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We present microlensing planet OGLE-2017-BLG-0173Lb, with planet-host massratio either $q\simeq 2.5\times 10^{-5}$ or $q\simeq 6.5\times 10^{-5}$, thelowest or among the lowest ever detected. The planetary perturbation isstrongly detected, $\Delta\chi^2\sim 10,000$, because it arises from a bright(therefore, large) source passing over and enveloping the planetary caustic: aso-called "Hollywood" event. The factor $\sim 2.5$ offset in $q$ arises becauseof a previously unrecognized discrete degeneracy between Hollywood events inwhich the caustic is fully enveloped and those in which only one flank isenveloped, which we dub "Cannae" and "von Schlieffen", respectively. Thisdegeneracy is "accidental" in that it arises from gaps in the data.Nevertheless, the fact that it appears in a $\Delta\chi^2=10,000$ planetaryanomaly is striking. We present a simple formalism to estimate the sensitivityof other Hollywood events to planets and show that they can lead to detectionsclose to, but perhaps not quite reaching, the Earth/Sun mass ratio of $3\times10^{-6}$. This formalism also enables an analytic understanding of the factor$\sim 2.5$ offset in $q$ between the Cannae and von Schlieffen solutions. TheBayesian estimates for the host-mass, system distance, and planet-hostprojected 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 estimatesof 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$\mu=6\,\masyr$ will permit imaging of the lens in about 15 years or at firstlight on adaptive-optics imagers on next-generation telescopes. These willallow to measure the host mass but probably cannot resolve the planet-hostmass-ratio degeneracy.

OGLE-2017-BLG-0173Lb: Low Mass-Ratio Planet in a "Hollywood" Microlensing Event