The concept of a natural satellite orbiting another natural satellite, often called a sub-satellite, captures the imagination. This suggests a hierarchical structure extending one layer deeper than the typical planet-moon system observed in our solar system. Whether a moon can host its own moon is fundamentally a problem of orbital mechanics and gravitational influence. The physics governing the stability of such a system are highly specific, placing strict requirements on the size and distance of the sun, the planet, and the moon.
The Current Status of Sub-Satellites
As of today, scientists have not discovered any naturally occurring, stable sub-satellites orbiting a moon in our solar system or among the exomoons detected around other stars. This observational reality is consistent across the hundreds of moons orbiting the gas giants like Jupiter and Saturn, where a sub-satellite might theoretically have the best chance of survival. Even searches around large, planet-sized moons such as Saturn’s Titan and Jupiter’s Ganymede have yielded no candidates for a stable, long-term sub-satellite.
Searches rely on telescopic observation and orbital analysis of existing spacecraft data. Detecting a small object orbiting a moon that is itself orbiting a distant planet is difficult. The chaotic gravitational environment around most moons suggests any potential sub-satellite would be small and short-lived. This lack of discovery suggests that either formation conditions are extremely rare, or destabilizing forces are overwhelmingly strong.
The Gravity Problem: Why Moons Don’t Have Stable Moons
The primary obstacle preventing moons from having their own stable satellites is the immense gravitational pull of the host planet. A stable orbit requires a delicate balance where the moon’s gravity must be the dominant force acting on the smaller orbiting body. This region of gravitational dominance around any celestial body is defined by its Hill sphere.
For a sub-satellite to exist, it must orbit entirely within the moon’s Hill sphere. The moon’s Hill sphere is often small and distorted because the host planet constantly exerts a massive gravitational tug on the system. For example, while the Earth’s Moon has a Hill sphere where its gravity is stronger than Earth’s, this zone is often insufficient to guarantee long-term stability for an orbiting object.
Orbital stability is further complicated by the planet’s powerful tidal forces, which create constant gravitational disturbances. This perturbation continually stretches and distorts the sub-satellite’s orbit. Even if a small object were captured by the moon, the planet’s continuous influence would quickly destabilize the new orbit.
A sub-satellite must also orbit far enough away from its moon to avoid being torn apart by gravitational stress, a concept related to the Roche limit. This requirement places a minimum distance for a stable orbit. The Hill sphere defines the maximum stable distance, while the Roche limit defines the minimum stable distance. This combination often leaves a narrow, or non-existent, zone where a sub-satellite could maintain an orbit over billions of years, leading to ejection or collision.
Temporary Moons and Co-orbital Objects
While stable sub-satellites are not observed, there are related astronomical phenomena that represent near-miss scenarios for complex orbital dynamics. These objects are generally categorized as temporary companions that are not truly gravitationally bound over the long term. One type is a temporary satellite, sometimes called a “mini-moon,” which is a small asteroid or debris fragment briefly captured by a planet’s gravity.
Earth occasionally captures such objects, like asteroid 2020 CD3, which orbited our planet for a few years before being ejected back into a solar orbit. These mini-moons are temporarily held by the planet, but their capture is transient, lasting only decades or centuries, not the geological timescales required for a true moon.
Another distinct group are quasi-satellites, which appear to orbit a planet but are not gravitationally bound to it. These objects, such as asteroid 469219 Kamoʻoalewa, primarily orbit the Sun but maintain a synchronized path with the planet. Their motion is dominated by the Sun’s gravity, making them co-orbital companions rather than true satellites. These temporary objects demonstrate complex gravitational interplay but do not meet the criteria for a stable, long-term sub-satellite.