Presentation #216.01 in the session “Exoplanets and Systems: Giant Planet Atmospheres 3”.
Hydrogen-rich atmospheres are common for giant planets. However, some recently discovered giant exoplanets, such as NGTS-4b and TOI-849b, appear to have very thin atmospheres. For the smaller sub-Neptune class, the population appears to decrease sharply at 3R(Earth), where R(Earth) is Earth’s radius. A modeling study suggested a possible role of the interior for the “radius cliff” (Kite et al, 2019). In these planets, hydrogen is in contact with the core at high pressure-temperature (P-T) conditions. In H2O-rich giant planets, reaction between metallic iron and water would form Fe-H alloys: 3Fe + H2O = 2FeH + FeO, according to high-pressure experiments. Therefore, it is important to understand alloying behavior between Fe and H for a wide range of planet types. Recent experiments have shown that H solubility in Fe increases dramatically at high pressure through formation of polyhydrides, reaching H/Fe = 5 in molar ratio. However, stability of the iron polyhydrides remains uncertain at the high temperatures relevant for planetary interiors. Instead, close-packed Fe-H alloys could be more stable at high temperatures. It has been thought, however, that H/Fe of close-packed Fe-H alloy is limited to 1. Through a series of high P-T experiments and density functional theory calculations, we found that Ni promotes the solubility of H substantially for the close-packed Fe metal at pressures above 30 GPa and temperatures over 2700 K, reaching H/Fe = 1.5-2.0. We also found that polyhydrides become stable at high temperatures with further compression above 80 GPa. Owing to more efficient packing, planets with H-storing core will have much smaller radii than ones with H degassed to the atmosphere, affecting the mass-radius relations substantially. This H ingassing and core storage would also be important for understanding the “dilute core” of Jupiter, the “radius cliff” in the sub-Neptune population at 3R(Earth), and some massive dense exoplanets with thin H atmosphere.