Sending a mission to the solar gravitational lens (SGL) is the most effective way of actually directly imaging a potentially habitable planet, as well as its atmosphere, and even possibly some of its cities. But, the SGL is somewhere around 650–900 AU away, making it almost four times farther than even Voyager 1 has traveled—and that's the farthest anything human has made it so far.

It will take Voyager 1 another 130+ years to reach the SGL, so obviously traditional propulsion methods won't work to get any reasonably sized craft there in any reasonable timeframe. A new paper by an SGL mission's most vocal proponent, Dr. Slava Turyshev of NASA's Jet Propulsion Laboratory, walks through the different types of propulsion methods that might eventually get us there—and it looks like we would have a lot of work to do if we plan to do it anytime soon. It is available on the arXiv preprint server.