Sub-Neptunes, or exoplanets 2–4 times Earth’s radius, are abundant in our galaxy. Models indicate that these exoplanets have rocky cores (the non-volatile interior) blanketed by envelopes of either hydrogen (dry gas dwarfs) or water (water worlds).
In our own solar system, the water worlds of Uranus and Neptune orbit far from the sun, where temperatures are low enough for water to condense. This has led to the idea that water-rich planets form in the outer orbits of planetary systems, beyond what is known as the snow or ice line. They may then migrate inwards, to orbit closer to their star.
In recent years, however, large numbers of potentially water-rich exoplanets have been discovered in very close orbits. This is difficult to reconcile with the idea that such worlds can only form beyond the snow line.
The latest research by scientists from Arizona State University, The University of Chicago and the Open University of Israel suggests that water could be produced through chemical reactions at the boundary between a dry planet’s rocky core and hydrogen-rich atmosphere. This finding calls into question the idea that a planet’s composition is linked to where it formed.
Researchers used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. Their results were published in the journal Nature.
To explore the potential high pressure and temperature interactions between the hydrogen in the envelope and silicate in the core of dry planets at the core-envelope boundary, the team used the unique capabilities of the University of Chicago’s GeoSoilEnviroCARS beamline at 13-ID-D of the APS. This beamline’s high pressure, high temperature diamond anvil cell setup is designed to probe materials in-situ at extreme conditions to answer geochemical and geophysical questions across the pressure and temperature range of Earth and other planets.
Read more on the APS website
Image: The high pressure, high temperature diamond anvil cell experiments suggest that reactions between dense hydrogen fluid and molten silicates on dry planets could generate substantial amounts of water. This hints at a potential way for dry, hydrogen-rich planets to evolve into watery worlds, challenging conventional planetary formation theory.