What defines a planet's 'habitable zone'?

A planet's 'habitable zone' is defined as the region around a star where conditions are just right for liquid water to exist on a planet's surface. This zone is often referred to as the "Goldilocks Zone," as it is neither too hot nor too cold, allowing for stable temperatures that can support liquid water—a critical ingredient for life as we know it.

The proximity of a planet to its star significantly influences its surface temperature and, consequently, the potential for water to remain liquid. If a planet is too close to the star, the intense heat may cause any water present to evaporate, while if it is too far, the cold could freeze water into ice. The 'habitable zone' is, therefore, a key concept in astrobiology and the search for extraterrestrial life, as the presence of liquid water is often seen as a primary indicator of a planet's ability to support life.

In contrast, options related to moons, extreme temperatures, or the existence of gas planets do not define the criteria for a habitable zone, as they do not focus on the conditions necessary for liquid water or the potential for life. These factors may influence a planet’s environment but are not part of the core definition of the habitable