A transmission spectrum captured by NASA’s James Webb Space Telescope reveals the presence of water (H2O) and the possible presence of sulfur dioxide (SO2) and carbon monoxide (CO), but no signs of carbon dioxide (CO2) or methane (CH4), in the atmosphere of the hot sub-Neptune exoplanet 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 obscured 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 atmosphere, 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).
Credits:
Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI)
Sub-Neptunes. Shrouded in mystery… and haze.
These are one of the most common types of exoplanets in our galaxy, but we don’t have one in our solar system to study. Webb is able to help us to learn more about them. Sub-Neptunes were discovered initially by the Kepler mission. They are much smaller than gas-giant planets, and typically cooler than “hot Jupiters” - which makes them more challenging to observe than larger gas giants. Prior to Webb, observations of their atmospheres didn’t show the chemical fingerprints that would tell us about their compositions. Astronomers believed this could mean many sub-Neptunes were hidden beneath clouds or haze.
Enter Webb and its extreme sensitivity and ability to see detail in the chemical fingerprints that other telescopes can’t discern. Astronomers picked planet TOI-421b to observe because previous data showed that it may not be as hazy as some other candidates. It’s also a bit hotter than others, which allowed them to test the theory that planets over a certain temperature range might be less prone to haze or clouds.
Indeed Webb saw a clear atmosphere that allowed astronomers to see TOI-421’s chemical fingerprints and determine the composition of its atmosphere, which includes water vapor, and possibly carbon monoxide and sulfur dioxide. Additionally, a large amount of TOI-421b’s atmosphere is made up of hydrogen, which was the biggest surprise to scientists. Astronomers thought most sub-Neptunes might have heavy-molecule atmospheres, and TOI-421b’s is the opposite, meaning it may have formed and evolved differently from the cooler sub-Neptunes observed previously. That its atmospheric composition mimics its host star also makes it more similar to the relationship between the gas giants in our solar system and our Sun. In fact, unlike many other sub-Neptunes, TOI-421b orbits a Sun-like star, rather than a smaller, cooler red dwarf star. This begs the question - are sub-Neptunes diverse? Or are those orbiting sun-like stars all similar to TOI-421b?
Read more: go.nasa.gov/3GEaoeS
Artist illustration: NASA, ESA, CSA, Dani Player (STScI)
Image description: Illustration showing a large fuzzy blue planet in the foreground, and an orange-yellow star in the background. The side of the planet facing the star is lit, and the side facing away is dark. The boundary between the lit and dark sides is fuzzy. The atmosphere is almost homogeneous in color and texture, with extremely subtle variations and no sign of a surface. The star is slightly more orange than the Sun. The black background of space is scattered with white points of light.
iss073e0008524 (May 1, 2025) --- This photograph from NASA spacewalker Nichole Ayers looks to the forward portion of the International Space Station and highlights the Kibo laboratory module with the Japanese robotic arm, the SpaceX Dragon cargo spacecraft docked to the Harmony module's space-facing port, and the partially obscured SpaceX Dragon crew spacecraft docked to Harmony's forward port. The International Space Station was orbiting 260 miles above the Pacific Ocean east of the Hawaiian island chain.
iss073e0008485 (May 1, 2025) --- NASA astronaut and Expedition 72 Flight Engineer Anne McClain is pictured near one of the International Space Station's main solar arrays during a spacewalk to upgrade the orbital outpost's power generation system and relocate a communications antenna.
iss073e0008235 (May 1, 2025) --- NASA astronaut and Expedition 72 Flight Engineer Anne McClain points a camera towards herself and takes a "space-selfie" during a spacewalk to upgrade the orbital outpost's power generation system and relocate a communications antenna. Reflected in her helmet's visor is fellow spacewalker and NASA astronaut Nichole Ayers.