Final answer:
The habitable zone is where liquid water could exist on a planet's surface, increasing the likelihood of life. However, being in this zone is not alone sufficient for habitability.
Step-by-step explanation:
The habitable zone around a star is defined as the region where liquid water can potentially exist on the surface of terrestrial-sized planets. This zone is signified by the ideal range of distances from a star where the temperature is such that water can remain in liquid form, which is essential for life as we know it. Being in the habitable zone, however, does not guarantee a planet's habitability, as other conditions must also be met, such as having an atmosphere capable of supporting life.
A critical factor in the evolution of the habitable zone is the changing luminosity of a star over its lifetime. For instance, as the Sun's power output has increased over the past 4 billion years, the habitable zone has gradually moved outward, leading to the concept of the continuously habitable zone. This narrower zone is where conditions remain suitable for life over the star's life span. Additionally, modern technology has allowed for the discovery of exoplanets within their star's habitable zone, significantly improving our chances of finding potentially habitable worlds.