  Surveys reveal that terrestrial- to Neptune-sized planets (1 $< R <$ 4
R$_{\rm{Earth}}$) are the most common type of planets in our galaxy. Detecting
and characterizing such small planets around nearby stars holds the key to
understanding the diversity of exoplanets and will ultimately address the
ubiquitousness of life in the universe. The following fundamental questions
will drive research in the next decade and beyond: (1) how common are
terrestrial to Neptune-sized planets within a few AU of their host star, as a
function of stellar mass? (2) How does planet composition depend on planet
mass, orbital radius, and host star properties? (3) What are the energy
budgets, atmospheric dynamics, and climates of the nearest worlds? Addressing
these questions requires: a) diffraction-limited spatial resolution; b)
stability and achievable contrast delivered by adaptive optics; and c) the
light-gathering power of extremely large telescopes (ELTs), as well as
multi-wavelength observations and all-sky coverage enabled by a comprehensive
US ELT Program. Here we provide an overview of the challenge, and promise of
success, in detecting and comprehensively characterizing small worlds around
the very nearest stars to the Sun with ELTs. This white paper extends and
complements the material presented in the findings and recommendations
published in the National Academy reports on Exoplanet Science Strategy and
Astrobiology Strategy for the Search for Life in the Universe.
