Without water, life is highly unlikely, as far as scientists understand it. The entire idea of a habitable zone around other stars is based on a planet's potential to have liquid water on its surface. But where does this water come from?
One explanation is migration. The idea is that planets formed in one region of the protoplanetary disk where water was present, then migrated to their current positions in habitable zones.
When it comes to Earth, the idea that Earth formed first then received its water later has been paramount. Bodies like comets and asteroids from the colder regions of the Solar System delivered water ice to Earth, leading to its eventual habitability. That idea has been challenged in recent years, with research showing that Earth already had the necessary hydrogen and oxygen and that the planet formed its own water.
New research examined this idea in terms of sub-Neptunes, a very numerous type of exoplanet. It's titled "Building wet planets through high-pressure magma–hydrogen reactions," and it's published in the journal Nature. The lead author is Harrison Horn, a postdoctoral researcher at the Lawrence Livermore National Laboratory.
Sub-Neptunes range from about 1.5 to 4 times the radius of Earth. They have substantial hydrogen and helium-rich atmospheres surrounding rocky cores, much different from Earth, with its relatively small atmosphere. Our Solar System doesn't have any sub-Neptunes, but they're common elsewhere. "Close-in transiting sub-Neptunes are abundant in our Galaxy," the researchers write in their article.
Many of these worlds are found very close to their stars. Models based on these planets show that sub-Neptunes fall into two categories. In one category, the planets contain a discernible amount of hydrogen, and they're called dry planets. In the other, they contain a discernible amount of water, called wet planets. In each case, the hydrogen or water surrounds a rocky, metallic core. "Water-rich sub-Neptunes have been believed to form farther from the star and then migrate inwards to their present orbits," the researchers explain.
But maybe that's not accurate. Maybe neither migration nor delivery is required.
Water is not exactly an extraordinary molecule. It's just hydrogen, the most abundant element in the entire Universe, and oxygen, which is the third most abundant element in the Universe, by mass. Oxygen is a swinger; it likes to combine with all sorts of other elements. It's abundant in Earth's crust where it makes up 46% of its mass and is present in silicate minerals and minerals containing various other metals. It's also abundant elsewhere in the Solar System, like on Mars and Venus.
Oxygen should be widespread on other rocky planets, too, including sub-Neptunes. If it is, and if those planets have hydrogen atmospheres, there could be a pathway for water to form. In this new research, Horn and his co-researchers turned to laboratory experiments to find out.
“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,” lead author Horn said in a press release. “We show that water does not need to come from further out in the solar system. It can be produced within a planet itself.”
In their experiments, Horn and his fellow researchers recreated the conditions at the boundary between the hydrogen atmospheres and molten cores of sub-Neptunes. "In a sub-Neptune planet with a rocky core and a substantial amount of hydrogen, the CEB (core-envelope boundary) is the most likely region to experience reactions between dense fluid hydrogen and silicate melt," the authors write. Temperatures and pressures there are extremely high, and they recreated them using a laser-heated diamond anvil cell. These devices use a pair of diamonds to subject small samples to extreme pressures, and the laser provides the heat.
The experiments showed that the molten silicate can react with hydrogen. The reaction releases oxygen that combines with hydrogen to produce water.
"Here we report experimental evidence of reactions between warm, dense hydrogen fluid and silicate melt that release silicon from the magma to form alloys and hydrides at high pressures," the researchers write. "We found that oxygen liberated from the silicate melt reacts with hydrogen, producing an appreciable amount of water up to a few tens of weight per cent."
Sub-Neptunes aren't all exactly the same, so the amount of water produced will depend on both their individual compositions and their conditions. But the results show that wet and dry Neptunes may not form through different mechanisms. Instead, dry Neptunes can transition into wet ones.
"These reactions can generate a spectrum of water contents in hydrogen-rich planets, with the potential to reach water-rich compositions for some sub-Neptunes, implying an evolutionary relationship between hydrogen-rich and water-rich planets," the researchers explain. "Therefore, detection of a large amount of water in exoplanet atmospheres may not be the optimal evidence for planet migration in the protoplanetary disk, calling into question the assumed link between composition and planet formation location."
What's more, these reactions could persist for billions of years and create vast volumes of water. According to the researchers' calculations, "for a 5ME rocky planet with a 5 wt% H2 envelope, the temperature of the core remains sufficiently high to maintain the molten state of silicates for billions of years, which could, in turn, make the continuation of the observed reactions possible for billions of years."
The reactions could also extend well below the interface. "Moreover, because pressure greatly enhances H2 solubility in magma and can result in miscibility between hydrogen fluid and oxide melts24, hydrogen can reach greater depths below the CEB (core-envelope boundary)," the authors explain in their research. "Vigorous convection in the molten core could also transport hydrogen further to greater depths. For all of these reasons, the reactions discussed can continue into the deep interior."
There are many variables involved. The exact composition of the planet makes a huge difference. Not just in terms of how much oxygen and hydrogen it contains, but in terms of other chemicals involved in the reactions, like sulphur, silica, and magnesium. The Mg:Si ratio is particularly important, and the authors show that changes in that ratio can boost water production by as much as 100%. "Therefore, the large variation in Mg:Si ratios observed in exoplanetary systems will result in variation in endogenic water production among planets with rocky interiors and hydrogen-dominated atmospheres," the authors explain.
Conventional planet formation theory says that water-rich planets form outside of their solar systems' snow lines. But there's growing observational evidence that water-rich sub-Neptunes exist in orbits close to their stars. Migration scenarios have been employed to explain these planets, but they may not be necessary. "Endogenic water production through hydrogen–magma reactions observed in our experiments provides a straightforward process to build water-rich sub-Neptunes inside the snow line," the authors write in their conclusion.
"Our new experimental findings challenge the assumed link between planet formation location and composition," the researchers conclude.
“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.