought to be smaller than Neptune, but much larger than Earth. Webb has detected methane and carbon dioxide in K2-18b's atmosphere, as well as a lack of ammonia. 

Atmosphere composition of K2-18b. Credit: NASA/ESA/CSA/Ralf Crawford, and Joseph Olmsted, STScl

Intriguingly, Webb detected Dimethyl Sulfide (DMS) in the planet's atmosphere. On Earth, DMS is produced by marine phytoplankton. Now this doesn't mean K2-18b has life (at least not yet). The DMS observation isn't as robust as the others, and will need follow-up observations to confirm its presence. Remember the 2020 Venus phosphine detection? 

Phosphine is known to be created by Earth microbes, and when it was detected in substancial numbers in Venus's atmosphere, many people thought that meant that Venus has life. Furthur detections have lowered the amount of phosphine thought to present, and the little that is there is now believed to come from volcanic activity and other non-biological phenomina.

These detections were made with only 2 transits of K2-18b (the planet orbits once every 32.9 Earth days). In 2019, the Hubble Space Telescope detected water vapour in K2-18 b's atmosphere, but it required a vastly longer observation time. More observations of the K2-18 system are sure to come in the near future.

The Target

Old illustration of K2-18b. Credit: NASA/ESA/M. Kornmesser

• Date Discovered: 2015
• Planet Type: Temperate Hycean
• Radius: 14751 Kilometres
• Atmospheric Composition: Hydrogen, Carbon Dioxide, Methane, Water, and Dimethyl Sulfide (tentative)
• Mass: 5.3272x10^25 Kilograms (8.92 Earths)
• Density: 3.96 g/cm^3
• Orbital Period: 32.9 Days
• Distance From Earth: 124 Light Years (38 Parsecs)