National Geographic : 2009 Dec
KEPLER SEARCH AREA SCUTUM-CENTAURUS ARM NORMA ARM SAGITTARIUS ARM SAGITTARIUS ARM FAR 3KPC ARM NEAR 3KPC ARM OUTER ARM PERSEUS ARM ORION SPUR THE SUN "There will be nothing to hinder an infinity of worlds," wrote Greek philosopher Epicurus some 23 centuries ago. Now, with ever clearer views of distant space, such thinking is back in vogue. More than 370 planets outside our solar system have already been discovered. An overhead view of our Milky Way galaxy (artist's conception, right) gleams with more than 100 billion stars---and billions of those are likely to possess planetary systems of their own. Astronomers zooming in on those points of light with more sensitive instruments expect to discover other Earthlike planets--- some, perhaps, with all the makings for life. GALACTIC NEIGHBORS Most of the exoplanets found to date orbit stars within 400 light-years of our solar system, which lies at the center of the white square (left and enlarged at right). NASA's Kepler satellite is now monitoring stars up to 6,000 light-years distant along the Orion Spur in search of Earth-size planets. Sunlike stars 600 to 1,800 light-years away are thought to be the most promising. THE MILKY WAY The arms of our galaxy spiral off a central bar of stars 27,000 light-years long. Our solar system lies in a partial arm, the Orion Spur. In this view, based on infrared images from NASA's Spitzer Space Telescope, regions where new stars are forming appear in red.