All These Worlds Are Yours - Part 2

Of course, life might already be out there. It is hard to imagine (for non-sci-fi authors at least) lifeforms that call deep space their home, or thrive on the surface of a star, so planets are the place to look. Gas giants like Jupiter and Saturn lack a stable platform for organic molecules to meet and react making life less likely on the largest of planets. Liquid matter appears to be essential for biological processes - as a solvent, lubricant, transport mechanism or temperature regulator. Water is a perfect (although probably not unique) solution to this requirement, so a planetary temperature that allows for liquid water is optimal. Gases and liquids will not remain long on an object that lacks a strong enough gravitational field, so we can perhaps rule out the small rocky bodies such as asteroids and tiny moons like those of Mars (Deimos and Phobos). Gases are important for without our atmosphere trapping some of the heat from the Sun, Earth would be too cold to support life as we know it. "Goldilocks" planets are so called because they are not too hot nor too cold. They have to be the right distance away from the right kind of star and be the right kind of size.
Improvements in astronomy have lead to the detection of planets orbiting other stars. Spotting these extra-solar planets is incredibly difficult so they are sought through indirect means. Measurements are taken of the dimming of a star as a planet passes in front of it or the slight positional wobble caused by the planet's gravitational pull. Large planets close to their stars are the easiest to detect with these methods, so we've found mostly blazingly hot Jupiter-sized objects orbiting at Mercury-like distances. As our equipment and measurements improve, we are getting closer to seeing Earth-size objects at Earth-like distances.