We present microlensing planet OGLE-2017-BLG-0173Lb , with planet-host mass ratio either q \simeq 2.5 \times 10 ^ { -5 } or q \simeq 6.5 \times 10 ^ { -5 } , the lowest or among the lowest ever detected .
The planetary perturbation is strongly detected , \Delta \chi ^ { 2 } \sim 10 , 000 , because it arises from a bright ( therefore , large ) source passing over and enveloping the planetary caustic : a so-called “ Hollywood ” event .
The factor \sim 2.5 offset in q arises because of a previously unrecognized discrete degeneracy between Hollywood events in which the caustic is fully enveloped and those in which only one flank is enveloped , which we dub “ Cannae ” and “ von Schlieffen ” , respectively .
This degeneracy is “ accidental ” in that it arises from gaps in the data .
Nevertheless , the fact that it appears in a \Delta \chi ^ { 2 } = 10 , 000 planetary anomaly is striking .
We present a simple formalism to estimate the sensitivity of other Hollywood events to planets and show that they can lead to detections close to , but perhaps not quite reaching , the Earth/Sun mass ratio of 3 \times 10 ^ { -6 } .
This formalism also enables an analytic understanding of the factor \sim 2.5 offset in q between the Cannae and von Schlieffen solutions .
The Bayesian estimates for the host-mass , system distance , and planet-host projected separation are M = 0.39 ^ { +0.40 } _ { -0.24 } M _ { \odot } , D _ { L } = 4.8 ^ { +1.5 } _ { -1.8 } { kpc } , and a _ { \perp } = 3.8 \pm 1.6 { AU } .
The two estimates of the planet mass are m _ { p } = 3.3 ^ { +3.8 } _ { -2.1 } M _ { \oplus } and m _ { p } = 8 ^ { +11 } _ { -6 } M _ { \oplus } .
The measured lens-source relative proper motion \mu = 6 { mas } { yr } ^ { -1 } will permit imaging of the lens in about 15 years or at first light on adaptive-optics imagers on next-generation telescopes .
These will allow to measure the host mass but probably can not resolve the planet-host mass-ratio degeneracy .