For the first time, astronomers have used NASA’s James Webb Space Telescope to peer through the thick haze of a sub-Neptune planet’s atmosphere, offering new clues about one of the universe’s most mysterious and common types of worlds.
Sub-Neptunes may be the most common type of exoplanet, but their thick atmosphere makes them difficult to observe. Now, thanks to JWST’s close-up of exoplanet TOI-421 b, scientists are finally getting a good look at the small gas planets.
The Inscrutable sub-Neptunes
“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said principal investigator Eliza Kempton of the University of Maryland, College Park. “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.”
Sub-Neptunes are astronomical discoveries that were not theoretically predicted, like the black hole, but were only found by observing hard data. The Kepler space telescope, launched in 2009 and active through 2018, provided astronomers with their first glimpses of such planets. This relatively recently known exoplanet type appears extremely common, leaving scientists to determine the reasons for their origin and ubiquity. A significant impediment to answering those questions was that scientists had little information about the difficult-to-observe planets.
Several times the size of Earth, sub-Neptunes are still smaller than other planets, such as gas giants, and generally cooler. Additionally, scientists could not utilize the spectra analysis tools commonly utilized to study exoplanets, as sub-Neptunes have a flat, featureless spectrum, leaving researchers with no data to identify their atmospheric composition, suggesting a dense, heavy cloud cover of unknown makeup.
James Webb Space Telescope Observes TOI-421 b
“Why did we observe this planet, TOI-421 b? It’s because we thought that maybe it wouldn’t have hazes,” said Kempton. “And the reason is that there were some previous data that implied that maybe planets over a certain temperature range were less enshrouded by haze or clouds than others.”
Researchers believe that sunlight and methane generate the impenetrable haze through complex photochemical reactions occurring on exoplanets with temperatures below 1,070 °F. TOI-421 b carries temperatures well beyond that threshold, at 1,340 °F, dissipating the methane for a clear atmosphere observable by the James Webb Space Telescope.
“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” said the University of Maryland’s Brian Davenport, a third-year Ph.D. student who conducted the primary data analysis. “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.”
Atmospheric Spectra From James Webb Space Telescope
Spectral analysis indicated water vapor, accompanied by potential signs of carbon monoxide and sulfur dioxide in TOI-421 b’s atmosphere. Notably absent were any signs of methane or carbon dioxide, a factor which seemed to suggest a surprising abundance of hydrogen.
“We had recently wrapped our mind around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmospheres, so that had become our expectation, and then we found the opposite,” said Kempton.
Understanding Sub-Neptunes
All of these features, which are in contrast to standard expectations for a sub-Neptune, only add to the mystery of these planets as astronomers attempt to puzzle out why TOI-421 b is so unique. Scientists suggest that the odd planet formed and evolved unusually for a sub-Neptune. One particularly interesting feature is how the exoplanet’s hydrogen-rich atmosphere reflects its host star’s composition.
“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,” said Kempton.
The planet itself isn’t the only thing unusually hot about TOI-421 b. Most sub-Neptunes orbit relatively small and cool red dwarf stars, while TOI-421 b circles a larger, hotter star, similar to Earth’s sun. Going forward, astronomers hope to observe more of these hot sub-Neptunes and their Sun-like stars to determine how common such a pairing is.
“We’ve unlocked a new way to look at these sub-Neptunes,” said Davenport. “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 paper “TOI-421 b: A Hot Sub-Neptune with a Haze-free, Low Mean Molecular Weight Atmosphere” appeared on May 5, 2025, in The Astrophysical Journal Letters.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.