Why does Earth have only one satellite while other planets have hundreds. Physicist's explanation

Our planet Earth has only one satellite, the Moon, but there are planets that have hundreds of satellites, or like Venus, none. There is a logical physical explanation for this.

Physicist from the American University of Quinnipiac Nicole Granucci explains that with the help of gravitational force, objects attract other objects. And the larger the object, the greater the force of attraction it has, writes The Conversation.

"I am a physics teacher who subscribes to current theories that explain why some planets have satellites and some do not. There are currently two main theories that explain this phenomenon. Moons are either entrained by gravity if they are within the so-called Hill sphere radius of a planet, or they form with the solar system," Nicole Granucci explained.

What is gravity and the Hill sphere radius, and does it all work?

Gravity or gravitation, attraction is the property of bodies with mass to be attracted to each other. Therefore, some objects are able to attract other objects nearby. And the larger the object, the greater the force of attraction it has. It is the Earth's gravitational force that attracts people and keeps us from flying off into space.

The solar system is dominated by the large gravitational force of the Sun, which holds all the planets in orbit. The Sun is the most massive object in our solar system, so it has the greatest gravitational influence on objects such as planets.

For a satellite to orbit a planet, it must be close enough to the planet. Instead, the planet must exert enough force to keep it in orbit. The minimum distance that a planet can hold a satellite in orbit is called the radius of Hill's sphere.

The Moon orbiting the Earth is a good example of how Hill's sphere radius works. The Earth revolves around the Sun, but the Moon is close enough to the Earth that the Earth's gravitational pull grabs it. The Moon revolves around the Earth, not the Sun, because it is within the Hill sphere radius of the Earth.

Small planets, such as Mercury and Venus, have tiny Hill sphere radii because they cannot exhibit a large gravitational force. So any potential satellites, near small planets, will be attracted by the Sun.

Mars has two satellites, Phobos and Deimos. Scientists are still debating whether they come from asteroids that passed close to the radius of the Mars Hill sphere and were captured by the planet, or formed at the same time as the solar system. More evidence supports the first theory, since Mars is close to the asteroid belt.

Jupiter, Saturn, Uranus, and Neptune have large Hill sphere radii because they are much larger than Earth, Mars, Mercury, and Venus, and are farther from the Sun. Their gravitational pull can attract and keep more natural satellites in orbit. For example, Jupiter has 95 satellites and Saturn has 146.

Moons form along with the solar system

According to physicist Nicole Granucci, another theory suggests that some satellites formed at the same time as their solar system.

Solar systems begin as a large disk of gas orbiting around the sun. As the gas revolves around the sun, it condenses into planets and satellites that revolve around them. Then all the planets and satellites orbit in the same direction.

The presented video shows how the planets of our solar system were formed. The dark rings on the disk symbolize the formation of planets and satellites. Gas later condenses into planets, natural satellites and asteroids.

But only a few satellites in our solar system were probably created this way. Scientists hypothesize that the inner satellites of Jupiter and Saturn were formed at the origin of our solar system because they are very old. The other satellites in our solar system, including the outer satellites of Jupiter and Saturn, were probably entrained by the gravity of their planets.

The special origin of Earth's moon

However, Earth's moon is special, according to scientists, because it probably formed in a different way. Scientists believe that once upon a time, a large object the size of Mars collided with Earth. And during that collision, a large piece flew away from the Earth into its orbit and became the Moon.

Scientists are considering a special origin for Earth's moon because they have discovered a type of rock called basalt in the soil on the moon's surface. Lunar basalt looks similar to basalt found inside Earth.

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