TESS's First Bound Microlensing Planet: A Binary Microlensing Event Revealing a Planetary Companion toward the Galactic Plane

Abstract

We report the discovery of Gaia23bra b, the first gravitationally bound microlensing planet detected by the Transiting Exoplanet Survey Satellite (TESS). Initially flagged as a single-lens event by the Gaia Science Alerts system, Gaia23bra was serendipitously observed by TESS over two consecutive sectors. During those TESS sectors, the light curve of the event displayed caustic-crossing features characteristic of a binary-lens event. Joint modeling of Gaia and TESS photometry with pyLIMA, supplemented by stellar parameter inference using pyLIMASS, suggests a K dwarf ($M_L=0.79_{-0.17}^{+0.19}{M}_{\odot }$) hosting a Jovian planet with $M_P=1.63_{-0.38}^{+0.42}{M}_{\mathrm{Jup}}$ at a projected separation of $a_{\perp ,\min }\approx 4.8\mathrm{AU}$. This result underscores the synergy between high-cadence photometry and long-baseline monitoring for robust microlensing characterization. Its location along the Galactic Plane highlights TESS's unexpected capacity for microlensing science through its all-sky coverage and its potential to detect planets in regions beyond the Galactic Bulge.

16 pages, 12 figures

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