The search for extraterrestrial life begins with the assumption that it would require a homeworld broadly similar to Earth in a few key characteristics. One of these is temperature, so astronomers estimate a “habitable zone” radiating out from each star, based on how hot the star is. If a planet orbits too close, it would fry; too far, and it would freeze. Also, orbiting at the wrong distance could subject the surface to either an excess or shortage of radiation from its sun.
Another great unanswered question is the role played by the size of a planet upon its biosphere. Gravity is an issue, as is the stability of the atmosphere – not much hope for life to evolve on a world racked by perpetual tornadoes, an atmosphere so thick or thin that no organism could breathe it, or a lack of water that isn’t frozen solid. This is especially true if one wishes to find complex organisms capable of returning any greeting we might send their way.
High-powered equipment has allowed astronomers to discover hundreds of planets thus far, but they’ve only just located one of approximately Earth size (actually a little bigger) that orbits within the habitable zone of its star (a dim red dwarf that doesn’t give off much heat by stellar standards, so the planet orbits much closer than Earth does to our Sun.) Unfortunately, because its star is so dim, and it’s 490 light-years away, it will be difficult to determine if it has an atmosphere comparable to ours, within the broad parameters that might support life.
Space.com has details of the discovery of Kepler-186f, the first solid candidate for “another Earth”: