Hydrogen May Push Some Exoplanets off a Cliff
https://eos.org/articles/hydrogen-may-push-some-exoplanets-off-a-cliff]
Excerpt: With the discovery of more than 5,000 confirmed exoplanets, astronomers understand more and more about what kinds of planets exist and why. But the data deluge has also thrown into relief the kinds of planets that don’t seem to exist. In particular, there is a steep decrease in the abundance of planets larger than approximately 3 Earth radii, a pattern nicknamed the “radius cliff.” ...new research published in the Planetary Science Journal has shown how high-pressure, high-temperature chemical reactions might put a cap on planet growth. ...Missions like NASA’s Kepler and Transiting Exoplanet Survey Satellite have revealed a curious mystery: Planets 3 times Earth’s size are about 10 times more abundant than planets that are only slightly larger. ...In a first-of-its-kind experiment, researchers placed a thin foil of pressed metal oxides into tiny presses called diamond anvil cells ...to study what happens under the high-temperature, high-pressure conditions expected at the atmosphere–rocky core interface on sub-Neptunes. ...hydrogen not only freed iron from its oxides but also reacted with it, forming an alloy. ...That ability to sequester hydrogen, mostly as the iron-hydrogen alloy sinking into the metallic part of the core, could limit growth of an exoplanet’s atmosphere, resulting in the observed radius cliff....