For a planet to support life, the planet must be located within its star's habitable zone. That is, it must be located at a distance where the temperature would allow water to remain in liquid form.
At present we've identified about 250 extrasolar planets, and only two have appeared to be located close to or within this zone. And most of these extrasolar planets are about the size of Jupiter, not Earth. So, is Earth's size and orbit unique? Unlikely. The problem is that planets the approximate mass and orbit distance of Earth are too difficult to detect using our current space technology.
However, telescopes in the works right now are being designed specifically to overcome the obstacles astronomers face detecting Earth-like planets. One such project is NASA's Terrestrial Planet Finder, or TPF. TPF's capabilities will include blocking out the glare of stars, making visible relatively tiny Earth-like planets orbiting them.
TPF will be able to search the habitable zones of stars located up to fifty light years away, analyzing the composition of planets' atmospheres. It will look for gasses that signal the possibility of life, gasses such as oxygen, water vapor, methane, and carbon dioxide. And TPF will be capable of taking pictures that are one-hundred times more detailed than the photographs of the Hubble Space Telescope.
Many astronomers are hopeful that these new telescopes will lead to a groundbreaking discovery of life elsewhere in the universe. Regardless of what discoveries they may yield, TPF and other space telescopes in development promise to bring exciting changes to space exploration.