Most of the more than 300 exoplanets found thus far have been detected by the periodic Doppler shifting of the host star’s spectrum as an orbiting planet tugs it to and fro. A few have revealed themselves by periodically dimming a star’s perceived brightness as they transit across its face. And gravitational microlensing has uncovered a handful (see Physics Today, April 2006, page 22). But none of those techniques is practical for planets that orbit their stars at distances larger than about 5 astronomical units. (1 AU is the mean distance between Earth and the Sun.)
There is, however, a search technique that gets easier with increasing distance between star and planet—namely, direct imaging. A planet less than about 10 AU (the orbital radius of Saturn) from its star is lost in the star’s glare. And beyond 10 AU, reflected starlight would generally be too dim for imaging....