Of the approximately 1,500 exoplanets found so far, only a few are in the so-called habitable zone around their parent star. It's also called the "Goldilocks zone," a coveted sweet-spot where the amount of radiation coming from a star is "just right" for liquid water to exist on the surface of a rocky planet.
For example, Earth is within the present habitable zone around our sun, which is a comparatively narrow swath 18 million miles across.
Enthusiasm about finding other worlds nestled inside other stars' habitable zones is so strong that in 2008 a radio message was beamed to a planet known to exist in the habitable zone of the red dwarf star Gliese 581, located 20 light-years away.
The signal contains 501 greetings that were selected through a competition on a social networking site.
The tight noose defined by a habitable zone is one of the important variables in the famous Drake Equation, which tries to estimate the number of extraterrestrial civilizations in our galaxy.
The habitable zone concept was widely popularized in the mid 1970s by astronomer Michael Hart. He used it to argue that Earth was the only home for intelligent life in the galaxy. In fact, Hart's own calculation from the Drake Equation came up with a probability of .000000000000000000000000000000000001 percent of there being intelligent life elsewhere in the galaxy.