How sub-Neptunes turn into water worlds by losing their hydrogen

Star system: K2-135 | Distance from the Sun: 96.7 light years

A new study using the BOREAS model reveals how sub-Neptune planets can transform their atmospheres over hundreds of millions of years. By analyzing how different elements escape into space, researchers found that lighter hydrogen is often stripped away while heavier oxygen remains trapped. This process, known as chemical fractionation, can gradually turn a gas-poor envelope into a water-rich atmosphere. The findings suggest that planets like GJ 9827 d may have started with very different compositions, evolving into the water-rich worlds we observe today due to the intense radiation from their host stars.

Scientific study

Oxygen left behind: Atmospheric Enrichment due to Fractionation in Sub-Neptunes using BOREAS (external link)