• JWST detected water vapor in the atmosphere of the hot sub-Neptune exoplanet TOI-421 b, challenging previous expectations of hazy, featureless atmospheres for this type of planet.
• The atmosphere of TOI-421 b is rich in hydrogen and lacks methane or carbon dioxide, resembling the composition of its host star and differing from cooler sub-Neptunes.
• This discovery provides a new avenue for studying hot sub-Neptunes, offering insights into their formation and evolution and explaining why they don't exist in our solar system.

The James Webb Space Telescope (JWST) has successfully peered through the atmosphere of the exoplanet TOI-421 b, a hot sub-Neptune located approximately 244 light-years from Earth. This marks a significant achievement as sub-Neptunes, a common type of exoplanet, have previously been difficult to observe due to their dense, hazy atmospheres. The data obtained from JWST revealed the presence of water vapor, potential signs of carbon monoxide and sulfur dioxide, and a surprising abundance of hydrogen, offering new insights into the composition and formation of these mysterious worlds.

• What: The James Webb Space Telescope has detected water vapor and other gases in the atmosphere of the hot sub-Neptune exoplanet TOI-421 b.
• Who: NASA's James Webb Space Telescope, Eliza Kempton (University of Maryland), Brian Davenport (University of Maryland), NASA Webb Mission Team.
• When: Observations were made recently and the findings were published on May 5, 2025, in The Astrophysical Journal Letters.
• Where: Exoplanet TOI-421 b, located approximately 244 light-years from Earth.

The discovery of water vapor and a hydrogen-rich atmosphere on TOI-421 b represents a significant step forward in the study of exoplanets. The fact that TOI-421 b lacks methane and carbon dioxide, unlike other observed sub-Neptunes, suggests that its formation and evolution may be different. The planet's high temperature and its orbit around a Sun-like star also contribute to its unique characteristics. This discovery challenges previous assumptions about sub-Neptune atmospheres and opens new avenues for research into these common yet mysterious exoplanets.

I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets.

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.

If you just took the same gas that made the host star, plopped it on top of a planet's atmosphere, and put it at the much cooler temperature of this planet, you would get the same combination of gases. That process is more in line with the giant planets in our solar system, and it is different from other sub-Neptunes that have been observed with Webb so far.

The James Webb Space Telescope's observation of TOI-421 b marks a significant advance in our understanding of sub-Neptune exoplanets. The detection of water vapor and a hydrogen-rich atmosphere, coupled with the absence of methane and carbon dioxide, challenges previous assumptions and highlights the diversity of exoplanetary atmospheres. Further research on hot sub-Neptunes orbiting Sun-like stars is needed to determine if TOI-421 b is a unique case or a representative of a broader trend, paving the way for a more comprehensive understanding of exoplanet formation and evolution.