A team of astronomers has measured the mass and size of the smallest exoplanet yet, a Mars-sized planet named Kepler-138b orbiting a red dwarf star about 200 light years from our solar system. Kepler-138b is the first exoplanet smaller than the Earth to have both its mass and its size measured. Kepler-138b is one of three planets that orbit the star Kepler-138 and that pass in front of it -- or transit -- on every orbit. Each time a planet transits the star, it blocks a small fraction of the star's light, allowing astronomers to measure the size of the planet. All three Kepler planets were identified by NASA's Kepler mission, which has discovered over a thousand planets around other stars. This video shows a mass-radius diagram based on measurements of 127 exoplanets. The video begins by showing a range of planets with masses up to that of Jupiter's, then gradually zooms toward the smaller masses and radii to display a comparison of the physical properties of the Kepler-138 planets relative to Earth, Venus, Mars and Mercury.