A balloon someone forgot to tie off deflates before your eyes. Planet GJ 3090b behaves much the same way: its gaseous envelope is leaking into space, and astronomers caught it in the act with their instruments.
The planet crosses in front of its star, and the starlight shines through its atmosphere. Within that light, astronomers found the shadow of helium, a gas escaping from the planet. The shadow turned out to be very deep, far deeper than on other similar planets. That means the atmosphere is escaping fast, like air rushing out of a punctured balloon.
Scientists compared their observations with data from the James Webb Space Telescope and realized the leak varies from night to night. On one night the planet lost more gas, on another less. Either way, the rate is extremely high: in 300 million years the envelope will be gone completely. For a planet that's a short span, like a single day in a human life.
The balloon from the opening line isn't deflating on its own. The star is doing the deflating, and the process is visible in fine detail. Now astronomers know how quickly small planets can lose their atmospheres, and they'll be hunting for other such "deflating" worlds.
The helium line at a wavelength of 10833 angstroms is so sensitive to conditions in the upper atmosphere that it's sometimes called a "thermometer for escaping gases." On GJ 3090b it turned out to be so deep that the planet looks like a miniature gas giant shedding material, even though it's only twice the size of Earth.
The rapid atmospheric loss of a young sub-Neptune echoes the fate of the planet Kamino from the Star Wars universe, whose ocean world is gradually becoming less habitable because of climate change. In reality, GJ 3090b is losing not water but its hydrogen-helium envelope, yet the motif of "a planet on the brink of irreversible change" is unmistakable.