29. A Proper Atmosphere
We cannot call a planet habitable if it does not have a proper atmosphere, it is truly among the big things a planet needs for us to live there. Our moon, for example, does not have an atmosphere at all and is, therefore, unable to sustain human life.
Meanwhile, a planet like Mars does have an atmosphere but it’s too thin and allows a lot of crucial things for human life to escape it. This means we cannot live on Mars currently either. Yet Earth does have an atmosphere and is, therefore, possible to sustain human life. It’s all about the atmosphere.