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Bruce Murray Space Image Library

Planet Kepler-10b transiting its host star (artist's depiction)

Planet Kepler-10b transiting its host star (artist's depiction)
Planet Kepler-10b transiting its host star (artist's depiction) Measuring 1.4 times the size of Earth, Kepler-10b was, at the time of its discovery in 2009, the smallest planet known outside our solar system. (Many smaller ones have since been discovered.) It is a rocky planet with a mass 4.6 times that of Earth and with an average density of 8.8 grams per cubic centimeter (which is significantly denser than Earth's 5.5). NASA / Kepler Mission / Dana Berry