Beneath the frozen crusts of distant icy worlds, liquid oceans may persist and even harbor life, well outside what we traditionally call the habitable zone. An exploration of this idea appeared in Nature Communications, expanding our picture of where life might exist in the cosmos.
Historically, astrobiologists define a habitable zone as the sweet spot around a star where a planet could keep liquid water on its surface. The core condition for life as we know it is the presence of liquid water. The zone must be warm enough to melt ice but not so hot that water boils away. That balance creates a fairly narrow corridor, and many Earth-like planets likely lie beyond it in a freezing outer region.
Researchers from Rutgers University conducted calculations suggesting that planets outside the classic warm belt could still host liquid water oceans. In their view, heat can originate not from stellar radiation but from internal processes. Geothermal energy, amplified by the rocky interior and high-pressure environments, can melt subsurface ice and create pockets where oceans persist. Crucially, atmospheric disposal of heat becomes less significant in these scenarios, allowing water to remain in liquid form even when surface temperatures would seem hostile. The study notes that even a modest geothermal flow, akin to what powers lunar volcanism on our Moon, might suffice to sustain such oceans over long timescales.
These hidden seas are not isolated from chemistry. Interactions between seawater and surrounding rocks drive chemical exchanges that could supply nutrients essential to hypothetical ecosystems. Simultaneously, the oceans would be shielded from harmful radiation by thick ice layers and the overlying crust, providing another potential safeguard for life. The dynamics become more intricate on super-Earths, where stronger gravity alters pressure regimes and expands the array of water ice phases. At high pressures, water can adopt numerous crystalline forms, which can assemble into layered structures that trap and preserve liquid water while chemically communicating with the rocky interior. This complexity widens the possible habitability conditions beyond what is familiar on our own planet. [Nature Communications]
The implications for red dwarf systems are especially noteworthy. In those systems, the classical habitable zone hugs close to the star, exposing nearby planets to intense tidal forces and stronger stellar winds. Such factors could strip atmospheres or destabilize climates, limiting surface habitability. Yet if subsurface oceans exist beyond the conventional zone, life could still thrive in environments shaped by internal heating and high-pressure ice. The idea invites a broader search strategy for astrophysicists who seek biosignatures and planetary histories in regions once dismissed as too cold or too hostile. The ongoing discussion emphasizes that habitable conditions may be more diverse than previously imagined, opening new avenues for observing missions and theoretical work aimed at identifying worlds where life might persist under ice or beneath hidden oceans. [Nature Communications]