Roman FFP Revolution: Two, Three, Many Plutos

Roman microlensing stands at a crossroads between its originally charted path of cataloging a population of cool planets that has subsequently become well-measured down to super-Earths, and the path of free-floating planets (FFPs), which did not exist when Roman was chosen in 2010, but by now promises revolutionary insights into planet formation and evolution via their possible connection to a spectrum of objects spanning 18 decades in mass. Until now, it was not even realized that the 2 paths are in conflict: Roman strategy was optimized for bound-planet detections, and FFPs were considered only in the context of what could be learned about them given this strategy. We derive a simple equation that mathematically expresses this conflict and explains why the current approach severely depresses detection of 2 of the 5 decades of potential FFP masses, i. e. , exactly the two decades, M ₏₋ₔₓ₎ 3. 0, when normalized to the adopted Roman cadence =4/hr, and to source radius _*=0. 3\, as, lens-source proper motion =6\, mas/yr, and source impact parameter z=0. 5, which are all typical values. By contrast N=6 are needed for an FFP detection. Thus, unless is doubled, FFP detection will be driven into the (large-_*, small-) corner of parameter space, reducing the detections by a net factor of 2 and cutting off the lowest-mass FFPs.