We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio q2 = 0:9- 1:2 × 10-5 = 3-4 q⊕ at 1σ, which is the lowest mass-ratio microlensing planet to date. Together with two other recent discoveries (4. q=q⊕. 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 θE. 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, Amax ∼ 1450 for the underlying single-lens event, its light curve revealed a second companion with q3 ∼ 0:05 and j log s3j ∼ 1, i.e., a factor ∼ 10 closer to or farther from the host in projection. The measurements of the microlens parallax ∼E and the angular Einstein radius ∼E allow estimates of the host, planet and second companion masses, (M1;M2;M3) ∼ (0:3M⊙; 1:0M⊙; 17MJ ), the planet and second companion projected separations, (a⊥;2; a⊥;3) ∼ (1:5; 0:15 or 15) au, and system distance DL ∼ 1 kpc. The lens could account for most or all of the blended light (I ∼ 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.
- Gravitational lensing: Micro