Unexpected Atmosphere Composition of a Warm Mini-Neptune
Steam over a boiling pot slightly tints the light of a flashlight: it blocks some colors and lets others through. That's roughly how we study the air of distant planets.
A Young Sub-Neptune's Helium Exhale
A balloon someone forgot to tie off deflates before your eyes. That is roughly how GJ 3090b behaves: its hydrogen-helium envelope is leaking into space at a rate that surprised astronomers. The planet is only twice the size of Earth, yet it is shedding gas as if it wanted to get rid of everything superfluous as fast as possible.