I have heard that the only planets able to be seen outside the Solar system are Jovian-sized planets with the occasional detection of planets three times the Earth's size. But, as far as I know, we haven't seen any Earth-sized (due to the range of our telescopes?) extrasolar planets. When will we be able to detect them?
The smallest detected exoplanet (Kepler-37-b) is actually smaller in mass and size than Earth, so we already have a limited ability to detect these size exoplanets.
It is worth noting that there wasn't any new technology that allowed this advance, the paper linked to by NASA indicates that the same methods were used as they would usually use. Making it remarkable that they were even able to discover something this small.