The discovery of six exoplanets or extra-solar planets (planets outside the Solar System) orbiting a single sun-like star, dubbed Kepler-11, at a distance of about 2,000 light-years from Earth makes it the largest collection to be ever found. Those found earlier using ground-based detection methods were single exoplanets orbiting a star. The discovery by the Kepler spacecraft launched in March 2009 and reported recently online in Nature (“A closely packed system of low-mass, low-density planets transiting Kepler-11” by Jack J. Lissauer et al.,) became possible as the Kepler telescope continuously looks out for exoplanets transiting the more than 150,000 stars in a specific region of the sky in the Cygnus and Lyra constellations. A transiting planet causes a dip in the brightness of the star. Three to four transits causing the same dip in brightness, taking the same time to transit the star, and taking the same amount of time between successive transits are necessary for confirmation that the object is a planet. Though several thousand planets may be present in the region studied, the actual number that may eventually be found will be smaller as the orbital plane of the transits must be perfectly aligned with Kepler's line of sight. All the six exoplanets have orbits smaller than Venus's, with the orbits of the first five being smaller than Mercury's. The innermost planet must have a dense and rocky core as it has 4.6 times Earth's mass despite being only 1.4 times the size of the Earth. Most of the other planets have significant amounts of light gas.
The main objective of the Kepler mission is to find Earth-sized planets orbiting Sun-like stars and capable of supporting life. All the six exoplanets are bigger than Earth, with the largest ones comparable with Uranus and Neptune. Like Mercury and Venus, they are too close to the star to support life. According to the U.S. National Aeronautics and Space Administration (NASA), finding an Earth-sized planet in the habitable zone that is neither too close nor too far from the star may take at least three years; one transit would take nearly a year and three such transits are needed for planet confirmation. With Kepler only halfway into its mission, we cannot possibly expect discovery of an Earth-like planet before 2013. The Kepler finds add to our understanding of the universe. For instance, with ground-based instruments, generally only the radius and not the mass of the planets can be measured, and hence the density and composition would remain unknown; the size and mass of only three exoplanets smaller than Neptune used to be known. Kepler has added five more to that list.