Planets generally emit no light, but they reflect that of their parent star. This method aims at detecting this reflected light. It is a very difficult task because the nearby star is so bright that it overwhelms the image and hides the much fainter planet. Planned satellites like TPF from NASA and Darwin from ESA will use imaging techniques to look for terrestrial planets in the habitable zone. We are living in an exciting age, where discovering other worlds similar to our own, understanding how our Solar System formed, and even observing planets where life may be present, is now within our reach.
Infrared Wavelength Imaging
On the diagram below you can see an illustration showing why the infrared wavelength imaging method is one of the most effective in the search for extrasolar planets. Using the solar system as an example, you can see that at certain wavelengths in the infrared part of spectrum, farthest from the visible spectrum, the difference in brightness of planets and their parent star is not as high as it is in the visible part. In the visible spectrum, light from a star is so bright that it obscures any planets around it, but in the infrared spectrum, a planet's light can be spotted more easily.