Kepler's small habitable zone planets. Illustration comparing Earth and a range of small exoplanets in habitable zones, as discovered by NASA's Kepler space observatory. Next to Earth at top, Kepler-452b is one of the most Earth-like planet discovered, close in size and with a Class G star very similar to that of our Sun. The other exoplanets shown here are generally smaller and have smaller parent stars. Across middle are seven found around Class K stars. Across bottom are four found orbiting Class M stars. K-type stars are orange-yellow stars that are slightly cooler than the Sun. Class M stars are red dwarfs. The Kepler space telescope was launched in 2009. This image was published in 2015.