Abstract We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio \(q_2 = 0.9 - 1.2 \times 10^{-5} = 3-4 q_{\bigoplus}\) at 1σ, which is the lowest mass-ratio microlensing planet to date. Together with two other recent discoveries (\(4\lesssim q/q_{\bigoplus} \lesssim 6\)), it fills out the previous empty sector at the bottom of the triangular (log \(s\), log \(q\)) diagram, where \(s\) is the planet-host separation in units of the angular Einstein radius \(\theta_{\rm E}\). Hence, these discoveries call into question the existence, or at least the strength, of the break in the mass-ratio function that was previously suggested to account for the paucity of very low-\(q\) planets. Due to the extreme magnification of the event, \(A_{\rm max}\sim 1450\) for the underlying single-lens event, its light curve revealed a second companion with \(q_3 \sim 0.05\) and \(| \log s_3| \sim 1\), i.e., a factor \(\sim\) 10 closer to or farther from the host in projection. The measurements of the microlens parallax \(\pi_{\rm E}\) and the angular Einstein radius \(\theta_{\rm E}\) allow estimates of the host, planet and second companion masses, \((M_1, M_2, M_3) \sim (0.3 M_{\odot}, 1.0 M_{\bigoplus}, 17 M_J)\), the planet and second companion projected separations, \((a_{\perp,2}, a_{\perp,3}) \sim\) (1.5, 0.15 or 15) au, and system distance \(D_{\rm L} \sim 1\) kpc. The lens could account for most or all of the blended light (\(I \sim 19.3\)) and so can be studied immediately with high-resolution photometric and spectroscopic observations that can further clarify the nature of the system. The planet was found as part of a new program of high-cadence follow-up observations of high-magnification events. The detection of this planet, despite the considerable difficulties imposed by COVID-19 (two KMT sites and OGLE were shut down), illustrates the potential utility of this program.

Keywords planets and satellites: detection, gravitational lensing: micro, (stars:) brown dwarfs, planets and satellites: terrestrial planets

https://doi.org/10.1088/1674-4527/21/9/239