• JWST detected water vapor in the atmosphere of the hot sub-Neptune TOI-421 b, a planet notable for its lack of haze.
• The planet's atmosphere contains a significant amount of hydrogen and possibly carbon monoxide and sulfur dioxide, but no methane or carbon dioxide.
• TOI-421 b's atmosphere closely resembles the composition of its host star, suggesting a different formation process compared to cooler sub-Neptunes.

The James Webb Space Telescope (JWST) has peered into the atmosphere of TOI-421 b, a hot sub-Neptune exoplanet located approximately 244 light-years away. This planet is unique because, unlike many other sub-Neptunes, its atmosphere is relatively free of haze, enabling scientists to identify the presence of water vapor, as well as possible traces of carbon monoxide and sulfur dioxide. This discovery challenges previous assumptions about the composition and formation of sub-Neptune atmospheres and provides valuable insights into the diversity of exoplanets.

• What: The James Webb Space Telescope detected water vapor, possible carbon monoxide and sulfur dioxide, and a significant amount of hydrogen in the atmosphere of the exoplanet TOI-421 b.
• Who: The research team is led by Eliza Kempton of the University of Maryland, with significant contributions from Brian Davenport and NASA's Webb Mission Team.
• When: The findings were published on May 5, 2025, in The Astrophysical Journal Letters, based on JWST observations conducted recently.
• Where: TOI-421 b is located approximately 244 light-years from Earth in the constellation Lepus. The research was conducted by scientists at the University of Maryland and NASA's Goddard Space Flight Center.

The discovery of water vapor and a hydrogen-rich atmosphere in TOI-421 b challenges previous assumptions about sub-Neptune planets. The lack of haze in its atmosphere allowed for a clearer view than previously possible, leading to the detection of various gases. The similarity between the planet's atmosphere and its host star's composition suggests a unique formation process, potentially more akin to the gas giants in our solar system. Further observations of similar hot sub-Neptunes are needed to determine if TOI-421 b is an anomaly or represents a broader trend.

I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets. By studying their atmospheres, we're getting a better understanding of how sub-Neptunes formed and evolved, and part of that is understanding why they don't exist in our solar system.

We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere. Whereas with many of the other sub-Neptunes that had been previously observed, we know their atmospheres are made of something, but they're being blocked by haze.

We've unlocked a new way to look at these sub-Neptunes. These high-temperature planets are amenable to characterization. So by looking at sub-Neptunes of this temperature, we're perhaps more likely to accelerate our ability to learn about these planets.

The JWST's observations of TOI-421 b have opened a new window into the study of sub-Neptune exoplanets. The discovery of a haze-free atmosphere with water vapor and a composition similar to its host star provides valuable clues about the formation and evolution of these common yet mysterious planets. Ongoing research and future observations of other hot sub-Neptunes will help determine if these characteristics are unique to TOI-421 b or representative of a larger population, ultimately enhancing our understanding of planetary diversity.