The study shows that many hot sub-Neptunes slowly lose their original H-He atmosphere because of strong X-ray UV radiation from their star.
The atmosphere gets replaced by gases released from the interior, especially hydrogen and water — but not helium, because there is very little helium in the interior. So the planet’s atmosphere becomes helium-poor and water-rich, forming a secondary atmosphere.
This explains why many small exoplanets do not show helium in observations. Many small planets (<2.5 Earth radii) have already transitioned to secondary atmospheres and lost most of their helium.
Larger sub-Neptunes tend to retain more of their original H-He atmospheres, which means they keep higher helium abundances and therefore show higher helium escape rates.
Here the Atmospheric Escape Method is used. The researchers calculated how the atmosphere escapes due to strong X-ray and UV radiation (XUV) from the host star:
FXUV = stellar XUV radiation hitting the planet
RXUV = effective radius where XUV is absorbed
M˙esc = mass-loss rate
Source: https://www.arxiv.org/pdf/2511.15903
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