Scientists are hot on the trail of habitable, earth-like planets. So what are such planets? They are places human beings could live without any genetic manipulation. Higher forms of life characteristic of earth should thrive there.
Habitability requires a planet large enough to maintain a gravitational hold on its atmosphere, yet not so large as to generate a crushing atmospheric pressure. Planetary gravity should not be so weak that our muscles and bones atrophy, yet not so strong that easy movement is impossible. Average temperature should result in liquid water as the normal state, with oceans on the planetary surface. Plate tectonics should assure a stabile carbon concentration in the atmosphere, preventing a runaway increase in temperature. Large asteroids should not be constantly plunging through the atmosphere and striking the planetary surface. Radiation should not cause life to mutate incessantly, thus the need for a strong magnetic field to deflect the solar wind. The planet should not be tidally locked with its primary star such that one hemisphere always faces its sun and one is in perpetual darkness.
Habitable exoplanets need to orbit within a star's habitable zone, the region of space around a star where a planet would receive roughly the same energy as our Earth. This video explains the formation of habitable planets.
The graphic to the left illustrates the variation in habitable zone around different star types. The graphic shows how both the distance of the habitable zone from the star and its width are directly dependent on the star's mass. The width of the habitable zone can also be affected by assumptions, such as cloud cover, as shown in a paper related to the Kepler discoveries.