Hazes and chemistry shape the atmosphere of K2-18 b, a sub-Neptune exoplanet

Star system: K2-18 | Distance from the Sun: 124 light years

K2-18 b is a temperate sub-Neptune planet orbiting a red dwarf star, making it a key target for understanding atmospheres beyond our Solar System. Recent observations with the James Webb Space Telescope have provided new insights into its atmospheric composition and structure, helping to bridge the gap between planets like Neptune and the more commonly studied hot-Jupiters.

Researchers used a detailed computer model to study K2-18 b's atmosphere, focusing on how clouds, hazes, and energetic particles called photoelectrons affect its chemistry and temperature. They compared their results with JWST data from several independent teams. The study found that a hydrogen-rich atmosphere with a high amount of heavier elements best matches the observed data, regardless of how the data was processed.

The planet's atmosphere is shaped by complex chemical reactions that are not in balance, known as disequilibrium chemistry. Methane is the main molecule seen in the spectrum, with smaller amounts of carbon dioxide, carbonyl sulfide, and possibly ethylene. Photoelectrons increase the production of nitrogen-based molecules. Hazes cool the upper atmosphere, creating a cold layer where water can condense, which reduces the amount of water vapor. This cooling also explains why ammonia is missing from the observed spectra.

Key findings
- K2-18 b has a hydrogen-rich, high-metallicity atmosphere
- Methane dominates the observed spectrum, with other molecules present
- Hazes and clouds strongly affect temperature and chemistry
- Photoelectrons boost nitrogen molecule production
- No extra molecules are needed to explain the JWST data

Scientific study

The atmosphere of K2-18 b: The role of hazes, clouds and photoelectrons (external link)