The Two Forces That Keep the Planets in Motion Around the Sun

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Many people know that the planets in Earth's solar system move around the sun in orbits. This orbit creates the days, years and seasons on the Earth. However, not everyone is aware of why the planets orbit around the sun and how they remain in their orbits. There are two forces that keep the planets in their orbits.


Gravity is the primary force that controls the orbit of the planets around the sun. While each planet has its own gravity based on the size of the planet and the speed at which it travels, orbit is based on the gravity of the sun. The sun's gravity is just strong enough to keep the planets pulled toward it to create an orbit pattern but not strong enough to pull the planets into the sun. This is similar to the effect of the Earth on the orbit of the moon and satellites. The lesser gravity of the planets also helps to keep the planets from falling toward the sun.

The force of gravity is defined as:

F = Gm1m2 / r2

The m1 and m2 refer to the masses of the two objects involved in the interaction, G is the universal gravitational constant and r is the separation between the two objects. This shows that gravity gets stronger for bigger objects, and weaker the farther away they are from each other. If planets were bigger, the force between them and the sun would be larger and it would alter their orbits. Similarly, the equation shows that the distance of the planet from the sun is also a crucial factor in establishing an orbit.


The physical law that states that objects in motion have a tendency to remain in motion also plays a role in keeping the planets in orbit. According to Eric Christian, who works for NASA, the solar system was formed from a spinning gas cloud. This set the planets into motion from their birth. Once the planets were in motion, the laws of physics keep them in motion by virtue of inertia. The planets continue to move at the same rate in their orbits.

Gravity Working with Inertia

The gravity of the sun and the planets works together with the inertia to create the orbits and keep them consistent. The gravity pulls the sun and the planets together, while keeping them apart. The inertia provides the tendency to maintain speed and keep moving. The planets want to keep moving in a straight line because of the physics of inertia. However, the gravitational pull wants to change the motion to pull the planets into the core of the sun. Together, this creates a rounded orbit as a form of compromise between the two forces.

Velocity and Gravity

The speed, or velocity, of the planets plays a big role in their orbits, including the shape of the orbit. For a planet to remain in orbit around the sun and not fall into it, the planet must have a speed fast enough to keep it at a certain distance from the sun. The faster a planet moves, the further away from the sun it remains. If the planet travels too fast, though, the orbit may become more elliptical in shape, resulting in varying orbit shapes based on the varying speeds of the planets. However, none of the planets travel fast enough to break away from the sun's gravitational pull.


About the Author

Kimberly Turtenwald began writing professionally in 2000. She has written content for various websites, including Lights 2 You, Online Consultation, Corpus Personal Injury and more. Turtenwald studied editing and publishing at Wisconsin Lutheran College.