In the search for extraterrestrial life, the presence of water is a key signature of possible habitability. Identifying why some planets are wet and others are dry could help with the hunt.
In a recent study published in Nature, scientists including Lawrence Livermore National Laboratory (LLNL) postdoctoral researcher Harrison Horn demonstrate a novel pathway for producing significant quantities of water on sub-Neptune-sized exoplanets. The results turn planet formation and migration theories on their head.
"Our experiments are the first to look at interactions between hydrogen and silicates at the pressure-temperature conditions expected at their interface in sub-Neptune exoplanets," said Horn. "We show that water does not need to come from further out in the solar system. It can be produced within a planet itself."
Sub-Neptune exoplanets orbit other stars and have radii 2-4 times that of Earth. Despite having no analogue in our solar system, they are among the most common planets in the galaxy. Observations by NASA's Kepler mission show many sub-Neptunes orbiting close to their stars. Models suggest these planets split into two categories: wet and dry.
But temperatures and conditions that close to a star usually lead to rocky and dry planets. For water to be present on close-in sub-Neptunes, scientists believed something else had to be at play. Either the planets formed further out in the solar system, where temperatures were cool enough for water to condense, then migrated inward, or far-out objects like asteroids and comets delivered water to the surface.
Instead, Horn and his coauthors explored an interaction that could take place on the planet itself. In the laboratory, they used a laser-heated diamond-anvil cell to recreate the conditions that would exist on dry sub-Neptunes at the boundary between their hydrogen atmosphere and magma core. At these high temperatures and high pressures, they showed that the molten silicate can react with hydrogen, releasing oxygen that in turn reacts with the hydrogen to produce a significant amount of water.
The amount of water varies depending on the conditions and composition of the sub-Neptune. The findings suggest that, rather than forming via separate mechanisms, dry sub-Neptunes could evolve into their wet counterparts.
"These results help further our understanding of how planets form, a rapidly growing field in the era of space- and ground-based telescope exoplanetary search efforts," said Horn.
