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How can radioactive decay warm a planet outside its star’s habitable zone?

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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.

Radioactive decay warms a planet outside its star’s habitable zone by generating internal heat through the spontaneous disintegration of radioactive isotopes within the planet's interior.12

This process involves the decay of elements such as uranium-238, thorium-232, and potassium-40. As these nuclei decay, they release kinetic energy and radiation that is converted into heat. For planets located far from a star—or rogue planets that wander freely through space—this internal heat can be significant enough to maintain subsurface reservoirs of liquid water or, under specific conditions, potentially support liquid water on the surface.213

When combined with a thick atmosphere—such as one rich in carbon dioxide that traps heat—this internal energy source may create conditions suitable for life, even in the absence of stellar radiation.13