In our search for Extra-Solar Planets and life outside our solar system, we will be looking to find planets similar to Earth: with rocky (solid) surfaces, and with conditions (temp/press/chemistry) conducive to liquid water on its surface.

Some of the smallest planets in our Solar System are no bigger than many of the largest Jovian (gas-giant) moons! When we consider other worlds, we need to keep planetary satellites on our list of possible sites for life.

General Characteristics of our Solar System
But what do we know about other Solar Systems? Nothing. We must start with what we understand about our own, and work from there.

1) All the planets orbit in the same direction around the Sun. Almost all share the same rotational direction, consistent with the Sun's rotational direction. (except Venus rotates backwards, Pluto & Uranus have highly tilted rotational axes.)