On the Feasibility of Characterizing Free-floating Planets with Current and Future Space-based Microlensing Surveys

Abstract

Simultaneous space- and ground-based microlensing surveys, such as K2's Campaign 9 (K2C9) and $WFIRST$, facilitate measuring the masses and distances of free-floating planet (FFP) candidates. FFPs are identified as single-lens events with a short timescale, of-order 1 day. Measuring the mass of the lensing object requires determining the finite size of the source star $ρ$, as well as the microlens parallax $π_{\rm E}$. A planet that is bound to but widely separated from a host star can produce a light curve similar to that of an FFP. This tension can be resolved with high-resolution imaging of the microlensing target to search for the lens flux $F_l$ from a possible host star. Here we investigate the accessible parameter space for each of these components --- $π_{\rm E}$, $ρ$, and $F_l$ --- considering different satellites for a range of FFP masses, Galactic distances, and source star properties. We find that at the beginning of K2C9, when its projected separation from the Earth (as viewed from the center of its survey field) is $\lesssim$0.2 AU, it will be able to measure $π_{\rm E}$ for Jupiter-mass FFP candidates at distances larger than $\sim$2 kpc and to Earth-mass lenses at $\sim$8 kpc. At the end of its campaign, when $D_\perp = 0.81$ AU, it is sensitive to planetary-mass lenses for distances $\gtrsim$3.5 kpc, and even then only to those with mass $\gtrsim$M$_{\rm Jup}$. From lens flux constraints we find that it will be possible to exclude all stellar-mass host stars (down to the deuterium-burning limit) for events within $\sim$2 kpc. Together these indicate that the ability to characterize FFPs detected during K2C9 is optimized for events occurring toward the beginning of the campaign. $WFIRST$, on the other hand, will be able to detect and characterize FFPs with masses at least as low as super-Earths throughout the Galaxy during its entire microlensing survey.