Three super-Earth-size planets in two newly discovered planetary systems are in a “habitable zone” — a distance from their star suitable for supporting liquid water — and potentially could support life as we know it, according to NASA’s Kepler mission. “The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home,” said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA headquarters in Washington. “It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity." The Kepler space telescope, which simultaneously and continuously measures the brightness of more than 150,000 stars, is NASA's tenth Discovery Mission and the first mission capable of detecting Earth-size planets around stars like our sun. It has found five planets in the Kepler-62 system: 62b, 62c, 62d, 62e and 62f; and two planets in the Kepler-69 system: 69b and 69c. Kepler-62e, 62f and 69c are the super-Earth-sized planets. Relative sizes of Kepler habitable-zone planets discovered as of April 18, 2013. From left: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth (except for Earth, these are artists' renditions). Images courtesy of NASA Ames/JPL-Caltech. Two of the newly discovered planets orbit a star smaller and cooler than the sun. Orbiting its star every 122 days, Kepler-62e was the first of these habitable-zone planets identified. It orbits on the inner edge of the zone and is roughly 60 percent larger than Earth. Kepler-62f, with an orbital period of 267 days, is likely to have a rocky composition and is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet that is known to be in the habitable zone of another star. It was discovered by Eric Agol, associate professor of astronomy at the University of Washington and co-author of a paper on the discoveries published in Science (doi: 10.1126/science.1234702). The size of Kepler-62f has been measured, but its mass and composition have not. However, based on previous studies of rocky exoplanets similar in size, scientists have been able to estimate its mass by association. The third planet, Kepler-69c, is 70 percent larger than the size of Earth and orbits in the habitable zone of a star similar to our sun. Astronomers are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a sunlike star resembles that of our neighboring planet Venus. It is not yet known whether life could exist on the newfound planets, but the discovery signals that we are another step closer to finding a world similar to Earth around a star like our sun. The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring just two-thirds the size of the sun and only one-fifth as bright. At 7 billion years old, the star is somewhat older than the sun. The artistic concepts of the Kepler-62 planets are the result of scientists and artists collaborating to help imagine the appearance of these distant worlds. “We only know of one star that hosts a planet with life, the sun,” said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, Calif., and lead author of the Kepler-69 system discovery published in The Astrophysical Journal. “Finding a planet in the habitable zone around a star like our sun is a significant milestone toward finding truly Earthlike planets.” The two habitable zone worlds orbiting Kepler-62 have three companions in orbits closer to their star, two larger than the size of Earth, and one about the size of Mars. Kepler-62b, Kepler-62c and Kepler-62d orbit every five, 12 and 18 days, respectively, making them very hot and inhospitable for life as we know it. The five planets of the Kepler-62 system orbit a star classified as a K2 dwarf, measuring just two-thirds the size of the sun and only one-fifth as bright. At 7 billion years old, the star is somewhat older than the sun. It is about 1200 light-years from Earth in the constellation Lyra. A companion to Kepler 69c, known as Kepler 69b, is more than twice the size of Earth and whizzes around its star every 13 days. The Kepler-69 planets’ host star belongs to the same class as our sun, called G-type. It is 93 percent the size of the sun and 80 percent as luminous and is located approximately 2700 light-years from Earth in the constellation Cygnus. The artist's concept depicts Kepler-62e, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1200 light-years from Earth in the constellation Lyra. Kepler-62e orbits its host star every 122 days and is roughly 60 percent larger than Earth in size. Scientists do not know if Kepler-62e is a water world or whether it has a solid surface, but its discovery signals that we are another step closer to finding a world similar to Earth. When a planet candidate transits, or passes in front of the star from the spacecraft's vantage point, a percentage of light from the star is blocked. The resulting dip in the brightness of the starlight reveals the transiting planet's size relative to its star. Using the transit method, Kepler has detected 2740 candidates. Using various analysis techniques, ground telescopes and other space assets, 122 planets have been confirmed. Early in the mission, the Kepler telescope found primarily large, gaseous giants in very close orbits of their stars. These “hot Jupiters,” as they are known, have a size and very short orbital period that makes them easy to detect. Earth would take three years to accomplish the three transits required to be accepted as a planet candidate. As Kepler continues to observe, transit signals of habitable zone planets the size of Earth orbiting stars like the sun will begin to emerge. “Kepler has brought a resurgence of astronomical discoveries, and we are making excellent progress toward determining if planets like ours are the exception or the rule,” said William Borucki, Kepler science principal investigator at NASA’s Ames Research Center at Moffett Field, Calif., and lead author of the Science paper. Ames is responsible for Kepler’s ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development. Ball Aerospace & Technologies Corp. in Boulder, Colo., developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder. The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. The mission was funded by NASA’s Science Mission Directorate. For more information, visit: www.nasa.gov/kepler