Liquid water is essential for life, and planets where it’s freely available on the surface are rare. But a new study indicated that subsurface oceans—thick layers of liquid water covered by a frozen surface—might be much more common than previously thought.
A group of researchers led by planetary scientist Lujendra Ojha of Rutgers University crunched the numbers to reveal that our galaxy is likely brimming with planets hosting subsurface oceans like those on Enceladus, Europa, and Ganymede.
“Before we started to consider this subsurface water, it was estimated that around one rocky planet [orbiting] every 100 stars would have liquid water,” Ojha said. “The new model shows that if the conditions are right, this could approach one planet per star.”
Heat from Within
The traditional, idealized definition of a star’s habitable zone hinges on the amount of stellar radiation heating a planet from afar. A planet must be at the right distance from its star so that incoming radiation is strong enough to keep water from freezing but does not make the environment so toasty that all of the water evaporates. These conditions create a narrow band around a star where scientists think life has the potential to develop.
But defining a more realistic, less cookie-cutter habitable zone is more complicated than that. Although Venus and Mars theoretically fall within the Sun’s radiative habitable zone, only Earth has the right atmospheric conditions to have liquid water on its surface.
The only other places in the solar system where liquid water exists for sure (some moons of Jupiter and Saturn) are much farther away from the habitable zone. On these satellites, geothermal heat powered by tidal forces or radioactive decay can melt the ice sheets from below, a process known as basal melting. Similar processes heat up Earth and other solar system bodies from within, so it’s relatively safe to assume that these heating mechanisms exist on exoplanets, too, the team argued.
“It doesn’t matter if the planet is very close to the star or very far away from the star; there are alternative ways of creating liquid water,” Ojha said. Even if a planet is out of the radiative habitable zone, “basal melting by geothermal heat is a perfect solution for it to be a habitable world,” he added.
Inspired by this phenomenon, the researchers created simulations of the conditions on Earth-sized planets orbiting red dwarfs, which comprise up to 70% of the Milky Way’s 100 billion stars. These stars aren’t very bright and have narrow radiative habitable zones, but most rocky planets the size of Earth have been found around them.
Ojha and his colleagues simulated the evolution of subsurface oceans under many different conditions, playing with such parameters as internal heat flux, incoming stellar radiation, size, surface gravity, and the depth of the ocean. They found that planets with higher surface temperatures, thicker ice sheets, and higher surface gravity are more likely to form subsurface oceans.
They also found that under the right conditions, planets with an inner heat flow as low as that of the Moon (about 10 times lower than Earth’s) can potentially host a subsurface ocean. Planets with surface temperatures as low as 200 K, such as Trappist-1 g, might have them too. Because core-dwelling, heat-producing radioactive elements decay over very long periods of time, these oceans are likely long-lived, lasting for billions of years, providing a stable environment for life to potentially evolve.
Formation mechanisms get a bit tricky for extremely deep oceans, the team found. When pressure at depth exceeds 200 megapascals, roughly double the pressure at the deepest points in Earth’s oceans, the crystalline structure of ice changes, forming ice that is denser than liquid water and can therefore sink.
Even under these conditions, however, basal melting can produce liquid water sandwiched between the top and bottom ice layers.
“When you take these concepts and extrapolate [them] to exoplanets and do the physics, it turns out that it’s actually quite easy to create what we call geothermically sustained liquid water,” Ojha said.
The team presented these results at the Goldschmidt 2023 conference in Lyon, France, in July. The research on which the presentation was based was published in Nature Communications in 2022.
Redefining the Habitable Zone
Other scientists agreed that although subsurface oceans are likely to exist on exoplanets, the tougher question is whether these oceans can be habitable. “Life requires a complete biogeochemical cycle to replenish nutrients,” said Amy Barr Mlinar, a planetary scientist at the Planetary Science Institute and chief academic officer at the Summer Science Program, who wasn’t involved in the study. “Even if the planets have an ocean, they would still require some geological activity to bring nutrients from the surface or to periodically supply nutrients from the rock-water interface.”
Ojha plans to continue with more modeling work to determine whether subsurface oceans are likely in the inner part of the habitable zone, where planets are expected to be too irradiated to preserve any surface water. However, these planets would also tend to be tidally locked, meaning that the same side of the planet always faces the star. On the farside of these planets, temperatures could be frigid, and depending on how heat is transported from the warm side, water could freeze and form subsurface oceans as well.
At the same time, future observations could help scientists determine whether they are on the right track. “It would be interesting to see what constraints, if any, the James Webb Space Telescope could provide in support [of] or against the notion of subglacial oceans on exoplanets,” Ojha said.
—Javier Barbuzano (@javibarbuzano), Science Writer