What makes a planet habitable?

The answer

The 'habitable zone' is the region around a star where liquid water could exist on a planet's surface, but habitability requires much more. A planet needs a protective magnetic field to shield against stellar radiation, plate tectonics to recycle carbon and regulate climate, a substantial atmosphere to maintain surface pressure and temperature, and likely a large moon to stabilize its axial tilt. Even the host star matters - red dwarfs may tidally lock planets, leaving one side in permanent darkness. The more we learn, the more we appreciate how many conditions must align for a world to support life.