Last April, scientists with the European Southern Observatory (ES0) lead by Michael Mayor, reported the discovery of the lightest and smallest exoplanet to date. Referred to as Gliese 581e, the planet is the fourth identified in the Gliese system in which one of the other planets, Gliese 581d, could be be habitable. Per Mayor, the detection of a rocky, Earth-like planet in the ‘habitable zone’ is considered the "holy grail" of exoplanet research.For more on astrobiology, visit our Astrobiology Field Guide.