Free Astronomy Magazine July-August 2025

30 JULY-AUGUST 2025 ASTRO PUBLISHING derstanding of how sub-Neptunes formed and evolved, and part of that is understanding why they don’t exist in our solar system.” The existence of sub-Neptunes was unexpected before they were dis- covered by NASA’s retired Kepler space telescope in the last decade. Now, astronomers are trying to un- derstand where these planets came from and why are they so common. Before Webb, scientists had very lit- tle information on them. While sub- Neptunes are a few times larger than Earth, they are still much smaller than gas-giant planets and typically cooler than hot Jupiters, making them much more challeng- ing to observe than their gas-giant counterparts. A key finding prior to Webb was that most sub-Neptune atmospheres had flat or featureless transmission spectra. This means that when scien- tists observed the spectrum of the planet as it passed in front of its host star, instead of seeing spectral fea- tures – the chemical fingerprints that would reveal the composition of the atmosphere – they saw only a flat-line spectrum. Astronomers con- cluded from all of those flat-line spectra that at least certain sub-Nep- tunes were probably very highly ob- scured by either clouds or hazes. “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 T hough they don’t orbit around our Sun, sub-Neptunes are the most common type of exo- planet, or planet outside our solar system, that have been observed in our galaxy. These small, gassy plan- ets are shrouded in mystery…and often, a lot of haze. Now, by observ- ing exoplanet TOI-421 b, NASA’s James Webb Space Telescope is help- ing scientists understand sub-Nep- tunes in a way that was not possible prior to the telescope’s launch. “I had been waiting my entire career for Webb so that we could meaning- fully characterize the atmospheres of these smaller planets,” said prin- cipal investigator Eliza Kempton of the University of Maryland, College Park. “By studying their atmos- pheres, we’re getting a better un- The atmospheric makeup of small, hot, gassy sub-Neptune TOI - 421 b by NASA/ESA/CSA Claire Blome & Christine Pulliam