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Why do some planets have moons and others don’t? – Siddharth, age 6, Texas
On Earth, you can look up at night and see the moon shining brightly 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 instructor who has followed 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 the so-called Hill sphere radius of a planet, or they are formed together with a solar system.
The radius of the hill sphere
Objects exert an attraction on other nearby objects. The larger the object, the greater the attraction.
This gravity is the reason we all stay connected to the Earth instead of floating away.
The solar system is dominated by the sun’s strong gravity, which keeps all the planets in orbit. The sun is the most massive object in our solar system, meaning it has the greatest gravitational influence on objects such as planets.
For a satellite to orbit a planet, it must be close enough that the planet can exert enough force to keep it in orbit. The minimum distance required for a planet to maintain a satellite in orbit is called the Hill sphere radius.
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 revolves around the Earth, instead of the Sun, because it is within the radius of the Earth’s hill sphere.
Nicole Granucci
Small planets such as Mercury and Venus have a small Hill sphere radius because they cannot exert a large gravitational force. Any potential moons would likely be attracted to the Sun instead.
Many scientists are still investigating whether these planets may have had small moons in the past. During the formation of the solar system, they may have had moons that were blown away 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 Mars Hill’s 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 a larger radius from the Hill Sphere because they are much larger than Earth, Mars, Mercury and Venus and are further from the Sun. Their gravity can attract more natural satellites, such as moons, and keep them in orbit. For example, Jupiter has 95 moons, while Saturn has 146.
Moons form 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 rotate in the same direction.
But probably only a few moons in our solar system 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 remaining moons in our solar system, including the outer moons of Jupiter and Saturn, were likely gravitationally captured by their planets.
Earth’s moon is special because it was probably formed in a different way. Scientists believe that a large object the size of Mars collided with Earth long ago. During that collision, a large piece of Earth flew into its orbit and became 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 the 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.
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