ne = The number of "earths" per planetary system
All stars have a habitable zone where a planet would be able to maintain a temperature that would allow liquid water. A planet in the habitable zone could have the basic conditions for life as we know it.

What Makes a Planet "Earth-like"? "Life as we know it" requires liquid water. It is the medium within which terrestrial biochemistry occurs. Water can exist in its liquid state only under a certain range of temperature and pressure conditions. This constrains the mass and orbit of an "Earth-like" planet.

If a planet is too small, its gravity will not be able to hold a sufficiently dense atmosphere. Much of the atmosphere may "evaporate" into space or be blown into space by the impact of large asteroids.

If a planet is too large, its gravity may hold too much atmosphere for terrestrial biochemistry to occur. Under the intense pressure of a dense atmosphere, liquid water may exist, but at a very high temperature. Although there are examples of terrestrial organisms that live in hot springs and in the ocean floor hydrothermal vents, it's not clear whether they originated there or adapted to the extreme temperature environment. While we can't rule out the origin of life on a large planet, we know that life can evolve on a planet like the Earth.

Another Possibility for Liquid Water

Since the Pioneer and Voyager missions to Jupiter, scientists have been intrigued with the moon Europa. It has a smooth, icy surface with a fine network of dark lines. Early theories suggested that variable tidal forces cause by Jupiter and the other moons might heat the interior of Europa to the point where ice would melt. Some speculation suggested that there might be an ocean underneath the icy surface, and where there's water perhaps ... (This idea was even used in Arthur C. Clarke's novel 2010.)

Now the Galileo spacecraft has revealed possible evidence of liquid water on Europa.

Could life develop on Europa? Perhaps, but probably not advanced life. However, optimists might consider a larger Earth-sized moon, orbiting a Jovian planet in another solar system, a suitable site for life.

Ecoshells - Life Zones

• Now Available: "Circumstellar Habitable Zones, Proceedings of the First International Conference", Laurence R. Doyle (ed.), Travis House Publications, Menlo Park, California, 1996.
  ISBN 0-9650896-0-6

Searching for Earth-like Planets

Project Kepler.
A proposed Discovery class mission using photometry to detect the transit of an Earth-sized planet in front of its star.

Project Darwin.
A proposed European Space Agency space-based infrared interferometer that would detect Earth-like planets around nearby stars and then search for a spectroscopic signature of life, e.g., ozone in the atmosphere.

ExNPS - Exploration of Neighboring Planetary Systems.
A NASA space-based interferometer program that will search nearby stars for earth-like planets.