Recently Sahu et al. , using the Hubble Space Telescope to monitor stars in the direction of the old ( \sim 12 ~ { } { Gyr } ) globular cluster M22 , detected six events in which otherwise constant stars brightened by \sim 50 \% during a time of \lesssim 1 ~ { } { day } .
They tentatively interpret these unresolved events as due to microlensing of background bulge stars by free-floating planets in M22 .
Using simple analytic arguments , I show that if these spike events are due to microlensing , the lensing objects are unlikely to be associated with M22 , and unlikely to be part of a smoothly distributed Galactic population .
Thus either there happens to be a massive , dark cluster of planets along our line-of-sight to M22 , or the spike events are not due to microlensing .
The lensing planets can not be bound to stars in the core of M22 : if they were closer than \sim 8 ~ { } { AU } , the lensing influence of the parent star would have been detectable .
Moreover , in the core of M22 , all planets with separations \gtrsim 1 ~ { } { AU } would have been ionized by random stellar encounters .
Most unbound planets would have escaped the core via evaporation which preferentially affects such low-mass objects .
Bound or free-floating planets can exist in the outer halo of M22 ; however , for reasonable assumptions , the maximum optical depth to such a population falls short of the observed optical depth , \tau \sim 3 \times 10 ^ { -6 } , by a factor of 5-10 .
Therefore , if real , these events represent the detection of a significant free-floating Galactic planet population .
The optical depth to these planets is comparable to and mutually exclusive from the optical depth to resolved events measured by microlensing survey collaborations toward the bulge , and thus implies a similar additional mass of lensing objects .
If the spatial and kinematic distributions of the two populations are the same , there are > 10 ^ { 3 } planets per bulge microlens .
Such a population is difficult to reconcile with both theory and observations .