Detecting Exomoons in Free-Floating-Planet Events from Space-based Microlensing Surveys

TL;DR

This paper evaluates the potential of space-based microlensing surveys, specifically CSST and Roman, to detect exomoons orbiting free-floating planets by simulating light curves and analyzing detection efficiencies across various parameters.

Contribution

It introduces a detailed simulation framework to assess the detectability of exomoons around free-floating planets in upcoming space-based microlensing surveys, highlighting the sensitivity limits and optimal conditions.

Findings

CSST can detect Earth-mass satellites around Neptune-like free-floating planets.

Sensitivity extends to Moon-mass satellites at certain separations.

Roman shows greater sensitivity, especially for M-dwarf sources.

Abstract

When a planet is ejected from its star-planet system due to dynamical interactions, its satellite may remain gravitationally bound to the planet. The Chinese Space Station Telescope (CSST) will be capable of detecting a large number of low-mass free-floating planet events (FFPs) from a bulge microlensing survey. We assess the feasibility of detecting satellites (a.k.a., exomoons) orbiting FFPs by simulating CSST light curves and calculating the detection efficiency as a function of satellite-to-planet mass ratios (q) and projected separations (s) in units of the Einstein radius. For a Neptune-class FFP in the Galactic disk with a Sun-like star as the microlensed source, CSST can detect Earth-mass satellites over a decade of separations (from ~0.01 to ~0.1 AU) and has sensitivity down to Moon-mass satellites (q~1e-3) at s~1. CSST also has some sensitivity to detect Moon-mass satellites at s~2 (~0.02 AU) orbiting an Earth-mass FFP in the disk. CSST has substantially reduced sensitivity for detecting satellites when the source star is an M dwarf, compared to a Sun-like source. We also calculate the satellite detection efficiency for the dedicated microlensing survey of the Roman Space Telescope (Roman), which demonstrates greater sensitivity than CSST, particularly for M-dwarf sources. Notably, some of the Neptune-Earth systems detectable by CSST and Roman may exhibit significant tidal heating.