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Why do some planets have moons and others do not? – 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, it wouldn’t be like that. Not all planets have moons – so why do some planets have moons, but not others?
I’m a physics instructor who has followed the current theories that explain why some planets have moons and others don’t.
First, a moon is called a natural satellite. Astronomers refer to satellites as objects in space that orbit larger bodies. Since it is not a man-made moon, it is a natural satellite.
Currently, there are two main theories as to why some planets have moons. Moons are captured gravitationally if they are within what is known as the Sun’s spherical radius of the planet, or are formed together with a solar system.
Radius of the sphere of the Hill
Objects exert an attractive gravitational force on other nearby objects. The larger the object, the greater the pulling force.
This gravitational force is the reason why we all stay underground on 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 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 keep a satellite in orbit is called the Hill’s spherical radius.
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’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 around the Earth, rather than the Sun, because it is within the spherical radius of the Earth’s Hill.
Small planets such as Mercury and Venus have tiny spherical radii, because they cannot exert a large gravitational pull. The Sun would probably pull in any potential moons.
Many scientists are still looking into whether these planets may have had small moons in the past. Back during the formation of the solar system, they may have had moons that were battered by collisions with other space objects.
Mars has two moons, Phobos and Deimos. Scientists are still debating whether these came from asteroids that passed through the spherical radius of Mars and captured the planet, or whether they were created 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 spherical radii, because they are much larger than Earth, Mars, Mercury and Venus and are further from the Sun. Their gravitational attraction can attract and keep in orbit more natural satellites such as moons. For example, Jupiter has 95 moons, while Saturn has 146.
Moons forming with 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 rotates around the sun. As the gas revolves around the sun, it condenses into planets and moons that revolve around them. The planets and moons then rotate in the same direction.
But probably only a few moons in our solar system were 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. Their planet probably gravitationally captured the rest of the moons in our solar system, including the outer moons of Jupiter and Saturn.
Earth’s Moon is special because it is probably another form. Scientists believe that a large Martian object collided with Earth a long time ago. During that collision, a large chunk flew off the Earth and into its orbit and became the Moon.
Scientists think 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 the Earth.
Ultimately, why certain planets have moons is still widely debated, but factors such as a planet’s size, gravitational pull, the radius of the Sun’s sphere and how its solar system formed may play a role.
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This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Nicole Granucci, Quinnipiac University
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Nicole Granucci does not work for, consult with, share in, or be funded by any company or organization that would benefit from this article, nor has she disclosed any relevant affiliations after her academic appointment.
