Is it enough for a planet to be in a habitable zone for life to exist? Any planet standing there would have to receive strong radiation from its parent star. It will eventually melt the planet’s atmosphere. (But the thick clouds on Venus reflect solar radiation, which is why they shine). The movement of molten iron around the center of the earth gives the earth a strong magnetic field . Thus, a large portion of this radiation from the Sun slides off the Earth’s magnetic field.
When Mars had a magnetic field, it had an ocean and an atmosphere. In the image below, Venus and Mars below are trying to block the sun’s radiation through the ionosphere, and the Earth’s magnetic field is covering the Earth like a large blanket.