Why a star can pull a moon away from its planet
A moon is bound to its planet, but the star also tugs on it. When the moon is far enough from the planet, the star's gravitational pull can overcome the planet's hold, and the moon's orbit becomes unstable [1]. Numerical simulations show that the outermost stable orbit shrinks as the planet's orbit becomes more eccentric—more stretched into an oval—and as the planet becomes less massive relative to the star [1].
There is a subtle exception. For planets that are relatively massive compared with their star, simulations find a detached island of stability at intermediate eccentricities (roughly 0.2 to 0.7), linked to a gravitational resonance called the evection resonance [1]. In that narrow range, a moon can survive farther out than the classical stability limit would suggest. The simulations also show that the starting orbital phase of the moon relative to the star can shift the stability threshold by up to about 10%, so the exact boundary is not razor-sharp [1].
Tides: the slow inward spiral
Stars also raise tides on planets, and those tides can drain orbital energy from a moon. In close-in systems, tidal interactions can make a moon's orbit decay over time, and in extreme cases the moon can be destroyed [2]. The effect depends on the star's mass and the planet–moon distance: moons are more stable when the planet is farther from the star, when the planet is heavier, or when the star is lighter [3].
The moon's own tides matter too. Earlier estimates of moon lifetimes ignored how a massive moon tugs back on its planet's rotation; including that lunar tide allows significantly longer lifetimes for large moons around Neptune-mass and super-Earth planets [3]. Based on these tidal models, the first exomoon around a Sun-like star is expected to be found at a planet–star distance of about 0.4–0.6 AU, or about 0.2–0.4 AU for a smaller, cooler M-type star [3].
Stars can also interfere before a moon forms
The star's influence begins even earlier, when a moon-forming disk of debris surrounds a young planet. Simulations of collisions around rocky planets with orbital periods of 1–300 days show that stellar perturbations can disrupt the development and survival of such disks compared with identical collisions in isolation [5]. In other words, a star can limit not just where an existing moon can orbit, but whether a sizable moon can assemble in the first place.
What this means for finding moons beyond our solar system
These results give observers a practical map. The stability limits from simulations can be used to constrain where to look for exomoons and which planetary systems are worth targeting [1]. Tidal models add a second filter: moons around very close-in giant planets are likely to be tidally perturbed or destroyed, while moons farther out around lighter stars have better odds [2][3]. The picture is not that stars simply strip moons away—they set a moving boundary that depends on the planet's mass, the moon's distance, the star's mass, and the shape of the planet's orbit.
Sources used in this answer
On the stability limits for moons orbiting planets: A numerical investigation
N-body simulations show that a moon's outermost stable orbit shrinks with increasing planetary eccentricity and decreasing planet-to-star mass ratio, with a detached stability island at intermediate eccentricities linked to the evection resonance, and orbital phase can shift the stability threshold by up to about 10%.
Tidal evolution and detectability of close-in star-planet and planet-moon systems
Stellar tidal interactions can cause close-in planets and their moons to lose orbital energy, potentially leading to orbital decay and destruction of the moon in extreme cases.
Outcomes and Duration of Tidal Evolution in a Star-Planet-Moon System
Moons are more stable when the planet–moon system is farther from the star, the planet is heavier, or the star is lighter, and including lunar tides significantly lengthens lifetimes for massive moons.
Interaction of Gravitational Field and Orbit in Sun-planet-moon system
The Sun's gravitational pull on the Moon is about 2.2 times that of Earth, but the planet and moon act as a single gravitational unit orbiting the Sun, which prevents the Moon's orbit from being broken off.
A Formation-Stage Bottleneck for Exomoons around Close-in Rocky Planets
Simulations of collisions around rocky planets with orbital periods of 1–300 days show that stellar perturbations can significantly affect the development and survival of moon-forming disks.