On Earth, you can look up at night and see the moon shining brightly from hundreds of thousands of miles away. But if you go to Venus, that won’t be the case. Not every planet has a moon – so why do some planets have several moons while others have none?
I’m a physics instructor who follows the current theories that explain why some planets have moons and others don’t.
First, the moon is called a natural satellite. Astronomers call satellites 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 within the so-called Hill sphere radius of a planet, or formed along 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 force of attraction.
This gravitational pull is the reason we all stay grounded to Earth instead of floating away.
The solar system is dominated by the great gravitational pull 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 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 support a satellite in orbit is called the Hill radius.
The radius of the Hill sphere is based on the mass of both the larger and smaller object. The Moon orbiting the Earth is a good example of how the Hill’s sphere radius works. The Earth orbits the Sun, but the Moon is close enough to the Earth that the Earth’s gravitational pull captures it. The moon orbits the earth, not the sun, because it is within the radius of the Earth’s Hill sphere.
Smaller planets like Mercury have a small Hill sphere radius because they cannot exert much gravitational pull. Any potential moons will 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 ejected from 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 spherical radius of Mars’ Hill 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 they are farther from the sun. Their gravitational pull can attract and hold 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 that orbits the sun. As the gas revolves around the sun, it condenses into planets and moons that revolve around them. Then all the planets and moons rotate in the same direction.
But only a few moons in our solar system were probably created 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 remaining moons in our solar system, including the outer moons of Jupiter and Saturn, have likely been gravitationally captured by their planets.
Earth’s moon is special because it probably formed differently. Scientists believe that a long time ago, a large object the size of Mars collided with Earth. During this collision, a large piece flew off the Earth and entered its orbit and became the moon.
Scientists hypothesize that the moon formed this way because they found a type of rock called basalt in the soil on the moon’s surface. Basalt on the Moon looks the same as basalt found in Earth’s interior.
Ultimately, why some planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, the radius of the Hill sphere, and how its solar system formed may play a role.
