In a significant milestone on the road to finding Earth's "twin" elsewhere in the galaxy, NASA's Kepler mission has confirmed its first planet in the habitable zone of a distant sun-like star.
Kepler-22b -- Comfortably Circling within the Habitable Zone
"This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth.
Kepler-22's star is a bit smaller than our sun, so its habitable zone is slightly closer in. The diagram shows an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star. It's about 2.4 times the size of Earth."
Image credit: NASA/Ames/JPL-Caltech
Diagram and descriptive text source: http://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler-22b-diagram.html