For a long time, we thought our Solar System was the template for the planets we'd find in the Universe. Inner, rocky worlds dominate the hottest part of the Solar System, with large, gaseous planets orbiting much farther out.
The largest rocky planet was Earth; the smallest gas giant was Uranus; the mass difference between the two was a factor of 17, with Uranus having four times Earth's radius.
So it was quite a surprise when exoplanet discoveries started rolling in. Not only can planets of various sizes and masses appear anywhere in a solar system's orbit, but of all the mass-and-size combinations out there, the most common type of planet is one we don't have at all: a Super-Earth.
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