  Detection and orbit determination for thousands of planets with periods up to
about 40 years would be obtained by astrometry from two Gaia-like missions,
results which cannot be obtained by any other mission, planned or proposed. A
billion stars of all spectral types will be surveyed. A comprehensive knowledge
about heavy planets with these periods, reaching well beyond the snow line of
any system, will lead to a better understanding of the formation and evolution
of planets, also in the habitable zone.
