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

Why do some planets have moons and a few don’t? – Siddharth, 6 years old, Texas

On Earth, you may look up at night and see the moon shining brightly from a whole lot of 1000’s of miles away. But that wouldn't be the case on Venus. Not every planet has a moon – so why do some planets have multiple moons while others have none?

I’m a physics teacher who has followed current theories describing why some planets have moons and others don't.

First, a moon is named a natural satellite. Astronomers call Satellites as objects in space that orbit larger celestial bodies. Since the moon was not created by man, it’s a natural satellite.

Currently, there are two essential theories as to why some planets have moons. Moons are either captured by gravity after they are throughout the so-called Radius of the Hill Sphereor they arise along with a solar system.

The radius of the Hill sphere

Objects exercise a Gravitational force on other objects nearby. The larger the thing, the greater the gravitational pull.

This gravitational force is the rationale why all of us stay on Earth and don’t float away.

The solar system is dominated by the strong gravitational force of the sun, which keeps all of the planets in its orbit. The sun is probably the most massive object in our solar system, meaning it has the best gravitational influence on objects resembling planets.

For a satellite to orbit a planet, it should be close enough for the planet to exert enough force to maintain it in orbit. The minimum distance a planet needs to maintain a satellite in orbit is named the radius of the Hill sphere.

The radius of the Hill sphere is predicated on the mass of the larger and smaller object. The moon orbiting the earth is a great example of how the radius of the Hill sphere works. The earth orbits the sun, however the moon is so near the earth that it’s captured by the earth's gravity. The moon orbits the earth and never the sun since it is throughout the radius of the earth's hill sphere.

A diagram showing the Earth with a large radius around it and a circle representing the Moon within that radius, and Mercury with a small radius around it.
The Earth has a bigger Hill sphere radius than Mercury.
Nicole Granucci

Small planets like Mercury and Venus have a tiny Hill sphere radius because they can’t exert a big gravitational force. Possible moons would probably be interested in the Sun as an alternative.

Many scientists still wonder if these planets could have had small moons prior to now. During the formation of the solar system could have had moons which were thrown away by collisions with other space objects.

Mars has two moons, Phobos and Deimos. Scientists still debate whether these were formed by asteroids that passed near the radius of the Martian hill sphere and were captured by the planet, or whether or not they formed similtaneously the solar system. Further evidence supports the primary theorybecause Mars is near the asteroid belt.

Jupiter, Saturn, Uranus and Neptune have larger Hill sphere radii because they’re much larger than Earth, Mars, Mercury and Venus and are farther away from the Sun. Their gravitational forces can attract and hold more natural satellites resembling moons in orbit. For example Jupiter has 95 moonswhile Saturn has 146.

Moons that form with a solar system

Another theory assumes that some moons were formed similtaneously their solar system.

Solar systems begin with a big disk of gas rotating around a sun. As the gas rotates across the sun, condenses into planets and moons that orbit around themThe planets and moons then all rotate in the identical direction.

This animation shows how the planets of our solar system were formed. The dark rings within the disk represent the formation of the planets and moons. Eventually the gas condenses into planets, natural satellites and asteroids.

But probably only just a few moons in our solar system were formed in this manner. Scientists predict that Jupiter and Saturn inner moons were formed throughout the formation of our solar system because they’re so old. The other moons of our solar system, including the outer moons of Jupiter and Saturn, were probably captured by the gravity of their planets.

The Earth's moon is special since it probably formed differently. Scientists consider that an extended time ago a big object the dimensions of Mars collided with the Earth. In this collision, a big piece of flew from Earth into its orbit and have become the moon.

This NASA animation shows a simulation of the formation of our moon throughout the collision.

Scientists suspect that the moon was formed in this manner because they found a style of rock called basalt within the soil on the lunar surface. The basalt of the moon looks the identical like basalt occurs contained in the earth.

Ultimately, the query of why some planets have moons remains to be widely debated, but aspects resembling a planet's size, its gravitational pull, the radius of its Hill sphere, and the formation of its solar system could play a task.

image credit : theconversation.com