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Fig. 3. Mass–radius diagram for planets with masses less than 20 times the mass of the Earth, along with theoretical curves for different compositions. Planets are color-coded by incident flux of starlight as a multiple of our Sun’s output. Kepler planets are shown by filled circles, other exoplanets are shown by open squares. For comparison, Venus and Earth (lower left), Uranus (center), and Neptune (center-right) are indicated by their planetary symbols. The black curve is for Earth-like composition, the blue curve is for pure water, and the orange curve is for an envelope of a 1% hydrogen to helium ratio atop a core of (bulk) Earth composition. Credit: Eric Lopez, University of California Lick Observatory