An Earth-mass planet in a time of COVID-19: KMT-2020-BLG-0414Lb

We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio q ₂ = 0.9-1.2 × 10^-5 = 3-4 ⊕ 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, A _max ~ 1450 for the underlying single-lens event, its light curve revealed a second companion with q ₃ ~ 0.05 and |log s ₃| ~ 1, i.e., a factor ~ 10 closer to or farther from the host in projection. The measurements of the microlens parallax π _E and the angular Einstein radius θE allow estimates of the host, planet and second companion masses, (M ₁, M ₂, M ₃) ~ (0.3 M _⊙, 1.0 M _⊕, 17 M_J ), the planet and second companion projected separations, (a _⊥,2, a _⊥,3) ~ (1.5, 0.15 or 15) au, and system distance D _L ~ 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.