MOA-2022-BLG-091Lb and KMT-2024-BLG-1209Lb: Microlensing planets detected through weak caustic-crossing signals

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
² Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
³ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁴ Institute of Natural and Mathematical Science, Massey University, Auckland 0745, New Zealand
⁵ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, China
⁶ University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁷ Department of Astronomy, Ohio State University, 140 West 18th Ave., Columbus, OH 43210, USA
⁸ University of Science and Technology, Daejeon 34113, Republic of Korea
⁹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁰ Center for Astrophysics | Harvard & Smithsonian 60 Garden St., Cambridge, MA 02138, USA
¹¹ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea
¹² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹³ Villanova University, Department of Astrophysics and Planetary Sciences, 800 Lancaster Ave., Villanova, PA 19085, USA
¹⁴ Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan
¹⁵ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
¹⁶ Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁷ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁸ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁹ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
²⁰ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
²¹ Oak Ridge Associated Universities, Oak Ridge, TN 37830, USA
²² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
²³ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
²⁴ Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
²⁵ University of Canterbury Mt. John Observatory, PO Box 56, Lake Tekapo 8770, New Zealand
²⁶ School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 6997801, Israel

^★ Corresponding author: leecu@kasi.re.kr

Received: 19 February 2025
Accepted: 28 May 2025

Abstract

Aims. The light curves of the microlensing events MOA-2022-BLG-091 and KMT-2024-BLG-1209 exhibit anomalies with very similar features. These anomalies appear near the peaks of the light curves, where the magnifications are moderately high, and are distinguished by weak caustic-crossing features with minimal distortion while the source remains inside the caustic. To achieve a deeper understanding of these anomalies, we conducted a comprehensive analysis of the lensing events.

Methods. We carried out binary-lens modeling with a thorough exploration of the parameter space. This analysis revealed that the anomalies in both events are of planetary origin, although their exact interpretation is complicated by different types of degeneracy. In the case of MOA-2022-BLG-091, the main difficulty in the interpretation of the anomaly arises from a newly identified degeneracy related to the uncertain angle at which the source trajectory intersects the planet–host axis. For KMT-2024-BLG-1209, the interpretation is affected by the previously known inner-outer degeneracy, which leads to ambiguity between solutions in which the source passes through either the inner or outer caustic region relative to the planet host.

Results. Bayesian analysis indicates that the planets in both lens systems are giant planets with masses about two to four times that of Jupiter, orbiting early K-type main-sequence stars. Both systems are likely located in the Galactic disk at a distance of around 4 kiloparsecs. The degeneracy in KMT-2024-BLG-1209 is challenging to resolve because it stems from intrinsic similarities in the caustic structures of the degenerate solutions. In contrast, the degeneracy in MOA-2022-BLG-091, which occurs by chance rather than from inherent characteristics, is expected to be resolved by the future space based Roman RGES microlensing survey.