  Exoplanets smaller than Neptune are common around red dwarf stars (M dwarfs),
with those that transit their host star constituting the bulk of known
temperate worlds amenable for atmospheric characterization. We analyze the
masses and radii of all known small transiting planets around M dwarfs,
identifying three populations: rocky, water-rich, and gas-rich. Our results are
inconsistent with the previously known bimodal radius distribution arising from
atmospheric loss of a hydrogen/helium envelope. Instead, we propose that a
density gap separates rocky from water-rich exoplanets. Formation models that
include orbital migration can explain the observations: Rocky planets form
within the snow line, whereas water-rich worlds form outside it and later
migrate inward.
