Searching for Free-Floating Planets with TESS: Results from Sectors 61-65

TL;DR

The paper investigates the detectability of terrestrial-mass free-floating planets (FFPs) via gravitational microlensing in the TESS dataset, exploiting high-cadence Full-Frame Images to search for short-duration events. Using a large-scale BLS-like search and a multi-stage vetting pipeline, the authors analyze 7.5 million light curves from TESS Sectors 61–65, finding one strong, microlensing-like event (TIC-107150013) whose FSPL interpretation yields a low-mass lens (approximately $0.3$–$0.5\,M_\oplus$) but whose implied event rate is inconsistent with simple abundance models. Extensive tests against false positives—stellar flares, binary interactions, and asteroid contamination—show that while a rare symmetric flare cannot be completely ruled out, microlensing remains the preferred explanation for the primary signal given current data, with the FSPL model providing the best fit. The study highlights the challenges of space-based microlensing backgrounds and demonstrates how cross-sector, Gaia-informed light curves can refine interpretations, informing future missions such as the Roman Space Telescope’s GBTDS and Earth 2.0. Even in the case of a non-detection, the work provides meaningful constraints on the local FFP population and offers a blueprint for optimizing yield calibrations and vetting strategies for upcoming space-based microlensing surveys.

Abstract

Though free-floating planets (FFPs) may outpopulate their bound counterparts in the terrestrial-mass range, they remain one of the least explored exoplanet demographics. Due to their negligible electromagnetic emission at all wavelengths, the only observational technique able to detect these worlds is gravitational microlensing. Microlensing by terrestrial-mass FFPs induces rare, short-duration magnifications of background stars, requiring high-cadence, wide-field surveys to detect these events. The Transiting Exoplanet Survey Satellite (TESS), though designed to detect close-bound exoplanets via transits, boasts a Full-Frame Image cadence as short as 200 seconds and has monitored hundreds of millions of stars, providing a unique dataset in which to search for rare short-duration transients. We have performed a preliminary search for FFP microlensing in 7.5 million light curves from TESS Sectors 61 - 65. We find one short-duration event with a light curve morphology consistent with expectations for a low-mass FFP, but in tension with the expected FFP abundance in this mass range. We consider possible false positive interpretations of this event such as stellar flares, hearbeat binaries, and centrifugal breakout. We find that all interpretations pose some challenges, and discuss the possibility that the event may constitute a first example of a new class of pernicious false positives that future space-based microlensing efforts will encounter. Our ongoing search through the TESS dataset will significantly support the upcoming hunt for rogue worlds with dedicated space-based microlensing surveys, and our results may be used alongside these surveys to place interesting constraints on the spatial distribution of FFPs in the Galaxy.

Figures (14)

• Figure 1: Expected FFP detection rate per dex in mass, across all currently available TESS sectors from QLP (Sectors 1 -- 71). The range of values correspond to different underlying mass functions, based on the central best-fit values describing the Sumi et al. power law and the $\pm1\sigma$ uncertainties sumi2023freefloating. TESS is most sensitive to FFP events between $0.01 < M/M_{\oplus} < 0.1$.
• Figure 2: Examples of BLS-detected brightening events corresponding to flares (left) and scattered light (right). The flare event has a skew shape parameter $\alpha = 137$ based on a fit to a skew normal distribution (red line), which indicates strong asymmetry. The scattered light event reflects the most common type of microlensing false positive in TESS data, representing $\sim$80% of BLS-detected events.
• Figure 3: Pixel-level data demonstrating typical signatures for asteroids (left) compared to isolated events (right), using examples associated with TIC-141479395 and TIC-432012378, respectively. These data were produced by subtracting the mean out-of-event image from each image within $1.5\tau$ of the center of a given event, and finding the maximum value of each pixel.
• Figure 4: Detrended QLP light curves from Sector 61 (top) and Sector 62 (bottom) for TIC-123147666, which feature a brightening event that was initially identified in Sector 61 as a potential microlensing event before repeating events were identified in other TESS sectors. This event is likely caused by tidal distortion in a highly eccentric binary star system. The heartbeat nature of the event is more obvious in Sector 62, further emphasizing the need for multiple sectors when performing microlensing searches with TESS.
• Figure 5: Pre-detrended QLP light curves from Sectors 7, 34, 61, 87, and 88 for TIC-107150013. There are no events with comparable significance to the event in Sector 61 at $\sim2987$ days. The increase in noise in later sectors is due to their shorter cadences, which are associated with higher per-point uncertainties. Sector 7 was observed at 30-min cadence, Sector 34 was observed at 10-min, and Sectors 61--88 were observed at 200-sec. The light curves potentially indicate stellar variability over timescales on the order of tens of days, but TESS alone is not able to constrain the amplitude and period of this variability due to systematic differences between TESS orbits.