👽 Possible life even beyond the famous "habitable zone"

In the search for extraterrestrial life, astronomers have focused on a precise orbital band around stars, where temperatures allow water to remain liquid on the surface. This approach has guided the study of exoplanets, but a new concept is now emerging, vastly expanding the panorama.

A study recently published in The Astrophysical Journal proposes a deep revision of these conventional boundaries. Astrophysicist Amri Wandel and his team used climate models to examine how heat is distributed on planets under extreme conditions. Their calculations reveal that the presence of liquid water does not depend solely on distance from the star, but also on internal or atmospheric mechanisms capable of warming areas previously considered inhospitable.


Take, for example, tidally locked planets, which always show the same face to their star. One hemisphere is in perpetual daylight, the other in eternal night. It was thought these worlds were too hot on one side and too cold on the other to host liquid water. Yet, models indicate that the heat accumulated on the day side can be transported by the atmosphere to the night side.

This could maintain local temperatures above freezing, and thus liquid water, even on planets orbiting very close to their star.

Similarly, planets located well beyond the classic habitable zone, where it is too cold for surface water, could hide liquid reservoirs beneath thick layers of ice. The internal heat of these worlds or melting at the base of ice caps could create lakes beneath the frozen surface.

These isolated environments would offer stable conditions, shielded from stellar radiation, and would multiply the number of potential candidates for hosting life forms.

This expanded vision finds an interesting echo with the latest observations from the James Webb Space Telescope. It detected water vapor in the atmosphere of some hot exoplanets orbiting red dwarfs—worlds that older models placed outside the habitable zone. These discoveries support the idea that water can persist under conditions previously considered too extreme.


By rethinking the criteria for the habitable zone, this study therefore encourages broadening the field of investigation. Worlds once ruled out could ultimately deserve careful examination. This does not guarantee the presence of life, but means the places to look for it are likely far more numerous than previously thought.

Subsurface oceans in the Solar System and beyond

The principle that liquid water could exist beneath a thick layer of ice finds examples in our own Solar System. Moons such as Europa, around Jupiter, or Enceladus, around Saturn, are striking examples. The heat generated by tidal forces, due to the gravitational pull of the giant planet they orbit, keeps their internal oceans liquid beneath an icy crust.

This mechanism can apply to many cold exoplanets located far from their star. Even if the surface is frozen, internal heat from the planet's formation or from the decay of radioactive elements in its core could be enough to melt ice at depth. Thus, vast reservoirs of liquid water could form, isolated from the external stellar environment.

These subsurface oceans represent potentially stable environments over very long periods. Shielded from harmful stellar radiation and extreme temperature variations, they could even harbor, near potential hydrothermal vents, conditions conducive to the emergence of biological reactions.

The search for such worlds relies on indirect techniques, such as analyzing surface composition. The detection of water vapor geysers, similar to those observed on Enceladus, would thus be a major clue to the presence of such a hidden ocean on a distant exoplanet.