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Why can a planet outside a star’s habitable zone still contain liquid water underground?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

A planet located outside a star’s habitable zone can still maintain liquid water underground due to internal energy sources that generate heat independently of stellar radiation.1

These internal heat sources include:

Radioactive decay: The natural breakdown of radioactive elements within a planet’s interior can provide sufficient thermal energy to maintain subsurface liquid reservoirs.12
Tidal heating: Gravitational interactions between a planet and its host star, or between a planet and its moons, can generate significant heat through tidal friction.132

These mechanisms allow for the existence of subsurface liquid water even on worlds that are otherwise too cold for surface water to exist. For example, several moons in our solar system, such as Jupiter’s moon Europa, are believed to harbor long-lived liquid oceans beneath their icy surfaces due to these gravitational forces. Similarly, it has been conjectured that rogue planets—which wander through space without orbiting a star—could potentially sustain liquid water through a combination of internal radioactive heating and insulating atmospheres.123