How do internal energy sources on icy moons expand the definition of habitability zones?

They create habitable zones independent of the star's energy output

The discovery of potentially habitable subsurface oceans on icy moons like Europa and Enceladus significantly broadens the criteria used for locating life-bearing worlds. While exoplanet searches typically prioritize the Habitable Zone defined by stellar radiation, these moons demonstrate that internal energy sources, derived from tidal heating, can maintain liquid water far outside that zone. This suggests that habitable environments are not exclusively dependent on the amount of energy received from the host star, greatly increasing the statistical probability of life existing throughout the galaxy.