  Planets intermediate in size between the Earth and Neptune, and orbiting
closer to their host stars than Mercury does the Sun, are the most common type
of planet revealed by exoplanet surveys over the last quarter century. Results
from NASA's Kepler mission have revealed a bimodality in the radius
distribution of these objects, with a relative underabundance of planets
between 1.5 and 2.0 $R_{\oplus}$. This bimodality suggests that sub-Neptunes
are mostly rocky planets that were born with primary atmospheres a few percent
by mass accreted from the protoplanetary nebula. Planets above the radius gap
were able to retain their atmospheres ("gas-rich super-Earths"), while planets
below the radius gap lost their atmospheres and are stripped cores ("true
super-Earths"). The mechanism that drives atmospheric loss for these planets
remains an outstanding question, with photoevaporation and core-powered mass
loss being the prime candidates. As with the mass-loss mechanism, there are two
contenders for the origins of the solids in sub-Neptune planets: the migration
model involves the growth and migration of embryos from beyond the ice line,
while the drift model involves inward-drifting pebbles that coagulate to form
planets close-in. Atmospheric studies have the potential to break degeneracies
in interior structure models and place additional constraints on the origins of
these planets. However, most atmospheric characterization efforts have been
confounded by aerosols. Observations with upcoming facilities are expected to
finally reveal the atmospheric compositions of these worlds, which are arguably
the first fundamentally new type of planetary object identified from the study
of exoplanets.
