What is the crucial condition defined by a planet orbiting within its star's Habitable Zone (HZ)?
The range of orbits where liquid water could be maintained on the surface.
The Habitable Zone, often referred to as the Goldilocks zone, defines a specific orbital band around a star where the thermal energy received permits water to exist stably in its liquid state on the planet's surface. This is paramount because liquid water functions as an essential solvent, a medium required to facilitate the complex chemical reactions that underpin known biological processes. If a planet orbits too close, the intensity of stellar radiation causes all water to vaporize, leading to a runaway greenhouse effect, similar to Venus. If the orbit is too distant, the resultant low temperatures cause water to freeze entirely into solid ice, inhibiting surface biology.
