Why do some planets have moons? A physics expert explains why Earth has just one moon while other planets have hundreds

Curious children is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

On Earth, you can look up at night and see the Moon shining hundreds of thousands of miles away. But if you went to Venus, that wouldn’t be the case. Not all planets have a moon – so why do some planets have multiple moons and others have none?

I’m a physics instructor which followed current theories describing why some planets have moons and others do not.

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.

Currently, there are two main theories about why some planets have moons. Moons are gravitationally captured if they are inside what is called a planet Hill Sphere Radiusor they are formed together with a solar system.

The radius of the Hill sphere

Objects exert a gravitational force of attraction on other nearby objects. The larger the object, the greater the force of attraction.

This gravitational force is the reason we all stay anchored to Earth instead of floating.

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 like 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 for a planet to keep 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 larger object and the smaller object. The Moon orbiting the Earth is a good example of how the Hill sphere radius works. Earth orbits around the Sun, but the Moon is close enough to Earth for Earth’s gravitational pull to capture it. The moon orbits around the Earth, not the Sun, because it is within the radius of the Earth’s Hill sphere.

Small planets like Mercury and Venus have a small Hill sphere radius since they cannot exert a large gravitational pull. Any potential moons would likely be attracted to 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 which was knocked down 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 radius of Mars’ hill sphere and were captured by the planet, or whether they were formed at the same time as the solar system. More evidence supports the first theorybecause 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 maintain more natural satellites, such as moons in orbit. For example, Jupiter has 95 moonswhile Saturn has 146.

Moons forming with a solar system

Another theory suggests that some moons formed at the same time as your solar system.

Solar systems begin with a large disk of gas rotating around the sun. As the gas revolves around the Sun, it condenses into planets and moons that revolve around them. The planets and moons all rotate in the same direction.

But only a few moons in our solar system were likely created this way. Scientists predict that Jupiter and Saturn inner moons formed during the emergence of our solar system because they are so old. The rest of our solar system’s moons, including the outer moons of Jupiter and Saturn, were likely 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 this collision, a large piece flew from Earth and entered its orbit and became the Moon.

Scientists guess that the Moon formed this way because they found a type of rock called basalt in the soil on the lunar surface. The basalt of the Moon It looks the same like basalt found inside the Earth.

Ultimately, the question of why some planets have moons is still widely debated, but factors such as the planet’s size, gravitational pull, Hill sphere radius and how its solar system formed may play a role.

Hello, curious children! Do you have a question you would like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and city where you live.

And since curiosity has no age limit – adults, also tell us what you are wondering. We won’t be able to answer every question, but we will do our best.

This article was republished from The conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you understand our complex world. It was written by: Nicole Granucci, Quinnipiac University

See more information:

Nicole Granucci does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic appointment.