The planet GJ 3470 b has a rocky core and a thick hydrogen and helium atmosphere, and circles quite close to its dim red star. (Credit: NASA/ESA/L. Hustak)

The Kepler Space Telescope revealed planets outside our solar system by the thousands. With this wave of discoveries, astronomers realized something peculiar: the most common type of planet in the galaxy is one our solar system doesn’t have. It’s a planet between the size of Earth and Neptune, known either as a super-Earth or a sub-Neptun