HST pre-imaging of a free-floating planet candidate microlensing event

¹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
² Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea
³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁴ Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Department of Physics, University of Warwick, Coventry CV4 7 AL, UK
⁶ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁷ Department of Astrophysics and Planetary Sciences, Villanova University, 800 Lancaster Ave., Villanova, PA 19085, USA
⁸ University of Canterbury, School of Physical and Chemical Sciences, Private Bag 4800, Christchurch 8020, New Zealand
⁹ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
¹¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
¹² National University of Science and Technology (UST), Daejeon 34113, Republic of Korea
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁴ School of Science, Westlake University, Hangzhou, Zhejiang 310030, China
¹⁵ Department of Astronomy, Tsinghua University, Beijing 100084, China
¹⁶ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea
¹⁷ Center for Cosmology and AstroParticle Physics, Ohio State University, 191 West Woodruff Ave., Columbus, OH 43210, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 June 2025
Accepted: 25 October 2025

Abstract

High-cadence microlensing observations uncovered a population of very short-timescale microlensing events, which are believed to be caused by the population of free-floating planets (FFPs) roaming the Milky Way. Unfortunately, the light curves of such events are indistinguishable from those caused by wide-orbit planets. To properly differentiate both cases, one needs high-resolution observations that would allow one to resolve a putative luminous companion to the lens long before or after the event. Usually, the baseline between the event and high-resolution observations needs to be quite long (∼10 yr), hindering potential follow-up efforts. However, there is a chance to use archival data if they exist. Here, we present an analysis of the microlensing event OGLE-2023-BLG-0524, the site of which was captured in 1997 with the Hubble Space Telescope (HST). Hence, we achieve a record-breaking baseline length of 25 years. A very short duration of the event (t_E = 0.346 ± 0.008 d) indicates an FFP as the explanation. Based on the finite-source effects in the light curve, we measure an angular Einstein radius value of θ_E = 4.78 ± 0.23 μas, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Though we detect no potential companion to the lens in the HST imaging, we find that the HST imaging is insufficient to constrain beyond 25-48% of potential companions (depending on whether or not the occurrence rate of wide-orbit planets is correlated with the host mass); hence, we are unable to confidently confirm this event as an FFP. Despite this, our results show that archival high-resolution images should be available for many microlensing events, providing a potential new avenue to confirm FFP candidates in future observations.