As discussed in Exoplanet Science Strategy (National Academies of Sciences Engineering
and Medicine 2018), WFIRST (Akeson et al. 2019) 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 main sequence stars. In semimajor axis, these planets fall between the close-in
planets found by Kepler (Coughlin et al. 2016) and the wide separation gas giants seen by
direct imaging (e.g. Lagrange et al. 2009) and ice giants inferred from ALMA observations
(Zhang et al. 2018). Furthermore, the smallest planets WFIRST can detect are smaller
than the planets probed by radial velocity (Mayor et al. 2011; Bonfils et al. 2013) and Gaia
(Perryman et al. 2014) 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⊕ at separations of 1 to 5 au.
Fundamentally, the case for a microlensing survey from space has not changed in the past
20 years: going to space allows wide-field diffraction-limited observations that can resolve
main-sequence stars in the bulge, which in turn allows the detection and characterization of
the smallest microlensing signals including those from planets with masses at least as small
as Mars (Bennett & Rhie 2002). What has changed is that ground-based microlensing is
reaching its limits, which underscores the scientific necessity for a space-based microlensing
survey to measure the population of the smallest planets. Ground-based microlensing has
found a break in the mass-ratio distribution at about a Neptune mass-ratio (Suzuki et al.
2016; Jung et al. 2018), implying that Neptunes are the most common microlensing planet
and that planets smaller than this are rare. However, ground-based microlensing reaches
its detection limits at mass ratios only slightly below the observed break. The WFIRST
microlensing survey will measure the shape of the mass-ratio function below the break by
finding numerous smaller planets: ~ 500 Neptunes, a comparable number of large gas giants,
and ~ 200 Earths (if they are as common as Neptunes), and it can detect planets as small
as 0.1M⊕ (Penny et al. 2018). In addition, because it will also measure host star masses and
distances, WFIRST will also track the behavior of the planet distribution as a function of
separation and host star mass