Since the early 1990s astronomers have discovered more than 300 planets orbiting stars other than our sun, nearly all of them gas giants like Jupiter. Powerful space telescopes, such as the one that is central to NASA's recently launched Kepler Mission, will make it easier to spot much smaller rocky extrasolar planets, or exoplanets, more similar to Earth.

But seen from dozens of light years away, an Earth-like exoplanet will appear in telescopes as little more than a "pale blue dot," the term coined by the late astronomer Carl Sagan to describe how Earth appeared in a 1990 photograph taken by the Voyager spacecraft from near the edge of the solar system.