Researchers from Rutgers University in the United States, presenting at the Goldschmidt Geochemistry Conference in Lyon, explored the surprising possibility of terrestrial-like exoplanets that harbor groundwater beneath their icy mantles. The study was published in Nature Communications, signaling a notable advance in our understanding of where water might persist in distant worlds.
These exoplanets would orbit red dwarf stars and lie outside the conventional habitable zone. Yet, they could sustain subsurface oceans driven by internal geothermal heat. Across the field, it has long been thought that water should reside only on planets positioned within the habitable zone. When a planet lies beyond that region, surface liquid water would be unlikely. Nevertheless, there are notable exceptions observed within our own solar system: icy moons circling Jupiter and Saturn maintain subsurface oceans despite being outside the classical habitable zone. The researchers propose a similar mechanism could operate for planets orbiting red dwarfs, given a sufficiently generous distance and internal heat production.
To investigate this possibility, the team modeled the long term evolution of ice layers on exo-Earths to determine the conditions under which liquid water could persist at temperatures above the freezing point of ordinary ice. The results indicate that oceans can emerge at the base of thick ice sheets, even when geothermal activity is only moderate. Over billions of years, the slow decay of heat-producing radioactive elements can sustain enough internal warmth to keep pockets of liquid water from refreezing. This finding helps explain how a planet can host liquid water well beyond the traditional habitable zone.
The study’s conclusions emphasize that subglacial oceans, in contact with a planet’s rocky crust and shielded from intense stellar radiation by a deep ice cover, could create environmental niches potentially suitable for life. The boundary conditions that permit such oceans involve a delicate balance between geothermal heat flux, ice thickness, planetary radius, and the distribution of insulating layers that protect liquid water from harsh space radiation and surface temperature swings. In this framework, the presence of liquid water is not tied strictly to proximity to a star but rather to the planet’s internal heat budget and structural geology.
As the researchers note, the initial heat sources for newly formed planets often include radioactive isotopes with long half-lives, such as potassium-40, thorium-232, and uranium-238 and 235. These isotopes slowly release heat over geological timescales, helping to sustain subsurface oceans long after the planet’s formation. This insight broadens the spectrum of worlds that could potentially support life, extending beyond the classic habitable zone to a more nuanced view of habitability grounded in interior planetary physics and thermal history.
Overall, the work advances a compelling scenario in which life-supporting environments may exist beneath icy crusts on distant worlds. By highlighting the viability of long‑lasting subglacial oceans, the study invites a broader search for life in planetary systems around red dwarfs. It also challenges conventional wisdom about where water and, by extension, biosignatures might occur, prompting future observations and modeling efforts to test these ideas against real exoplanet data. The implications reach into the design of observational campaigns and the interpretation of signals from icy, geothermally heated planets that lie outside the traditional habitable zone, offering a fresh lens on the diverse architectures of planetary systems in our galaxy.