The Solar System contains a diverse array of planetary bodies, most of which are accompanied by one or more natural satellites, or moons. These moons are gravitationally bound to their host world. Gas giants like Jupiter and Saturn boast dozens of these companions, and even smaller worlds such as Mars and Earth host at least one. Only two of the Solar System’s eight major planets are exceptions to this common celestial arrangement.
The Solar System’s Moonless Planets
The only two planets in our Solar System that currently possess no natural satellites are Mercury and Venus. These two worlds are the innermost planets, orbiting closest to the Sun. Their lack of a moon makes them unique among the other planets, which all host at least one satellite.
Mercury is the smallest of the eight planets. Venus, the second planet out, is often called Earth’s twin due to its similar size. Despite intense scrutiny from numerous space probes, no celestial body has been found in a stable, long-term orbit around either of these two worlds.
Defining a Natural Satellite
A natural satellite is an astronomical body that is held in a stable orbit by the gravitational influence of a larger celestial object, such as a planet. These bodies are distinguished from artificial satellites, which are human-made, and they range dramatically in size, including large, spherical objects like Earth’s Moon and tiny, irregularly shaped objects.
The key defining factor is the stability of the orbit, which must be long-lasting and dominated by the planet’s gravity. This definition sets moons apart from planetary rings or temporary captured objects that may only orbit the planet for a short period before escaping. Even the small, potato-shaped moons of Mars, Phobos and Deimos, qualify as natural satellites because their orbits are gravitationally stable over astronomical timescales.
Why Proximity to the Sun Prevents Moon Capture
The reason Mercury and Venus are moonless stems from their close proximity to the Sun and the resultant gravitational dynamics. Every planet has a region of gravitational dominance known as the Hill Sphere, which is the volume of space where the planet’s gravity is stronger than the Sun’s. A moon must maintain an orbit entirely within its planet’s Hill Sphere to remain gravitationally bound.
For Mercury and Venus, their close distance to the massive Sun drastically shrinks the size of their Hill Spheres. The immense gravitational pull of the Sun exerts a disruptive force on any potential satellite orbit around these inner planets. Any object attempting to orbit Mercury or Venus at a distance would quickly be pulled away and captured into an orbit around the Sun instead.
Furthermore, the Sun’s strong tidal forces would destabilize the orbits of any moons that formed or were captured too close to the planet. These forces would either accelerate the moon away or cause its orbit to decay, leading to an eventual collision with the planetary surface. The combination of a small zone of gravitational influence and tidal forces makes the long-term retention of a moon around Mercury and Venus virtually impossible.