The Moon can have its own tiny moon — like a nesting doll, but in space. The outer doll is a planet, the middle one is a moon, the inner one is a small moon-satellite, or submoon. Isaac Newton showed that all bodies attract each other, and gravity holds them together. But gravity is not uniform: on the near side of the satellite it's stronger than on the far side. This difference creates tidal forces that squeeze and stretch the object, like clay. If a submoon gets too close to the main moon, it gets torn apart — possibly just like the dusty ring around Saturn's moon Rhea was torn apart. spacetime curvature explains this as a deformation of space itself.

Calculations show: Earth could host an asteroid-sized submoon. If it were bigger, the Moon would move 30% farther away from us. For exoplanet Kepler-1625b — a warm gas giant — submoons could be as large as Earth. For life to arise on such a moon, Water, an atmosphere, and plate tectonics are needed, meaning the mass should be about half of Earth's. But precisely at that 'half' mass the orbit becomes unstable — a paradox that might make habitable submoons extremely rare.