OGLE-2019-BLG-0960 Lb: the Smallest Microlensing Planet
Authors: Yee Jennifer C. , Zang Weicheng , Udalski Andrzej , Ryu Yoon-Hyun , Green Jonathan , Hennerley Steve , Marmont Andrew , Sumi Takahiro , Mao Shude ,   (Leading Authors), Gromadzki Mariusz , Mróz Przemek , Skowron Jan , Poleski Radoslaw , Szymański Michał K. , Soszyński Igor , Pietrukowicz Paweł , Kozłowski Szymon , Ulaczyk Krzysztof , Rybicki Krzysztof A. , Iwanek Patryk , Wrona Marcin ,   (The OGLE Collaboration), Albrow Michael D. , Chung Sun-Ju , Gould Andrew , Han Cheongho , Hwang Kyu-Ha , Jung Youn Kil , Kim Hyoun-Woo , Shin In-Gu , Shvartzvald Yossi , Cha Sang-Mok , Kim Dong-Jin , Kim Seung-Lee , Lee Chung-Uk , Lee Dong-Joo , Lee Yongseok , Park Byeong-Gon , Pogge Richard W. ,   (The KMTNet Collaboration), Bachelet Etienne , Christie Grant , Hundertmark Markus P. G. , Maoz Dan , McCormick Jennie , Natusch Tim , Penny Matthew T. , Street Rachel A. , Tsapras Yiannis ,   (The LCO and μFUN Follow-up Teams), Beichman Charles A. , Bryden Geoffery , Novati Sebastiano Calchi , Carey Sean , Gaudi B. Scott , Henderson Calen B. , Johnson Samson , Zhu Wei ,   (The Spitzer Team), Bond Ian A. , Abe Fumio , Barry Richard , Bennett David P. , Bhattacharya Aparna , Donachie Martin , Fujii Hirosane , Fukui Akihiko , Hirao Yuki , Silva Stela Ishitani , Itow Yoshitaka , Kirikawa Rintaro , Kondo Iona , Koshimoto Naoki , Alex Li Man Cheung , Matsubara Yutaka , Muraki Yasushi , Miyazaki Shota , Olmschenk Greg , Ranc Clément , Rattenbury Nicholas J. , Satoh Yuki , Shoji Hikaru , Suzuki Daisuke , Tanaka Yuzuru , Tristram Paul J. , Yamawaki Tsubasa , Yonehara Atsunori ,   (The MOA Collaboration)
Published: October 2021

Abstract
We report the analysis of OGLE-2019-BLG-0960, which contains the smallest mass-ratio microlensing planet found to date (q = 1.2–1.6 × 10−5 at 1σ). Although there is substantial uncertainty in the satellite parallax measured by Spitzer, the measurement of the annual parallax effect combined with the finite source effect allows us to determine the mass of the host star (M
L = 0.3–0.6 M
⊙), the mass of its planet (m

p
= 1.4–3.1 M
⊕), the projected separation between the host and planet (a
⊥ = 1.2–2.3 au), and the distance to the lens system (D
L = 0.6–1.2 kpc). The lens is plausibly the blend, which could be checked with adaptive optics observations. As the smallest planet clearly below the break in the mass-ratio function, it demonstrates that current experiments are powerful enough to robustly measure the slope of the mass-ratio function below that break. We find that the cross-section for detecting small planets is maximized for planets with separations just outside of the boundary for resonant caustics and that sensitivity to such planets can be maximized by intensively monitoring events whenever they are magnified by a factor A > 5. Finally, an empirical investigation demonstrates that most planets showing a degeneracy between (s > 1) and (s < 1) solutions are not in the regime (

∣
log
s
∣
≫
0

) for which the “close”/“wide” degeneracy was derived. This investigation suggests that there is a link between the “close”/“wide” and “inner/outer” degeneracies and also that the symmetry in the lens equation goes much deeper than symmetries uncovered for the limiting cases.