  [abridged] WFIRST is uniquely capable of finding planets with masses as small
as Mars at separations comparable to Jupiter, i.e., beyond the current ice
lines of their stars. These planets fall between the close-in planets found by
Kepler and the wide separation gas giants seen by direct imaging and ice giants
inferred from ALMA observations. Furthermore, the smallest planets WFIRST can
detect are smaller than the planets probed by RV and Gaia at comparable
separations. Interpreting planet populations to infer the underlying formation
and evolutionary processes requires combining results from multiple detection
methods to measure the full variation of planets as a function of planet size,
orbital separation, and host star mass. Microlensing is the only way to find
planets from 0.5 to 5M_E at 1 to 5au. The case for a microlensing survey from
space has not changed in the past 20 yrs: space allows wide-field
diffraction-limited observations that resolve main-sequence stars in the bulge,
which allows the detection and characterization of the smallest signals
including those from planets with masses at least as small as Mars. What has
changed is that ground-based (GB) microlensing is reaching its limits,
underscoring the scientific necessity for a space-based survey. GB microlensing
has found a break in the mass-ratio distribution at about a Neptune, implying
that these are the most common microlensing planet and that planets smaller
than this are rare. However, GB microlensing reaches its detection limits only
slightly below the observed break. WFIRST will measure the shape of the
mass-ratio function below the break by finding numerous smaller planets: 500
Neptunes, 600 gas giants, 200 Earths, and planets as small as 0.1M_E. Because
it will also measure host masses and distances, WFIRST will also track the
behavior of the planet distribution as a function of separation and host star
mass.
