Small planets come in two sizes

There’s also an interesting gap in the distribution of the planet sizes: researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission have noticed that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earths and super-Earths (similar to Kepler-452b – sometimes nicknamed Earth 2.0 or Earth’s Cousin, the first potentially rocky super-Earth planet discovered orbiting within the habitable zone of a star very similar to the Sun), and the mini-Neptunes (similar to Kepler-22b, which discovered in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. Image credit: NASA/Ames/Caltech/University of Hawaii (B. J. Fulton)

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