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.
Over two September evenings in 2024, the team watched GJ 3090b pass in front of its star. The WINERED spectrograph on the 6-meter Magellan II telescope collected starlight filtering through the planet's atmosphere. A deep shadow appeared in that light at a wavelength of 10833 angstroms: the fingerprint of helium in a special, excited state. The absorption reached 2 percent, which is a lot for such a small planet. Sub-Neptunes usually show this signal only faintly, but here it was so strong that its significance came to 23 sigma.
The width of the line revealed the temperature and speed of the gas. If the atmosphere were escaping slowly, like steam rising off a hot pot, the line would be narrow. Instead, it stretched to 25 kilometers per second, which looks more like a jet from a cylinder. This style of outflow points to photoevaporation: the star's ultraviolet and X-ray radiation heats the upper atmosphere and literally blows it away. No shift of the line was found, which means the stellar wind barely hinders the gas as it leaves.
A comparison with James Webb Space Telescope data added some intrigue. The WINERED signal turned out to be deeper by a factor of 8.5 relative to its uncertainty. This is not an error: the outflow changes from night to night. On the first night, absorption was 2.03 percent; on the second, 1.62. The planet breathes unevenly, like a balloon from which helium escapes sometimes faster, sometimes slower.
Modeling helped explain that the lower layers of the atmosphere are hidden by aerosols, tiny particles resembling haze. Researchers used to think the flat spectrum was due to an abundance of heavy elements, but the calculations showed that metallicity is unlikely to exceed 100 times solar. Aerosols conceal whatever lies beneath them, the way thick fog hides a road.
The planet is losing its gas blanket hundreds of times faster than previously thought. After 300 million years, all that will remain is a bare rocky core. By cosmic standards, that is an instant. GJ 3090b is a teenager going through a turbulent growth spurt, and astronomers were lucky to catch this moment.
The balloon from the opening line is not deflating for no reason. Its star is doing the deflating, and the process is visible in detail thanks to a high-resolution spectrograph. Someday such observations will become routine, and we will be able to compile a census of planets losing their atmospheres. For now, GJ 3090b is a rare frame from a movie about sub-Neptunes turning into super-Earths.
The helium line at a wavelength of 10833 angstroms is so sensitive to conditions in the upper atmosphere that it is sometimes called a "thermometer for escaping gases." In GJ 3090b it turned out to be so deep that the planet looks like a miniature gas giant losing material, even though it is 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 losing its habitability due to 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.