On Earth, you can look up at night and see the moon shining brightly hundreds of thousands of kilometers away. But if you went to Venus, it wouldn’t be like that. Not every planet has a moon – so why do some planets have several moons while others have none?
I am a physics instructor who followed the current theories that explain why some planets have moons and some don’t.
First, the moon is called a natural satellite. Astronomers refer to satellites as objects in space that orbit larger bodies. Since the moon is not man-made, it is a natural satellite.
There are currently two main theories as to why some planets have moons. Moons are either gravitationally captured if they are in a region called the Hill radius of a planet’s sphere, or they form along with the solar system.
The radius of the Hill sphere
Objects exert a gravitational pull on other nearby objects. The larger the object, the greater the force of attraction.
This gravitational force is why we all stay grounded to Earth instead of floating away.
The solar system is dominated by the great gravitational force of the Sun, which keeps all the planets in orbit. The Sun is the most massive object in our solar system, which means it has the greatest gravitational influence on objects such as the planets.
In order 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 for a planet 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 radius of the Hill sphere works. Earth orbits the Sun, but the Moon is close enough to Earth that Earth’s gravity captures it. The Moon orbits the Earth rather than the Sun because it is in the Hill radius of the globe.
Smaller planets like Mercury have a small Hill sphere radius because they cannot exert much gravitational force. Any potential moons would likely be attracted to the sun instead.
Many scientists are still looking to see if these planets may have had small moons in the past. During the formation of the solar system, they may have had moons that were knocked off by collisions with other space objects.
Mars has two moons, Phobos and Deimos. Scientists still debate whether they came from asteroids that passed close to the radius of the 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 first theory because Mars is close to the asteroid belt.
Jupiter, Saturn, Uranus, and Neptune have larger Hill sphere radii because they are much larger than Earth, Mars, Mercury, and Venus and are further from the Sun. Their gravitational forces 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 the 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 rotating around the Sun. As the gas rotates around the Sun, it condenses into the planets and moons that orbit them. The planets and moons then all rotate in the same direction.
But only a few moons in our solar system were likely 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 gravitationally captured by their planets.
Earth’s moon is strange because it probably formed in a different way. Scientists believe that a long time ago, a large object the size of Mars collided with Earth. During this collision, a large chunk flew out of Earth into its orbit and became the Moon.
Scientists estimate that the moon was formed this way because they found a type of rock called basalt in the soil on the moon’s surface. Moon basalt looks just like basalt found inside Earth.
Ultimately, the question of why some planets have moons is still widely debated, but factors such as the planet’s size, gravitational force, the radius of the Hill sphere, and how its solar system formed may play a role.