Sub-Neptunes, planets with radii between two and four Earth radii, make up most of the short-period worlds in the Galaxy, yet their structure and evolution remain debated. Transit observations in the metastable helium line (He*) allow direct tracking of atmospheric loss, and the line's width can distinguish photo-evaporation from mass loss driven by core heat. The work sits alongside recent studies of sub-Neptune and exoplanet atmospheres using space- and ground-based telescopes, but stands out for its record signal depth and its link to the system's young age. GJ 3090b had already been studied with JWST and the VLT, but it was WINERED's high spectral resolution that revealed outflow details previously out of reach.
The observations were carried out on the 6.5-meter Magellan II telescope with the infrared spectrograph WINERED, which delivers a resolution of about 68,000. Two consecutive transits of GJ 3090b were recorded in September 2024, yielding 64 and 80 spectra per night. The data were processed with the WARP pipeline, systematic differences between telescope positions were corrected, and a stellar template was built from out-of-transit frames. The helium-line signal was then described with a three-Gaussian model sharing a common shift and width, while noise was handled with a Gaussian process. Two independent hydrodynamic models, sunbather and pyTPCI, which treat gas heating and acceleration differently, were used to interpret the outflow.
Peak absorption in the helium line reached 2.033±0.086% on the first night and 1.619±0.122% on the second, corresponding to significances of 23.6σ and 13.3σ. That is far deeper than typical sub-Neptune signals and equivalent to an opaque layer at a distance of nearly four planetary radii. The line width of about 25 km/s points to a hot photo-evaporative flow rather than a slow wind driven by core heat. No line shift was detected, which indicates the absence of strong interaction with the stellar wind. Comparison with JWST/NIRISS data revealed an 8.5σ discrepancy: the WINERED signal is deeper, confirming that the outflow is variable. Modeling showed that the atmosphere's metallicity does not exceed roughly a hundred times solar, while mass loss reaches 10^11 grams per second.
The work demonstrates for the first time that a sub-Neptune with a radius of about two Earth radii can host such a powerful photo-evaporative outflow, filling more than half of its Roche lobe. This sets it apart from neighboring studies where helium signals were weaker or ambiguous, and it highlights the value of high spectral resolution for small planets. Finding an aerosol cover instead of high metallicity resolves a long-standing ambiguity in interpreting the flat lower-atmosphere spectra of GJ 3090b. Moreover, an envelope lifetime of just 300 million years puts the planet in the category of "teenagers" going through a critical evolutionary phase, which matters for understanding the formation of the radius valley.
The topic could move toward a systematic WINERED survey of sub-Neptunes: the consortium plans to measure helium absorption for more than thirty objects with a precision down to 0.2%. That will make it possible to statistically separate the contributions of photo-evaporation and core-heat-driven mass loss to the shaping of the radius valley. Further monitoring of GJ 3090b, especially during stellar flares, will help clarify how the star's variable radiation governs the outflow. Combining high- and low-resolution data, for instance from JWST and ground-based spectrographs, will become the standard for testing models of atmospheric evolution.
The results will influence models of small-exoplanet evolution and the interpretation of sub-Neptune spectra obtained by JWST. They also matter for planning future surveys of atmospheric loss on planets with radii from one and a half to four Earth radii.
The immediate step is to observe additional transits of GJ 3090b to confirm the outflow's variability and refine its geometry. In parallel, the consortium will keep building statistics on other sub-Neptunes to uncover the link between mass-loss rate and planetary properties.
The work speaks directly to the unresolved question of the radius valley's origin: which atmospheric-loss mechanisms turn sub-Neptunes into super-Earths, and in what proportion. It also contributes to the problem of interpreting flat transmission spectra, where it is still unclear what matters more: high metallicity or aerosols. Finally, the variability of the helium signal ties the study to the poorly understood role of stellar activity in atmospheric evolution.
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 leaking material, even though it is only twice the size of Earth.
The rapid atmospheric loss from a young sub-Neptune echoes the fate of the planet Kamino in the Star Wars universe, whose ocean world gradually becomes unfit for life 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.