Recent research has shed light on the differing number of moons among planets in our solar system. While gas giants like Jupiter and Saturn boast numerous moons, terrestrial planets such as Venus have none. This disparity raises questions about the formation and capture of natural satellites.
Two primary theories explain the existence of moons: gravitational capture and simultaneous formation with the solar system. The gravitational capture theory posits that moons are drawn into a planet's orbit if they enter its Hill sphere radius, where the planet's gravitational pull is strong enough to retain them.
The Hill sphere radius varies significantly among planets, influenced by their mass and proximity to the Sun. For instance, Mercury's small Hill sphere radius limits its ability to capture moons, as potential satellites may instead be drawn into the Sun's gravitational influence.
In contrast, larger planets like Jupiter and Saturn possess extensive Hill spheres, enabling them to capture numerous moons. Jupiter, for example, has 95 known moons, while Saturn has 146.
Additionally, some moons may have formed concurrently with their planets during the early solar system's gas disk phase. However, this is believed to apply to only a few moons, with many others likely captured from the asteroid belt or through other means.
Earth's Moon is particularly unique, having formed from debris resulting from a collision with a Mars-sized body. This theory is supported by the discovery of basaltic rocks on the Moon, similar to those found on Earth.
Understanding the origins and characteristics of planetary moons continues to be a subject of scientific inquiry, with ongoing debates regarding the processes that govern their formation and retention.