Hot sub-Neptune exoplanet TOI-421 b (NIRISS/NIRSpec transmission spectrum)

This is a transmission spectrum captured by the NASA/ESA/CSA James Webb Space Telescope that 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 727°C) have clear atmospheres that are not obscured by clouds and haze.

Hot sub-Neptunes are larger than Earth, smaller than Neptune, and orbit closer to their stars than Mercury orbits the Sun. While not found in our own Solar System, sub-Neptunes may be the most common type of planet in our Milky Way galaxy.

This spectrum was made by measuring the decrease in apparent brightness of different wavelengths (colours) 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 combines data captured in 2023 by Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph).

[Image description: Graphic titled “Exoplanet TOI-421 b Hot Sub-Neptune, NIRISS Single Object Slitless Spectroscopy, NIRSpec Bright Object Time-Series Spectroscopy” shows a graph of amount of light blocked on the y-axis versus wavelength of light on the x-axis.]