Kepler uses the transit method (unsure how astronomers find planets? Read Planet Hunters - How They Do It) because transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm) lasting for 1 to 16 hours. The change must be periodic if it is caused by a planet and all transits produced by the same planet will be the same change in brightness and last the same amount of time, so it's a reliable detection method.
Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. Knowing the temperature of a planet is key to whether or not the planet is habitable (not necessarily inhabited). Only planets with moderate temperatures are habitable for life similar to that found on Earth.
Of course, we are at one point in the universe so for a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The probability for an orbit to be properly aligned is equal to the diameter of the star divided by the diameter of the orbit. This is 0.5% for a planet in an Earth-like orbit about a Sun-like star. (For the giant planets discovered in four-day orbits, the alignment probability is more like 10%.) In order to detect many planets astronomers must look at thousands of stars, even if Earth-like planets are common.
Kepler looks at 100,000 stars in one region of the sky, in the Cygnus and Lyra constellations. The field of view is extremely large for an astronomical telescope. Most telescopes such as Hubble Space Telescope only view a small region at one time, about the size of a grain of sand held at arms length. Kepler covers a much larger field, about an hand at arm’s length or 2 dips from the Big Dipper. The field of view is overhead at midnight in the middle of summer for those in the mid-Northern latitudes, and earlier in the evening in late summer and fall. Credit: NASA
To do this, Kepler continuously points at a single star field in Cygnus-Lyra region and they 'roll' the spacecraft 90 degrees about the line-of-sight every 3 months to maintain the sun on the solar arrays and the radiator pointed to deep space. This allows it to monitor 100,000 main-sequence stars for planets and the mission will last for up to 6 years.
Yesterday, Kepler announced discovery of its first Earth-size planet candidates and its first candidates in the habitable zone. Five of the potential planets are near Earth-size and orbit in the habitable zone of smaller, cooler stars than our sun. Obviously they require follow-up observations to verify they are actual planets. Kepler also found six confirmed planets orbiting a sun-like star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our solar system.
"In one generation we have gone from extraterrestrial planets being a mainstay of science fiction, to the present, where Kepler has helped turn science fiction into today's reality," said NASA Administrator Charles Bolden. "These discoveries underscore the importance of NASA's science missions, which consistently increase understanding of our place in the cosmos."
The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data. The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size -- up to twice the size of Earth -- to larger than Jupiter.
Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates. Kepler-11, located approximately 2,000 light years from Earth, is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury's. The only other star with more than one confirmed transiting planet is Kepler-9, which has three. The Kepler-11 findings are published today in Nature.
All of the planets orbiting Kepler-11 are larger than Earth, with the largest ones being comparable in size to Uranus and Neptune. The innermost planet, Kepler-11b, is ten times closer to its star than Earth is to the sun. Moving outward, the other planets are Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and the outermost planet, Kepler-11g, which is half as far from its star as Earth is from the sun.
The planets Kepler-11d, Kepler-11e and Kepler-11f have a significant amount of light gas, which indicates that they formed within a few million years of the system's formation.
Citation: Jack J. Lissauer, Daniel C. Fabrycky, Eric B. Ford, William J. Borucki, Francois Fressin, Geoffrey W. Marcy, Jerome A. Orosz, Jason F. Rowe, Guillermo Torres, William F. Welsh, Natalie M. Batalha, Stephen T. Bryson, Lars A. Buchhave, Douglas A. Caldwell, Joshua A. Carter, David Charbonneau, Jessie L. Christiansen, William D. Cochran, Jean-Michel Desert, Edward W. Dunham, Michael N. Fanelli, Jonathan J. Fortney, Thomas N. Gautier III, John C. Geary, Ronald L. Gilliland, Michael R. Haas, Jennifer R. Hall, Matthew J. Holman, David G. Koch, David W. Latham, Eric Lopez, Sean McCauliff, Neil Miller, Robert C. Morehead, Elisa V. Quintana, Darin Ragozzine, Dimitar Sasselov, Donald R. Short, Jason H. Steffen, 'A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11', arXiv:1102.0291v1
Citation: William J. Borucki, David G. Koch, Gibor Basri, Natalie Batalha, Timothy M. Brown, Stephen T. Bryson, Douglas Caldwell, Jørgen Christensen-Dalsgaard, William D. Cochran, Edna DeVore, Edward W. Dunham, Thomas N. Gautier III, John C. Geary, Ronald Gilliland, Alan Gould, Steve B. Howell, Jon M. Jenkins, David W. Latham, Jack J. Lissauer, Geoffrey W. Marcy, Jason Rowe, Dimitar Sasselov, Alan Boss, David Charbonneau, David Ciardi, Laurance Doyle, Andrea K. Dupree, Eric B. Ford, Jonathan Fortney, Matthew J. Holman, Sara Seager, Jason H. Steffen, Jill Tarter, William F. Welsh, Christopher Allen, Lars A. Buchhave, Jessie L. Christiansen, Bruce D. Clarke, Jean-Michel Désert, Michael Endl, Daniel Fabrycky, Francois Fressin, Michael Haas, Elliott Horch, Andrew Howard, Howard Isaacson, Hans Kjeldsen, et al. (19 additional authors not shown), 'Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data', arXiv:1102.0541v1