How does the broad structure of a solar system contribute to the possibility of creation and development of life as we know it?

When we are going to look for suitable places in the universe where life could exist, we first have to know what 'life' is. Life could occur in the most exotic and unimaginable forms. Even the greatest scientists and biologists cannot give a conclusive definition of life, so we will not try to exceed them. We will make a few assumptions for the basic needs for life to create and develop. By doing this we will look at life as we know it, namely life on Earth:


* Life can only exist on a solid object, large enough to make sure its gravitational attraction is large enough to contain an atmosphere. We will assume such an object has to be a solid planet.
* On such an planet has to be liquid water.
* For development of life a stable environment is required, most important a reasonable stable climate.

Remember that we restrict our subject to the global solar system conditions, so we will not in detail take into account the atmospheric and geologic conditions of the planet itself.

If we take a look at our own solar system we see that in general it consists of three components:

* a star at the center.
* planets, orbiting this star in elliptical to almost circular trajectory.
* asteroids, comets, rocks and grain, either moving around the star in elliptical, parabolic or hyperbolic trajectories, or moving through the system randomly.


In every solar system there are several components that have a significant influence on the three criteria for a livable habitat (solid planet, liquid water, stable environment). We will discuss them one by one:

The central star
Only stable stars that contain a continuously habitable zone can contain a planetary system in which life could be possible. The best candidates are main sequence stars of type K and G. Those of type "late" M and (early) F could also contain life.

Continuously Habitable Zone
The CHZ around our sun is in the order of 0.5 AU in width. The widths of CHZs in other planetary systems are approximately the same if distances are expressed on a logarithmic scale. If a planetary system exists, the chance is at least 50% that an Earth-like planet lies inside the CHZ.

Meteorites and protecting giant-planets
Massive planets close to the inner solar system protect small, possibly life-containing planets from frequent bombardments of devastating meteorites in the early stage of the solar system as well as in its final stage.

Planetary formation and spacing
The current fashionable theory about the formation of planetary systems states that most of the stars will have a planetary system quite similar to ours. However, in accordance with resent observations, other theories state that a planetary system such as ours is a rather uncommon outcome.