  Planets are thought to form via accretion from a remnant disk of gas and
solids around a newly formed star. During this process material in the disk
either remains bound to the star as part of either a planet, a smaller
celestial body, or makes up part of the interplanetary medium; falls into the
star; or is ejected from the system. Herein we use dynamical models to probe
the abundance and properties of ejected material during late-stage planet
formation and estimate their contribution to the free-floating planet
population. We present 300 N-body simulations of terrestrial planet formation
around a solar-type star, with and without giant planets present, using a model
that accounts for collisional fragmentation. In simulations with Jupiter and
Saturn analogs, about one-third of the initial (~5 Mearth) disk mass is
ejected, about half in planets more massive than Mercury but less than 0.3
Mearth, and the remainder in smaller bodies. Most ejections occur within 25
Myr, which is shorter than the timescale typically required for Earth-mass
planets to grow (30-100 Myr). When giant planets are omitted from our
simulations, almost no material is ejected within 200 Myr and only about 1% of
the initial disk is ejected by 2 Gyr. We show that about 2.5 terrestrial-mass
planets are ejected per star in the Galaxy. We predict that the space-borne
microlensing search for free-floating planets from the Wide-Field Infra-Red
Space Telescope (WFIRST) will discover up to 15 Mars-mass planets, but few
free-floating Earth-mass planets.
