Exoplanets are often considered “potentially habitable” if there is the possibility of liquid water on their surfaces, but in reality, too much water may make it impossible for life to persist.
“On Earth, the oceans are in contact with rock. If there is too much water, high-pressure ice forms beneath the oceans, separating them from the rocky interior of the planet,” said Caroline Dohn, a geophysicist at ETH Zurich, who led the new study of exoplanet interiors.
This high-pressure ice prevents the exchange of minerals and compounds between the rocks and the water. In theory, that should make the oceans barren and lifeless. But Dorn’s team argues that even exoplanets with enough water to form such high-pressure ice could still host life if most of that water wasn’t stored in the surface oceans, but much deeper in the planet’s core. Water in the core can’t support life; it’s not even in molecular form there. But it does mean that a significant portion of the planet’s water isn’t on the surface, which makes the surface oceans a bit shallower and prevents high-pressure ice from forming at their bottom.
Young people of the planet
“If you look at the exoplanet community three to five years ago, everyone was [water] “Water only exists on the surface of planets,” Dorn says. In the absence of evidence to the contrary, scientists have simply assumed that exoplanets are built like Earth, with water mostly in oceans on the surface, and some water — about 40 million cubic kilometers — held deep in the crust. But in 2020, a team of scientists from University College London discovered that water is actually a part of the surface ocean. study They argue that the Earth was never designed that way in the first place.
Instead, a 2020 study argued that most of the water on Earth isn’t in the oceans or crust, but in the Earth’s core, which contains 30 to 37 times more water than all of the Earth’s surface oceans combined. “When the Earth is very young and hot, you have a magma soup where everything mixes together. The Earth’s mantle contains silicates, but it also has iron droplets that eventually sink and form the core,” Dorn said.
Some of the water in this magma soup will bind with silicates and eventually reach the surface ocean, while some will remain with the iron, which will sink to the core along with it. At the extremely high temperatures and pressures of the newborn planet, iron can bind about 70 times more water than silicates.
As a rule of thumb, if a planet is massive, most of its water will travel down to the core and stay there — and this could be a good thing for our hopes of finding life in the universe.
Give life a chance
Despite the great precision of modern instruments, including the James Webb Telescope, the only way we can infer an exoplanet’s water balance is through an indirect clue: its bulk density, calculated from our best estimates of its mass and radius.
Before Dorn’s work, whenever we encountered an exoplanet with exceptionally high water abundance, we assumed that the water was in a surface ocean, meaning that the ocean was very deep and under unusually high pressure, because the hard data we had only told us which parts of the planet were made of water, but nothing about its distribution.
“Now that water is thought to be stored in the core, there could be 10 times as much water on Earth before we reach these high pressures — an order of magnitude,” Dorn said. Most of the water sinks to the core along with the iron, leaving only a small amount near the surface to form the oceans we know and reduce pressure below the seafloor.
This means the pool of potentially habitable planets has increased significantly. Still, finding conclusive evidence that any of these planets could host life remains a major challenge.
