Free Astronomy Magazine July-August 2025

JULY-AUGUST 2025 Kempton. Aside from being hotter than other sub-Neptunes previously observed with Webb, TOI-421 b or- bits a Sun-like star. Most of the other sub-Neptunes that have been observed so far orbit smaller, cooler stars called red dwarfs. Is TOI-421b emblematic of hot sub- Neptunes orbiting Sun-like stars, or is it just that exoplanets are very di- verse? To find out, the researchers would like to observe more hot sub- Neptunes to determine if this is a unique case or a broader trend. They hope to gain insights into the for- mation and evolution of these com- mon exoplanets. “We’ve unlocked a new way to look at these sub-Neptunes,” said Daven- port. “These high-temperature plan- ets are amenable to characteriza- tion. So by looking at sub-Neptunes of this temperature, we’re perhaps more likely to accelerate our ability to learn about these planets.” A transmission spectrum captured by NASA’s James Webb Space Telescope reveals the presence of water (H 2 O) and the possible presence of sulfur dioxide (SO 2 ) and carbon monoxide (CO), but no signs of carbon dioxide (CO 2 ) or methane (CH 4 ), in the atmosphere of the hot sub-Neptune exo- planet TOI-421 b. The observations support the hypothesis that planets this hot (TOI-421 b is about 1,340°F) have clear atmospheres that are not ob- scured by clouds and haze. The spectrum was made by measuring the decrease in apparent brightness of different wavelengths (colors) of starlight as the planet transited, or moved across the face of the star. During a transit, some wavelengths of starlight are transmitted through the planet’s atmos- phere, while others are partially blocked. Because each molecule absorbs a unique combination of wavelengths, the transmission spectrum can be used to identify gases in the atmosphere. This spectrum was made by combining data captured in 2023 by Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph). [NASA, ESA, CSA, Joseph Olmsted (STScI)] Without haze or clouds, researchers expected to see a clear atmosphere – and they did! “We saw spectral features that we attribute to various gases, and that allowed us to deter- mine the composition of the atmos- phere,” 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-Nep- tunes that had been previously ob- served, we know their atmospheres are made of something, but they’re being blocked by haze.” The team found water vapor in the planet’s atmosphere, as well as ten- tative signatures of carbon monox- ide and sulfur dioxide. Then there are molecules they didn’t detect, such as methane and carbon diox- ide. From the data, they can also infer that a large amount of hydro- gen is in TOI-421 b’s atmosphere. The lightweight hydrogen atmos- phere was the big surprise to the re- searchers. “We had recently wrapped our mind around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmos- pheres, so that had become our ex- pectation, and then we found the opposite,” said Kempton. This sug- gests TOI-421 b may have formed and evolved differently from the cooler sub-Neptunes observed previously. The hydrogen-dominated atmos- phere is also interesting because it mimics the composition of TOI-421 b’s host star. “If you just took the same gas that made the host star, plopped it on top of a planet’s at- mosphere, and put it at the much cooler temperature of this planet, you would get the same combina- tion 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 !