Britannica Sources used in this answer

Why can a world outside a stellar habitable zone still possibly support life?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

A world located outside a stellar habitable zone can potentially support life because surface temperature is not solely determined by proximity to a star. Other factors, such as atmospheric composition, reflectivity, and heat circulation, play critical roles in maintaining conditions suitable for liquid water.1

Even in the absence of ideal stellar positioning, internal energy sources can provide the necessary heat to maintain liquid water, either on the surface or in subsurface reservoirs. These sources include:1

Radioactive decay: The decay of radioactive elements within a planet or moon’s interior serves as a significant source of internal heat.12
Tidal heating: Gravitational interactions with other celestial bodies can generate substantial heat through tidal friction.134

These mechanisms allow for the existence of subsurface biospheres, such as those hypothesized for Jupiter's moon Europa or potentially even rogue planets that wander through space without a host star. In the case of rogue planets, a thick atmosphere could further trap heat generated by these internal processes, potentially creating a habitable environment.3