Gas giants are the most readily detectable exoplanets but fundamental
questions about their system architectures, formation, migration, and
atmospheres have been unanswerable with the current generation of ground- and
space-based facilities. The dominant techniques to detect and characterize
giant planets − radial velocities, transits, direct imaging, microlensing,
and astrometry − are each isolated to a limited range of planet masses,
separations, ages, and temperatures. These windows into the arrangement and
physical properties of giant planets have spawned new questions about the
timescale and location of their assembly; the distributions of planet mass and
orbital separation at young and old ages; the composition and structure of
their atmospheres; and their orbital and rotational angular momentum
architectures. The ELTs will address these questions by building bridges
between these islands of mass, orbital distance, and age. The angular
resolution, collecting area, all-sky coverage, and novel instrumentation suite
of these facilities are needed to provide a complete map of the orbits and
atmospheric evolution of gas giant planets (0.3−10 MJup) across
space (0.1−100 AU) and time (1 Myr to 10 Gyr). This white paper highlights
the scientific potential of the GMT and TMT to address these outstanding
questions, with a particular focus on the role of direct imaging and
spectroscopy of large samples of giant planets that will soon be made available
with Gaia.Comment: White paper for the Astro2020 decadal surve
