Planetary mass as a function of orbital period for our simulated planets in the three inner disk-edge scenarios (FIX, OUT[M], and IN[C]; open symbols) and for the observed USP systems around late M dwarfs (0.08 < M_*/M_⊙ < 0.35; filled symbols). Blue circles denote isolated USP planets and red circles USP planets with detected companions. The vertical dashed line marks the 1-day boundary that defines USPs. Asymmetric error bars indicate the 1σ uncertainties on the observational mass estimates.