  By providing sensitive sub-arcsecond images and integral field spectroscopy
in the 25 - 400 micron wavelength range, a far-IR interferometer will
revolutionize our understanding of planetary system formation, reveal
otherwise-undetectable planets through the disk perturbations they induce, and
spectroscopically probe the atmospheres of extrasolar giant planets in orbits
typical of most of the planets in our solar system. The technical challenges
associated with interferometry in the far-IR are greatly relaxed relative to
those encountered at shorter wavelengths or when starlight nulling is required.
A structurally connected far-IR interferometer with a maximum baseline length
of 36 m can resolve the interesting spatial structures in nascent and developed
exoplanetary systems and measure exozodiacal emission at a sensitivity level
critical to TPF-I mission planning. The Space Infrared Interferometric
Telescope was recommended in the Community Plan for Far-IR/Submillimeter Space
Astronomy, studied as a Probe-class mission, and estimated to cost 800M
dollars. The scientific communities in Europe, Japan, and Canada have also
demonstrated a keen interest in far-IR interferometry through mission planning
workshops and technology research, suggesting the possibility of an
international collaborative effort.
