OGLE-2017-BLG-0448Lb: A Low Mass-Ratio Wide-Orbit Microlensing Planet?

The gravitational microlensing technique is most sensitive to planets in a Jupiter-like orbit and has detected more than 200 planets. However, only a few wide-orbit (\(s > 2\)) microlensing planets have been discovered, where \(s\) is the planet-to-host separation normalized to the angular Einstein ring radius, \(\theta_{\rm E}\). Here we present the discovery and analysis of a strong candidate wide-orbit microlensing planet in the event, OGLE-2017-BLG-0448. The whole light curve exhibits long-term residuals to the static binary-lens single-source model, so we investigate the residuals by adding the microlensing parallax, microlensing xallarap, an additional lens, or an additional source. For the first time, we observe a complex degeneracy between all four effects. The wide-orbit models with \(s \sim 2.5\) and a planet-to-host mass-ratio of \(q \sim 10^{-4}\) are significantly preferred, but we cannot rule out the close models with \(s \sim 0.35\) and \(q \sim 10^{-3}\). A Bayesian analysis based on a Galactic model indicates that, despite the complicated degeneracy, the surviving wide-orbit models all contain a super-Earth-mass to Neptune-mass planet at a projected planet-host separation of \(\sim 6\) au and the surviving close-orbit models all consist of a Jovian-mass planet at \(\sim 1\) au. The host star is probably an M or K dwarf. We discuss the implications of this dimension-degeneracy disaster on microlensing light-curve analysis and its potential impact on statistical studies.