Free Astronomy Magazine July-August 2025

31 JULY-AUGUST 2025 ASTRO PUBLISHING were less enshrouded by haze or clouds than others.” That temperature threshold is about 1,070 degrees Fahrenheit. Below that, scientists hypothesized that a complex set of photochemical reac- tions would occur between sunlight and methane gas, and that would trigger the haze. But hotter planets shouldn’t have methane and there- fore perhaps shouldn’t have haze. The temperature of TOI-421 b is about 1,340 degrees Fahrenheit, well above the presumed threshold. T his artist’s concept shows what the hot sub-Neptune exoplanet TOI-421 b could look like. TOI-421 b orbits a Sun-like star roughly 244 light-years from Earth in the constellation Lepus (the Hare). The planet is in between Earth and Neptune in terms of size, mass, and density, and its star is slightly smaller and cooler than the Sun. However, because the planet orbits extremely close to the star (just 0.056 astronomical units, or 5.6% of the distance between Earth and the Sun), it is very hot. Spectroscopic observations of TOI-421 b captured by NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near Infrared Spec- trograph) on NASA’s James Webb Space Telescope show strong evidence for water (H 2 O) and the possible presence of sulfur dioxide (SO 2 ) and carbon monoxide (CO) in the planet’s hydrogen-rich atmosphere. Unlike cooler sub-Neptunes, TOI-421 b appears to have a clear atmosphere that is not shrouded in clouds and haze. This artist’s concept is based on spectroscopic data gathered by Webb, as well as previ- ous observations from other telescopes on the ground and in space. Webb has not captured any images of the planet. [NASA, ESA, CSA, Dani Player (STScI)]