  The extraordinary astrometric accuracy of radio interferometry creates an
important and unique opportunity for the discovery and characterization of
exo-planets. Currently, the Very Long Baseline Array can routinely achieve
better than 100 microarcsecond accuracy, and can approach 10 microarcsecond
with careful calibration. We describe here RIPL, the Radio Interferometric
PLanet search, a new program with the VLBA and the Green Bank 100 m telescope
that will survey 29 low-mass, active stars over 3 years with sub-Jovian planet
mass sensitivity at 1 AU. An upgrade of the VLBA bandwidth will increase
astrometric accuracy by an order of magnitude. Ultimately, the colossal
collecting area of the Square Kilometer Array could push astrometric accuracy
to 1 microarcsecond, making detection and characterizaiton of Earth mass
planets possible.
  RIPL and other future radio astrometric planet searches occupy a unique
volume in planet discovery and characterization parameter space. The parameter
space of astrometric searches gives greater sensitivity to planets at large
radii than radial velocity searches. For the VLBA and the expanded VLBA, the
targets of radio astrometric surveys are by necessity nearby, low-mass, active
stars, which cannot be studied efficiently through the radial velocity method,
coronagraphy, or optical interferometry. For the SKA, detection sensitivity
will extend to solar-type stars. Planets discovered through radio astrometric
methods will be suitable for characterization through extreme adaptive optics.
  The complementarity of radio astrometric techniques with other methods
demonstrates that radio astrometry can play an important role in the roadmap
for exoplanet discovery and characterization.
