TESS Free-floating Planet Candidate Is Likely a Stellar Flare

TL;DR

This study reinterprets a TESS-detected brightening event initially thought to be a free-floating planet candidate as a stellar flare caused by magnetic activity, highlighting potential false positives in microlensing surveys.

Contribution

It demonstrates that stellar flares can mimic microlensing events in TESS data, emphasizing the importance of archival data for accurate interpretation.

Findings

Archival OGLE data shows starspot-induced variability in TIC 107150013.

Similar stellar flares are identified that mimic microlensing signals.

Stellar activity can lead to false positives in space-based microlensing surveys.

Abstract

The discovery of a terrestrial-mass free-floating planet candidate in the light curve of the star TIC 107150013 observed by the Transiting Exoplanet Survey Satellite (TESS) has recently been announced. A short-duration (~0.5 day), low-amplitude (~0.06 mag) brightening in the TESS light curve was interpreted as a short-timescale gravitational microlensing event. However, the purported event occurred far from the Galactic center and the Galactic plane (l~ 239 deg, b ~ -5 deg), on a relatively nearby (~3.2 kpc) star, making the microlensing interpretation unlikely. Here, we report the archival photometric observations of TIC 107150013 collected by the Optical Gravitational Lensing Experiment (OGLE) from 2018 through 2020. The archival OGLE light curve reveals periodic variability indicative of starspots on the surface of the star. The presence of starspots indicates magnetic activity of the star, which may also manifest as stellar flares. We interpret the brightening of TIC 107150013 seen in the TESS data as the stellar flare. We present similar flaring stars detected in the archival OGLE data, mimicking short-timescale, low-amplitude microlensing events. Such stars may be a source of non-negligible false positive detections in the planned space-based microlensing surveys.