  High-precision astrometry at the sub-microarcsecond level opens up a window
to study Earth-like planets in the habitable zones of Sun-like stars, and to
determine their masses. It thus promises to play an important role in exoplanet
science in the future. However, such precision can only be acquired from space,
and requires dedicated instrumentation for a sufficient astrometric
calibration. Here we present a series of concepts designed for handling this
task. STARE is a small satellite concept dedicated to finding planets in the
very nearest stellar systems, which offers a low-cost option toward the study
of habitable planets. The NEAT concept is a set of two formation-flying
satellites with the aim to survey the 200 nearest Sun-like stars for Earths in
the habitable zone. Finally, THEIA is a proposal for an ESA M-class mission,
with a single-unit telescope designed for both dark matter studies as well as a
survey for habitable Earth-like planets among the 50 nearest stars. The
concepts illustrate various possible paths and strategies for achieving
exquisite astrometric performance, and thereby addressing key scientific
questions regarding the distribution of habitability and life in the universe.
