  The Solar System includes two planets --- Mercury and Mars --- significantly
less massive than Earth, and all evidence indicates that planets of similar
size orbit many stars. In fact, one of the first exoplanets to be discovered is
a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets
have inspired new ideas about planet formation and evolution, and these
"sub-Earths" should be no exception: they include planets with masses between
Mars and Venus for which there are no Solar System analogs. Advances in
astronomical instrumentation and recent space missions have opened the
sub-Earth frontier for exploration: the Kepler mission has discovered dozens of
confirmed or candidate sub-Earths transiting their host stars. It can detect
Mars-size planets around its smallest stellar targets, as well as exomoons of
comparable size. Although the application of the Doppler method is currently
limited by instrument stability, future spectrographs may detect equivalent
planets orbiting close to nearby bright stars. Future space-based microlensing
missions should be able to probe the sub-Earth population on much wider orbits.
A census of sub-Earths will complete the reconnaissance of the exoplanet mass
spectrum and test predictions of planet formation models, including whether
low-mass M dwarf stars preferentially host the smallest planets. The properties
of sub-Earths may reflect their low gravity, diverse origins, and environment,
but they will be elusive: Observations of eclipsing systems by the James Webb
Space Telescope may give us our first clues to the properties of these small
worlds.
