Astronomers have long searched for life within a rather narrow ring around a star, the “habitable zone,” where a planet should be neither too hot nor too cold for liquid water. A new study argues that this ring is too strict: on tidally locked worlds that keep one face in daylight and the other in permanent night, heat may still circulate enough for liquid water to persist on the dark side, even when the planet orbits closer to cool M- and K-dwarf stars than conservative climate models allow. The study also points outward: liquid water could exist far beyond the classical outer edge, hidden beneath ice as subglacial or intraglacial lakes, meaning the number of worlds worth checking for water, and potentially life-friendly conditions, may be much larger than the traditional map suggests.

For years, astronomers have relied on a simple rule of thumb when searching for life beyond Earth: look for planets in the “habitable zone,” the narrow range around a star where liquid water can exist on a planet’s surface. In our own solar system, that zone lies roughly between the orbits of Earth and Mars.

But many of the planets now being discovered do not fit neatly into this framework, orbiting stars quite different than our sun, at distances closer than the inner edge of the habitable zone or further out.

In a new study published in The Astrophysical Journal astrophysicist Prof. Amri Wandel from the Hebrew University asks what happens when scientists break the assumptions built into traditional habitability models.

The focus is on tidally locked exoplanets, worlds that always face their star with the same hemisphere. These planets experience permanent daylight on one side and permanent night on the other, a configuration often considered to challenge surface liquid water and life.

Wandel’s analysis suggests otherwise.

Using an analytical climate model that tracks temperature across the surface of such planets, the study shows that worlds orbiting M- and K-dwarf stars could sustain liquid water on their night side, even when they orbit significantly closer to their star than classical habitable-zone models allow. Heat transported from the day side can keep parts of the night side above freezing, expanding the range of environments where water may persist.

This extended definition of the habitable zone may help explain recent observations by the James Webb Space Telescope, which detected water vapor and other volatile gases in the atmospheres of warm, close-in Super-Earths orbiting M-dwarf stars—planets previously thought to lie outside the safe range for surface water.

The study also looks in the opposite direction, beyond the outer edge of the habitable zone. Even on cold planets far from their stars, liquid water could exist beneath thick ice layers, in the form of intraglacial lakes or subglacial melting, further widening the habitable zone and enhancing the number of worlds that may support water-based environments by a large factor.

By revisiting the assumptions behind the habitable zone and recalculating its boundaries, this research reframes where astronomers might look for conditions suitable for life, suggesting that potential habitats may exist on planets once ruled out.