text: We are only beginning to be able to detect planets within an order of magnitude of the mass of Earth. Just a couple months ago, we found the smallest rocky planet yet discovered outside the solar system, orbiting a star 20,000 light years away. It's about 5 times the mass of Earth, and has a larger orbit (and orbital period), about 2.6 AU (and orbits in about 10 years). It's also very cold, about 50 K (-370 F), as it orbits a red dwarf about a fifth the mass of the sun.\nIt was discovered with a new technique called gravitational microlensing, which can only be used occasionally. However, as this planet is one of three so far detected by microlensing, scientists think it implies that earthlike planets may be much more common than previously realized.\n\nThis isn't much of an answer to your question, but, honestly, the closest (non-earth) planet which is capable of sustaining intelligent life could orbit one of the two primary stars in the Alpha Centauri system (the nearest star system to the sun, at 4.3 light years away; there are three stars there, the larger Alpha Centauris A and B, and the distant companion Proxima Centauri) for all we know. The only way we'd have to find such a planet (without the presence of extremely rare astronomical conditions which allow microlensing) is the spectroscopic method, which has a difficult time finding any planets which are not a) extremely massive, and b) orbiting extremely close to their star. \n\nThe two primary Alpha Centauri stars could have rocky planets orbiting at less than 2 AU (Mars has an orbit of about 1.5 AU), and Alpha Centauri A is a metal rich G2 type star like our own sun, while Alpha Centauri B is a metal rich K1 star, which is in the band of spectral types which may be suitable for life-bearing planets.\n\nIn short, we don't know. They could be everywhere, including practically on our doorstep, or they could be relatively rare, and not found for thousands of light years from Earth. We'll have to wait until our methods of finding planets improves to know for sure.