What makes a planet capable of supporting life?

A planet needs several key conditions to support life as we know it. Liquid water is considered essential, which requires the planet to orbit within its star's habitable zone where temperatures allow water to exist on the surface. The planet also needs a stable atmosphere thick enough to regulate temperature and shield against harmful radiation, along with a magnetic field to deflect solar wind. Other important factors include a stable orbit, a rocky surface or ocean, the presence of carbon and other organic chemistry building blocks, and a relatively stable star that doesn't bombard the planet with lethal flares. Internal geological activity also helps cycle nutrients and regulate atmospheric composition over long timescales, as Earth's plate tectonics do.