Astronomers aren't yet able to directly detect new worlds. Instead, they look for the telltale signs of a planet's presence — especially its gravitational pull on a star. A planet's orbit around a sun is not a one-sided relationship; there's an equal effect in the opposite direction, which causes a star to "wobble." Astronomers have used this evidence of planetary influence on stars to build a list of more than one hundred known extrasolar planets.   But this isn't the only way to look for...