File:Comparing the size of Earth, Mars, and exoplanets of Kepler-20 and Kepler-42.jpg

English: This chart compares the (at the time of their discovery) smallest known exoplanets, or planets orbiting outside the solar system, to our own planets Mars and Earth. Astronomers using data from NASA's Kepler mission and ground-based telescopes recently discovered the three smallest exoplanets known to circle another star, called Kepler-42b, Kepler-42c and Kepler-42d (formerly KOI-961.01, KOI-961.02 and KOI-961.03). The smallest of these, Kepler-42d, is about the size of Mars with a radius of only 0.57 times that of Earth. Not long ago, in Dec. of 2011, the Kepler team announced the discovery of Kepler-20e and Kepler-20f -- the first Earth-size planets ever found outside the solar system. All five of these small exoplanets have toasty orbits close to their stars, and do not lie in the more temperate habitable zone. The ground-based observations contributing to the Kepler-42 discoveries were made with the Palomar Observatory, near San Diego, California, and the W.M. Keck Observatory atop Mauna Kea in Hawaii.