A planet is habitable (lat.: habitare, “live”), if it can sustain life for a long period of time. Such a planet requires:
enough energy for metabolism and reproduction. The energy either can be harvested from the sun by photosynthesis, or can be used from other sources (e.g. thermal gradients or tidal forces).
a liquid. Life as we know it is based on liquid water, but there are speculations about other liquids, which may have similar functions.
an element of which complex molecules can be build (in our case, this is carbon). If life could also be based on similar elements is speculative.
To detect life on other planets (especially beyond our solar system), we first concentrate on the planetary surface. To do that, we start from two directions: On one hand, we search for direct and indirect hints (biosignatires) and proofs (biomarkers) for existing life, like oxygen or ozone in the atmosphere. To select particular planets (out of billions in the milky way) for the investigation, we categorize them by their size, distance to their star and its luminosity on the other hand. A small range of temperature on a planetary surface is necessary to keep water liquid. Therefore, we concentrate on planets in a certain distance to their star (in the “habitable zone” of the star). Water on a planet closer to the star would vaporize, water on a planet further away from the star would freeze. The habitable zone of hotter stars is further away from the star than the habitable zone of cooler stars. This estimation of course is very vague, because greenhouse gases can impact the outer boundary of the habitable zone strongly.
Which other properties influence the planetary habitability? What about plate tectonics, a magnetic field, or a moon? Are important properties more likely on small or big, on cold or hot planets? The answers to these questions help us to select the best planets for searching for extraterrestrial life.