The necessary conditions for the appearance and the development of life on a planet can be sum up in one word:Balance
We may say that the appearance of the life results from a natural propention from the matter to organize itself into increasingly complex structures, from the time on some favorable conditions are met.
These conditions are those of a balance, a "moderated" zone between the extremes, too hot or too cold temperature, too massive or too light planet, too far or too close from the star.
Around many star, there is an orbital zone which is theoretically propitious for the life, from where very high probability that the life exists on many other planets in the universe.
Condition #1: Mass of the star
When the star is too massive, its longevity is lower than 1 billion year. Therefore, the giant stars die before complex life development. Conversely, the stars of mass 10 times lower than the sun are not massive enough to start the process of thermonuclear fusion. An average star as the Sun is thus favorable.
Condition #2: Mass of the planet
The mass of planet determines the composition of the atmosphere. Gravity selects the atoms retained on the planet, and those which can escape towards space.
If the planet is too massive, it completely retains the lightest gases like hydrogen and helium, which creates an atmosphere made of methane or amoniac, as on Jupiter, Saturn, Uranus or Neptune.
If the planet is not massive enough , it lets escape hydrogen but also the heavier gases essential to the life as oxygen, as well as the water which evaporate in space. The surface of such planets without protective atmosphere is fully exposed to solar nocive radiations as gamma rays or ultraviolet light. In the solar system, Mercure is an example of this type of planet.
Mercury: too small
Jupiter: too big
Condition #3: Distance from the star
The distance from the star conditions the received quantity of solar radiations and so the following parameters:The température, which determine the presence of liquid water, and the general bearability of the planet
The light available for the vegetation
The amount of harmful radiations to the life and the stability of the ADN (ultraviolets, gamma rays)
If the Earth would have been more close to the Sun of 4%, its fate would have been that of Venus: a furnace.
If it had been more distant from 1 or 2%, its destiny would have been that of Mars, a frozen planet. The band of space favorable to the life around the star is thus relatively narrow.
Venus: too hot
Mars: too cold
Condition #4: Composition of the planet
Water, oxygen, carbon, iron, are some of the key elements for the life such as we know it on Earth (based on carbon and water chemistry). However, we cannot exclude that some different life forms may develop from other chemical elements such as silicon or methane.
The internal composition of planet and its nucleus will condition the presence or of a magnetosphere which contribute to protect the planet from the dangerous solar and space radiations.
The magnetosphere is generated by the movements of the molten steel in the heart of planet.
Some basic facts, numbers, and dimension scales about universe, matter, and biologic life