The first two Earth-sized exoplanets found by Kepler are shown here in comparison to Earth and Venus. Kepler-20e has a diameter of 6,900 miles, meaning it is 0.87 times the size of Earth and 0.92 times the size of Venus. Kepler-20f has a diameter of 8,200 miles, meaning it is only 3 percent larger than Earth. They are part of a five-planet system orbiting the star Kepler-20. All five would fit within the orbit of Mercury in our solar system.