How space works
Habitable zones: what the label tells us
A habitable zone describes a range of possible surface-water conditions around a star. It does not certify that a planet has water, air, or life.
Updated
The question starts with liquid surface water
In the usual stellar habitable-zone definition, researchers ask where a rocky planet with a suitable atmosphere could maintain liquid water on its surface. The answer depends on models of atmospheric greenhouse warming, clouds, reflectivity, and the star's light. Different assumptions produce different inner and outer boundaries.
Earth provides one example of an inhabited planet, not a universal climate template. Venus and Mars show how neighboring rocky worlds can follow different paths. A position between chosen model boundaries is a reason to study a planet more carefully. It is not a direct observation of an ocean, and it is not proof that a world could support human visitors.
The star changes the relevant distance
A dim red dwarf's habitable zone lies closer to the star than the Sun's. A more luminous star can warm a planet farther away. Irradiation compares the starlight received per unit area, often expressed relative to Earth. It is more useful than orbital distance alone for a first comparison of planetary energy input.
Stars also differ in spectrum, activity, and history. A planet close to a red dwarf may have experienced strong early irradiation, flares, and tidal effects. Those processes can alter its atmosphere and water inventory. Two planets receiving similar total energy today need not have had similar pasts or similar atmospheric escape rates.
Temperature needs its assumptions attached
An equilibrium temperature estimates the radiation balance under specified assumptions. It depends on the amount of absorbed starlight and on how efficiently energy is redistributed. It usually leaves out greenhouse warming. The resulting number is not a thermometer reading from a planet's ground.
A planet's measured atmospheric emission, cloud-top brightness temperature, equilibrium estimate, and surface temperature are different quantities. If the archive supplies an equilibrium temperature, the Codex labels it accordingly. A familiar-looking number near Earth's temperature should not be promoted into a claim of mild surface conditions without evidence about pressure, composition, and circulation.
Habitability and inhabited are separate questions
A world might have conditions compatible with some form of life yet have no life. Detecting a potentially biological gas would require checking nonbiological explanations, the host star, atmospheric chemistry, and the measurement itself. Strong conclusions need converging evidence.
Subsurface oceans offer another complication: liquid water can exist below ice even far outside a star's conventional surface-water zone. Europa and Enceladus are important examples of worlds whose interiors receive additional energy. A habitable-zone map therefore answers one specific climate question. It does not provide a complete inventory of places where water or life might occur.