Kepler is a space observatory launched by NASA on March 7, 2009 to discover Earth-like planets orbiting other star (i.e., those one half to twice the size of the Earth).
The spacecraft is looking for planets that are in or near the habitable zone of their respective stars: they are close enough but not too far from their respective stars). The challenge is to find terrestrial planets that are close enough to their stars to allow liquid water and possibly life to exist.
The diagram below compares the planets of our inner solar system with those in this "neighboring" solar system that has five planets. The five planets of this other solar system orbit around a dwarf star that is is half the size and mass of our sun.
This discovery is the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distance from a star where liquid water might pool on the planet's surface.
You are invited to visit this new planet in NASA's simulation below.
The size of Kepler-186f is known to be less ten percent larger than Earth, but its mass and composition are not known. Kepler-186f orbits its star once every 130 days, receiving one- third the heat energy that Earth does from the sun. This places the planet near the outer edge of the habitable zone.
The inner four companion planets each measure less than fifty percent the size of Earth. Kepler-186b, Kepler-186c, Kepler-186d and Kepler-186, orbit every 4, 7, 13 and 22 days, respectively, making them very hot and inhospitable for life as we know it.
The Kepler space telescope infers the existence of a planet by the amount of starlight blocked when it passes in front of its star. From these data, a planet's radius, orbital period and the amount of energy received from the host star can be determined.
NASA Ames/SETI Institute/JPL-Caltech