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

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

Authors:

Hwang et al

Abstract:
We present the discovery of microlensing planet OGLE-2017-BLG-0173Lb, with planet-host mass ratio q=6.5×10−5, among the lowest ever detected. The planetary perturbation is nevertheless quite strongly detected, with Δχ2∼10,000, because it arises from a bright (therefore, large) source passing over, and partially enveloping, the planetary caustic. We present a simple formalism that can be used to estimate the sensitivity of other giant-source ("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 3×10−6. The best estimated Bayesian parameters for this system are host-mass M=0.42+0.40−0.24M⊙, planet mass, mp=9+11−6M⊕, and projected separation $a_\perp \sim 4\,\au$. The measured lens-source relative proper motion $\mu=6\,\masyr$ will permit imaging of the lens in about 15 years or at first light on adaptive-optics imagers on next-generation ("30 meter") telescopes, whichever comes first.