Much smaller than gas-giants and typically cooler than hot Jupiters, sub-Neptunes were extremely challenging to observe before the launch of NASA’s James Webb Space Telescope. Though they appear to be the most common type of exoplanet in our galaxy, you won’t find them in our solar system. Astronomers are trying to understand how these planets formed and evolved, why they are so common, and why they don’t orbit our Sun. 

Adding to the mystery, many sub-Neptunes appear to be very highly obscured by clouds and hazes, which have made it impossible to determine their atmospheric makeup. Now, by using Webb to study the hot sub-Neptune TOI-421 b, scientists have uncovered the chemical fingerprints of its atmosphere. Did this exoplanet form and evolve differently from the cooler, sub-Neptunes observed previously? Do TOI-421 b’s differences indicate a new classification of hot sub-Neptunes, or is it just that exoplanets are very diverse? By using Webb, researchers hope to find out.