Why don't moons have moons?

For this to happen there must be a great distance between the moon and the planet. A relatively large moon moving away from the planet may retain its own moon

James Norton

Astronomers state, with almost complete certainty, that no moon in the solar system has its own moons. However, from a physical point of view it is not impossible for the moon to have moons: after all, NASA managed to launch a spacecraft into orbit around the moon.

Although astronomers have discovered a number of asteroids with moons, a moon orbiting a planet with strong gravity would have difficulty maintaining its own natural satellite - says Seth Shostak, a senior astronomer at the non-profit SETI Institute. "There must be a great distance between the moon and the planet." A relatively large moon moving away from the star may retain its own moon.

It is possible that suitable conditions exist far from the solar system, but even though about 250 planets outside the solar system have been discovered so far, there is almost no chance of observing moons outside the solar system in the coming decades, certainly not moons of moons. This is because the existing methods for discovering planets - such as identifying the planet when it passes by its parent star - are mainly suitable for the discovery of giant planets, such as Jupiter, and not for the discovery of moons.

Even if astronomers succeed in discovering a moon with a moon, it will most likely not survive for long. "The forces of the planet will, over time, affect the stability of the orbit of the moon that orbits the moon, and will eventually deviate it from its orbit," says Webster Cash, a professor at the Center for Astrophysics and Space Astronomy at the University of Colorado. "The phenomenon of a moon orbiting a moon will not last long."