  Microlensing is the only method that can detect and measure mass of wide
orbit, low mass, solar system analog exoplanets. Mass measurements of such
planets would yield massive science on planet formation, exoplanet
demographics, free floating planets, planet frequencies towards the galaxy.
High res follow-up observations of past microlens targets provide a mass
measurement of microlens planets and hosts at an uncertainty of <20%. This will
be primary method for mass measurement with WFIRST. We advocate for the fact
that high resolution observations with AO, HST and JWST(in future) remain
necessary in coming decade to develop the methods, to determine the field and
filter selection, understand the systematics and to develop a robust pipeline
to release high quality data products from WFIRST microlensing survey such that
the astronomy community can promptly engage in the science. We also support
future high res obs with US ELTs with advanced Laser AO systems in context of
enhancing the science return of WFIRST microlensing survey.
  We endorse the 2018 Exoplanet Science Strategy report published by the
National Academy. This white paper extends and complements the material
presented therein. In particular, this white paper supports the recommendation
of the National Academy Exoplanet Science Strategy report that: NASA should
launch WFIRST to conduct its microlensing survey of distant planets and to
demonstrate the technique of coronagraphic spectroscopy on exoplanet targets.
This white paper also supports to the finding from that report which states "A
number of activities, including precursor and concurrent observations using
ground- and space-based facilities, would optimize the scientific yield of the
WFIRST microlensing survey."
