K2-138 is a Sun-like G-type star (about 0.94 solar masses, 5,356 K, 0.86 solar radii), 203 light-years from Earth. K2-138 is a near-resonant chain of sub-Neptunes discovered by citizen scientists, a classic peas-in-a-pod system. The system hosts 6 confirmed planets on orbits ranging from 2.4 days to 42.0 days. With 6 confirmed worlds, K2-138 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. K2-138 b skims its star closer than Mercury orbits the Sun. At 1.5 Earth radii, K2-138 b is a compact world comparable in size to rocky planets in our Solar System.