On Earth, you can look up at night and see the moon shining from hundreds of thousands of miles away. But if you went to Venus, that wouldn’t be the case. Not every planet has a moon – so why do some planets have multiple moons while others have none?
I am a physics teacher who follows current theories describing why some planets have moons and others do not.
First, a moon is called a natural satellite. Astronomers call satellites objects in space that orbit larger bodies. Because a moon is not made by humans, it is a natural satellite.
Currently, there are two main theories about why some planets have moons. Moons are either captured by gravity when they are within a planet’s so-called Hill radius, or they are formed together with a solar system.
The radius of the Hill sphere
Objects exert a gravitational pull on other nearby objects. The larger the object, the greater the gravitational pull.
This gravity is the reason we all stay connected to the Earth instead of floating away.
The solar system is dominated by the large gravitational force of the sun, which keeps all the planets in orbit around the earth. The sun is the most massive object in our solar system, which means that it has the largest gravitational influence on objects such as planets.
For a satellite to orbit a planet, it must be close enough for the planet to exert enough force to keep it in orbit. The minimum distance a planet needs to be from to keep a satellite in orbit is called the Hill radius of the sphere.
The radius of the hill sphere is based on the mass of both the larger object and the smaller object. The moon orbiting the Earth is a good example of how the hill sphere radius works. The Earth revolves around the Sun, but the Moon is so close to the Earth that the Earth’s gravity catches it. The moon orbits the Earth, instead of the sun, because it is within the radius of the Earth’s Hill sphere.
Smaller planets like Mercury have small Hill radiuses because they cannot exert a large gravitational pull. Any potential moons would likely be attracted to the Sun.
Many scientists are still investigating whether these planets had small moons in the past. During the formation of the solar system, they may have had moons that were knocked away by collisions with other space objects.
Mars has two moons, Phobos and Deimos. Scientists are still debating whether these came from asteroids that passed close to Mars’ Hill sphere and were captured by the planet, or whether they formed at the same time as the solar system. More evidence supports the former theory, as Mars lies close to the asteroid belt.
Jupiter, Saturn, Uranus and Neptune have a larger radius of the Hill Sphere because they are much larger than Earth, Mars, Mercury and Venus and are farther from the Sun. Their gravity can attract and keep more natural satellites, such as moons, in orbit. For example, Jupiter has 95 moons, while Saturn has 146.
Moons forming with a solar system
Another theory suggests that some moons formed at the same time as their solar system.
Solar systems begin with a large disk of gas orbiting a sun. As the gas orbits the sun, it condenses into planets and moons that orbit it. The planets and moons then all orbit in the same direction.
But only a few moons in our solar system probably formed this way. Scientists predict that the inner moons of Jupiter and Saturn formed during the formation of our solar system because they are so old. The rest of the moons in our solar system, including the outer moons of Jupiter and Saturn, were probably captured by their planets through gravity.
Earth’s moon is special because it probably formed in a different way. Scientists think that a long time ago, a large object the size of Mars collided with Earth. During that collision, a large chunk of Earth flew off and into its orbit, becoming the moon.
Scientists suspect that the moon was formed this way because they found a type of rock called basalt in the soil on the moon’s surface. The moon’s basalt looks the same as basalt found in Earth.
Ultimately, why some planets have moons is still much debated, but factors such as a planet’s size, gravity, the radius of the hill sphere, and how the solar system is formed may play a role.