  We propose to develop a wide-field and ultra-high-precision photometric
survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to
measure, for the first time, the occurrence rate and the orbital distributions
of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to
the Earth-Sun's L2 point. Six of these are transit telescopes with a field of
view of 500 square degrees. Staring in the direction that encompasses the
original Kepler field for four continuous years, this monitoring will return
tens of thousands of transiting planets, including the elusive Earth twins
orbiting solar-type stars. The seventh telescope is a 30cm microlensing
telescope that will monitor an area of 4 square degrees toward the galactic
bulge. This, combined with simultaneous ground-based KMTNet observations, will
measure masses for hundreds of long-period and free-floating planets. Together,
the transit and the microlensing telescopes will revolutionize our
understandings of terrestrial planets across a large swath of orbital distances
and free space. In addition, the survey data will also facilitate studies in
the fields of asteroseismology, Galactic archeology, time-domain sciences, and
black holes in binaries.
