KMT-2018-BLG-0029Lb: A Very Low Mass-Ratio Spitzer Microlens Planet

Abstract

At $q=1.81\pm 0.20\times 10^{-5}$, KMT-2018-BLG-0029Lb has the lowest planet-host mass ratio $q$ of any microlensing planet to date by more than a factor of two. Hence, it is the first planet that probes below the apparent "pile-up" at $q=5$--10 $\times 10^{-5}$. The event was observed by {\it Spitzer}, yielding a microlens-parallax $\ddot{I}{€}_{\mathrm{E}}$ measurement. Combined with a measurement of the Einstein radius $\hat{I}{¸}_{\mathrm{E}}$ from finite-source effects during the caustic crossings, these measurements imply masses of the host $M_{\mathrm{host}}=1.14_{-0.12}^{+0.10}{M}_{\odot }$ and planet $M_{\mathrm{planet}}=7.59_{-0.69}^{+0.75}{M}_{\oplus }$, system distance $D_L=3.38_{-0.26}^{+0.22}\mathrm{kpc}$ and projected separation $a_{\perp }=4.27_{-0.23}^{+0.21}\mathrm{au}$. The blended light, which is substantially brighter than the microlensed source, is plausibly due to the lens and could be observed at high resolution immediately.

18 pages, 10 Figures, re-submitted to JKAS