Extraterrestrial Life/Requirements For Life

Below, the requirements are listed.

Star
For a star to be able to host habitable planets, it must:
 * have a life span of at least one billion years.
 * not be a flare star.
 * not be too close to a cosmic explosion, such as a supernova

Planet
For a planet to be habitable by life similar to that on Earth, it must:
 * have a temperature that stays between -15°C, below which chemicals start reacting too slowly for life to occur and water freezes, and 115°C, above which protein and carbohydrate molecules, as well as genetic material, start to decompose and water evaporates.
 * be at just the right distance from the star so it gets enough energy from the star.
 * have continuous liquid water on its surface.
 * enough mass for radioactive heating, which allows for geological activity.
 * a thick atmosphere to protect the planet, stabilize its climate, and provide sufficient pressure to hold liquid water. However, it must not be so thick that it creates a greenhouse effect that makes the planet too hot.
 * not be too close to massive planets which can divert asteroids towards or strongly perturb its orbit.
 * have a massive planet well outside of its orbit that diverts dangerously large asteroids away from it.