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 ${\pi }_{\mathrm{E}}$ measurement. Combined with a measurement of the Einstein radius ${\theta }_{\mathrm{E}}$ 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.

arXiv comment18 pages, 10 Figures, re-submitted to JKAS