In any case, the underlying reason to look for Earth-like planets elsewhere is to come up with a reason to think that life is present throughout the universe (at the moment, we have absolutely no evidence that life exists elsewhere). Exoplanets have been known to exist for a couple of decades or so, but they are very hard to detect and the first ones we knew about were extremely large and could be called "failed stars".
There are a number of telescopes in orbit which are specifically designed to look for the extremely small effects which such planets have on their "suns".
http://www.astro.ubc.ca/MOST/ - This is a Canadian project, directed by Dr. Jaymie Matthews of UBC, which has detected some exoplanets as a serendipitous byproduct of its main mission (to measure tides and wave propagation in distant stars).
There are orbital telescopes specifically designed to look for exoplanets, and one in particular is designed to look for Earth-sized ones. Here's a link:
It has just succeeded in detecting one exoplanet which is just slightly larger than Earth, albeit unlikely to be earth-like. This is the news release from NASA:
(Artist's concept, NASA news release)