ASK Space

SOLAR SYSTEM

Why don’t Mercury or Venus have any moons?

Planets have a region around them where orbiting objects can be gravitationally bound. In planetary science this is called the ‘Hill sphere’, named after 19th-century American astronomer George Hill, who worked on the Moon’s orbit. The size of the Hill sphere depends on how massive a planet is in comparison to the Sun, and how close it is to the Sun.

Any bound object must orbit inside the Hill sphere. But for its orbit to be ‘stable’ it needs to orbit at less than about one-third of the radius of the Hill sphere. Therefore we expect that all moons must orbit at less than about a third of a Hill sphere radius.

There are other restrictions on where we expect to find moons. Earth’s Moon interacts tidally with Earth - it raises tides on Earth, and that gradually changes the Moon’s orbit. Because the Earth spins faster than the Moon orbits - the day is shorter than the month - the effect of this interaction is to move the Moon away from us at a rate of about four centimetres (1.5 inches) a year. A moon in orbit around Mercury or Venus - which spin slowly - would evolve in the opposite direction, with its orbit gradually shrinking. This happens at Mars, where Phobos orbits rapidly with a shrinking orbit,