Gliese 9827 d and TOI-238 b reveal secrets of mini-Neptune atmospheres

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

This study investigates the atmospheres of warm and hot mini-Neptunes, specifically Gliese 9827 d and TOI-238 b, which may have water- or metal-rich compositions. It examines how these compositions affect atmospheric photochemistry and mass loss rates, finding that higher water/metal content can significantly reduce atmospheric escape, making these atmospheres more stable over time.

Using the CHAIN model, researchers simulated the upper atmospheres of these sub-Neptunes, considering a range of compositions from hydrogen/helium with solar metallicity to highly water- and metal-enriched scenarios. This allowed them to understand how increasing water and metal fractions alter atmospheric processes.

They found that more water and metals increase both atmospheric cooling (due to metal line cooling and molecular cooling processes) and heating (metal line heating in dense atmospheric layers and metal ions). This complex interplay of heating and cooling mechanisms is key to understanding atmospheric stability.

The research suggests that for atmospheres with high water or metal fractions (water mass fractions of >=50% or metal enrichment over 100 times solar), atmospheric mass loss is significantly reduced. This indicates that such atmospheres are more stable and can retain their composition for longer periods. Furthermore, due to the preferential escape of hydrogen, the atmospheric metal-to-hydrogen ratio is expected to increase significantly over time, leading to an earlier cessation of hydrodynamic escape.

Key findings
- Increased water/metal content enhances atmospheric cooling and heating rates.
- High water/metal fractions significantly reduce atmospheric mass loss.
- Water-rich atmospheres generally show lower escape rates than H/He atmospheres with the same mean particle weight.
- Atmospheres with a high initial water fraction are more stable.
- Preferential hydrogen escape increases the atmospheric metal-to-hydrogen ratio, leading to earlier cessation of hydrodynamic escape.

Scientific study

Escape of Water- and Metal-enriched Atmospheres from compact Hot mini-Neptunes with CHAIN (external link)