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Why do some planets have moons and some don’t? – Siddharth, 6 years old, Texas
On Earth, you can look up at night and see the Moon shining from hundreds of thousands of kilometers away. But that wouldn’t be the case if you went to Venus. Not every planet has a moon; So why do some planets have more than one moon and others have none?
I am a physics teacher who follows current theories explaining why some planets have moons and others do not.
First, the moon is called a natural satellite. Astronomers call satellites objects that orbit larger objects in space. The Moon is a natural satellite because it is not man-made.
There are currently two main theories as to why some planets have moons. Moons are either gravitationally captured if they are in the so-called Hill sphere radius of a planet, or they form together with a solar system.
Hill sphere radius
Objects exert a gravitational force on other nearby objects. The larger the object, the greater the gravitational force.
This gravitational force is why we all remain attached to the Earth rather than 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 largest object in our solar system; This means it has the greatest gravitational effect on objects such as planets.
In order 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 orbit a satellite is called the Hill sphere radius.
Hill sphere radius is based on the mass of both the large object and the small object. The Moon in Earth orbit is a good example of how the Hill sphere radius works. The Earth revolves around the Sun, but the Moon is close enough to the Earth that the Earth’s gravitational pull catches it. Since the Moon is within the Hill sphere radius of the Earth, it orbits the Earth instead of the Sun.
Small planets such as Mercury and Venus have a small Hill sphere radius because they cannot exert a large gravitational force. Any potential satellites will likely be pulled in by the Sun.
Many scientists are still investigating whether these planets had small moons in the past. During the formation of the solar system, it may have had moons that were blown away as a result of collisions with other space objects.
Mars has two moons; Phobos and Deimos. Scientists are still debating whether they came from asteroids that passed near the Mars Peak 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 and farther from the Sun than Earth, Mars, Mercury, and Venus. Their gravitational pull can attract and keep more natural satellites, such as moons, in orbit. For example, Jupiter has 95 moons while Saturn has 146 moons.
Moons formed with the solar system
Another theory suggests that some moons formed at the same time as the solar system.
Solar systems begin with a large disk of gas orbiting the sun. As the gas revolves around the sun, it condenses into planets and moons orbiting them. So the planets and moons rotate in the same direction.
But probably only a few moons in our solar system were created this way. Scientists estimate that the inner moons of Jupiter and Saturn were 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, are likely gravitationally captured by their planets.
Earth’s Moon is special because it probably formed differently. Scientists believe that a large object the size of Mars collided with Earth long ago. During this collision, a large piece flew away from the Earth and entered its orbit, becoming the Moon.
Scientists guess 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 basalt on the Moon looks the same as the basalt found inside the Earth.
As a result, 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 the solar system formed may play a role.
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This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by Nicole Granucci Quinnipiac University
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Nicole Granucci does not work for, consult for, 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 duties.