On Earth, you can look up at night and see the moon shining from hundreds of thousands of miles away. But if you go 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’m a physics instructor who has followed the current theories that describe why some planets have moons and some don’t.
First, a moon is called a natural satellite. Astronomers refer to satellites as objects in space that orbit larger bodies. Since a moon is not man-made, it is a natural satellite.
Currently, there are two main theories as to why some planets have moons. Moons are either gravitationally captured if they are within what is called the planet’s hill-sphere radius, or they 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 attractive force.
This gravitational force is why we all stay grounded instead of flying away.
The solar system is dominated by the great gravitational force of the sun, which holds all the planets in orbit. The Sun is the most massive object in our solar system, which means it has the most 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 hold a satellite in orbit is called the radius of the hill sphere.
The radius of the hill sphere is based on the mass of the largest object and the smallest object. The Moon orbiting the Earth is a good example of how the Hill Sphere radius works. Earth revolves around the sun, but the moon is close enough to Earth that Earth’s gravitational pull grabs it. The moon revolves around the Earth, instead of the sun, because it is within the radius of the Earth’s hill sphere.
Smaller planets like Mercury have a small spherical radius, as they cannot exert a large gravitational pull. Instead, any possible moon is likely to be pulled away from the sun.
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 these came from asteroids that passed close to the Martian Hill sphere radius 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 farther from the sun. Their gravitational pulls can attract and hold more natural satellites such as moons in orbit. For example, Jupiter has 95 moons, while Saturn has 146.
Moons that form with a solar system
Another theory suggests that some moons formed at the same time as their own solar system.
Solar systems begin with a large disk of gas orbiting the sun. As the gas orbits 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 are likely to have formed this way. Scientists predict that the inner moons of Jupiter and Saturn formed during the emergence 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 special because it likely formed in a different way. Scientists believe that a long time ago, a large object the size of Mars collided with Earth. During that collision, a large chunk flew off the Earth and into its orbit and became the Moon.
Scientists assume that the moon was formed this way because they have found a type of rock called basalt in the soil on the surface of the moon. Moon basalt looks the same as basalt found inside Earth.
Ultimately, the question of why some planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, hill-sphere radius, and how its solar system formed may play a role. role.