  Despite the revolution in our knowledge resulting from the detection of
planets around mature stars, we know almost nothing about planets orbiting
young stars because rapid rotation and active photospheres preclude detection
by radial velocities or transits and because direct imaging has barely
penetrated the requisite range of high contrast and angular resolution. Of the
techniques presently under consideration for the coming decade, only
space-based astrometry offers the prospect of discovering gas giants (100 to >>
300 Mearth), lower mass systems such as icy giants (10 to 100 Mearth), and even
a few rocky, super-Earths 300 Mearth) orbiting stars ranging in age from 1 to
100 Myr. Astrometry will complement high contrast imaging which should be able
to detect gas giants (1~10 MJup) in orbits from a few to a few hundred AU. An
astrometric survey in combination with imaging data for a subsample of objects
will allow a detailed physical understanding of the formation and evolution of
young gas giant planets impossible to achieve by any one technique.
