An Earth-mass Planet in a Time of Covid-19: KMT-2020-BLG-0414Lb

Abstract

We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio $q_2=0.9$--$1.2\times 10^{-5}=3$--$4q_{\oplus }$ at $1\ddot{I}ƒ$, which is the lowest mass-ratio microlensing planet to date. Together with two other recent discoveries ($4\lesssim q/q_{\oplus }\lesssim 6$), 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 $\hat{I}{¸}_{\mathrm{E}}$. 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, $A_{\max }\sim 1450$ 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 $\ddot{I}{€}_{\mathrm{E}}$ and the angular Einstein radius $\hat{I}{¸}_{\mathrm{E}}$ allow estimates of the host, planet, and second companion masses, $(M_1,M_2,M_3)\sim (0.3M_{\odot },1.0M_{\oplus },17M_J)$, the planet and second companion projected separations, $(a_{\perp ,2},a_{\perp ,3})\sim (1.5,0.15\mathrm{or}15)$~au, and system distance $D_{\mathrm{L}}\sim 1$ 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.

23 pages, 5 figures; to be submitted to RAA