  An international group of scientists has begun planning for the Planet
Formation Imager (PFI, www.planetformationimager.org), a next-generation
infrared interferometer array with the primary goal of imaging the active
phases of planet formation in nearby star forming regions and taking planetary
system 'snapshots' of young systems to understand exoplanet architectures. PFI
will be sensitive to warm dust emission using mid-infrared capabilities made
possible by precise fringe tracking in the near-infrared. An L/M band beam
combiner will be especially sensitive to thermal emission from young exoplanets
(and their circumplanetary disks) with a high spectral resolution mode to probe
the kinematics of CO and H2O gas. In this brief White Paper, we summarize the
main science goals of PFI, define a baseline PFI architecture that can achieve
those goals, and identify key technical challenges that must be overcome before
the dreams of PFI can be realized within the typical cost envelope of a major
observatory. We also suggest activities over the next decade at the flagship US
facilities (CHARA, NPOI, MROI) that will help make the Planet Formation Imager
facility a reality. The key takeaway is that infrared interferometry will
require new experimental telescope designs that can scale to 8 m-class with the
potential to reduce per area costs by x10, a breakthrough that would also drive
major advances across astronomy.
