Kepler-20 is a Sun-like G-type star (about 0.95 solar masses, 5,495 K, 0.96 solar radii), 283 light-years from Earth. Kepler-20 e and f were the first confirmed Earth-sized exoplanets, alternating with larger worlds in a compact chain. The system hosts 6 confirmed planets on orbits ranging from 3.7 days to 77.6 days. With 6 confirmed worlds, Kepler-20 is a densely packed system whose resonances, spacing, and formation history are active research topics. Mutual gravitational tugs between the planets can slowly reshape their orbits, preserving clues to the system's assembly. Kepler-20 b skims its star closer than Mercury orbits the Sun. At 1.8 Earth radii, Kepler-20 b is a compact world comparable in size to rocky planets in our Solar System.