At 340 exoplanets and counting, astronomers tonight are much closer to discovering some more that might resemble our own. At 10:49:57 p.m. EST, a Delta II rocket lifted off from Launch Complex 17-B at Cape Canaveral Air Force Station. It sent the Kepler Telescope into an Earth-trailing heliocentric (Sun-centered) orbit with a period of 372.5 days.The Kepler telescope uses a 3-foot (0.95-meter) telescope with a 95-megapixel camera consisting of 42 charge-coupled devices (CCDs). It will monitor 100,000 stars with brightnesses between 9th magnitude and 16th magnitude in a field of view of 105 square degrees. The star field encompasses parts of the constellations Lyra the Harp and Cygnus the Swan.For at least the next 3.5 years, Kepler will monitor the stars’ brightnesses with a precision of 20 parts per million (ppm) to detect dimming caused by transiting Earth-sized planets. (A planet the size of Earth will dim the star by roughly 84 ppm.) The dimming will last from about 3 to 12 hours, and any dimming caused by a planet will be periodic.The Kepler mission will give astronomers a statistical measure of the occurrence of planets from Earth-sized to super-Earths to Neptune-sized orbiting within 2 astronomical units (AU) of normal stars. (The average Earth-Sun distance equals 1 AU.) For the first time, we may know how common other rocky worlds are.Because they expect to find Earth-sized planets, mission scientists built in an extension. If things are going well, expect Kepler to perform through 2014.Keep up with all the news from this mission at Astronomy.com’s Kepler news page.