How Rare are TESS Free-Floating Planets?

TL;DR

This study assesses TESS's capability to detect free-floating planets via microlensing, finding very low expected detection rates and suggesting future missions are needed for better understanding.

Contribution

It provides a detailed analysis of TESS's microlensing detection potential for FFPs, incorporating source properties and survey limitations, and estimates detection rates.

Findings

Expected TESS FFP detections are extremely low (about 0.0018 in Sector 61).

The reported TESS signal is likely due to stellar flare, not microlensing.

Future missions like Roman and Earth 2.0 could detect thousands of FFPs.

Abstract

Recently, Kunimoto et al. claimed that a short-lived signal in the Transiting Exoplanet Survey Satellite (TESS) Sector 61 database was possibly caused by a microlensing event with a terrestrial-mass free-floating planet (FFP) lens. In this study, we investigate TESS's ability to detect microlensing FFPs by considering the detailed source information (e.g., distance and radius), the TESS photometric accuracy, and finite-source effects. Using the FFP mass function from microlensing surveys toward the Galactic bulge, we find that only $0.0018$ microlensing events are expected to be detected in TESS Sector 61 for the entire planetary mass range. The reported signal is unlikely to be a real microlensing event, which is consistent with the evidence from the long-term OGLE data that the signal was likely due to a stellar flare. By extrapolating our result to fainter stars until $T = 16$ mag and adopting a possible optimized search algorithm, we find that only $\sim 1$ FFP events can be detected in the entire TESS mission within the first 7 years. Significant improvments of our understanding of FFPs still requires future satellite missions, such as Roman and Earth 2.0, which can detect thousands of FFPs.