# Which Force Causes Planets and Comets to Orbit the Sun?

You’ve probably heard that gravity is responsible for the motion of planets and comets but how does that work? We’ll look at two different forces involved: centripetal motion and Newton’s universal law of gravitation. What do these three forces have in common? Let’s explore them one by one. Which one is most important to our solar system? Which force is more important to the planets and comets?

## Gravity

How do planets and comets orbit the sun? Gravity is the main force governing the motions of solar system objects. The more massive the object the greater its gravitational force. Thus the closer it is to a large mass the larger its gravitational force is. However planets and comets orbit the sun more slowly so their orbits will not be perfectly circular.

As the Solar System became mostly cleared of debris planet building stopped. But meteorite impacts left scars on the surface of all solid objects. These scars can still be seen on the Moon and the Earth’s surface is riddled with craters which are remnants of meteorite impacts. About 65 million years ago impacts killed 90 percent of Earth’s species so their orbits must be quite stable to keep from colliding with each other.

The formation of planets and comets begins in a nebula. When the material in the nebula began spinning it began to accumulate. In time the material surrounding the sun flattened out and formed a disk. Then planetesimals accrete and stick together forming larger objects. Planetesimals eventually coalesced into larger bodies called planetesimals and eventually the entire system of planets and comets began to form.

Planets and comets orbit the sun because of gravity. Objects with mass experience gravitational force and the more mass an object has the greater its attraction to that object. Because of this planets and comets move in a circular path around the sun. However when the two objects are stationary their orbits do not match up exactly and they fall toward the star.

The Sun is the largest object in our solar system and therefore has greater gravitational attraction than any other object. Its gravitational attraction holds planets and comets in orbits that vary in length. Jupiter for example has many moons but no more. The same applies to jovian planets. However as a result of this effect planets and comets can only orbit the sun if they are near its center.

## Centripetal motion

Centripetal motion occurs when uniform circular motion is accelerated. This acceleration occurs because of the force that acts on the earth which is gravity. The earth’s gravitational attraction to the Sun produces this acceleration. The result is an orbit. Hence the orbital motion of planets and comets around the sun is constant and continuous. Hence the centripetal force keeps planets and comets in their orbits.

Centripetal force is the force that keeps objects moving in a circular motion. You can experience this force by spinning a toy plane. A string keeps a toy plane from moving in one direction. The string on the other hand causes the plane to fly in a circular motion. This causes centripetal force to act inwards which is perpendicular to the direction of motion.

When planets and comets orbit the sun they follow the same general orbital plane. However occasionally their orbital paths cross each other and collide. For instance comet Tempel-Tuttle’s orbit passes through the Earth’s orbit and debris from the tail falls into the Earth’s atmosphere as meteors. This phenomenon is known as the Leonid meteor shower.

A planet’s speed depends not on its mass but on its radius. The planets’ velocity does not depend on its mass and therefore it can fall with the same velocity. As a result a planet that orbits the sun has the same period as a pendulum falling in a circle. If it is falling in a circle it has a constant area of its orbital plane.

Whether the planets and comets orbit the sun depends on the amount of gravitational pull on them. Planets and comets tend to orbit the sun in a stable orbit over their solar system’s history. Smaller bodies however keep changing their orbits and eventually collide with other objects. The sun is the strongest gravitational attraction in the universe.

The first theory that has been proposed to explain the origin of the solar system was based on the concept of the planetesimal hypothesis. This hypothesis posited that the planets formed from a cloud of gas and dust surrounding the sun. Moreover the planets were cold spheres. Eventually these bodies were captured by the gravitational attraction of the planets.

## Passing neighbor stars

Planets and comets orbit the sun in different ways but passing neighbor stars affect the orbits of both in slightly different ways. The planets for example may be moved into a smaller orbit when they pass Jupiter. In contrast comets may be permanently dislodged from their orbits by passing a massive object like Neptune. These passing stars are responsible for the varying shapes of planets and comets in the solar system.

The closest star to our planets is Gl 710 which is believed to be a white dwarf. Its radial velocity indicates that it has a ninety percent chance of passing within 0.3 to 0.8 parsecs in 1.3 million years. Passing neighbor stars are the most probable source of cometary disruption but the details of their orbits are unclear. But there are other factors that may cause planets and comets to orbit the sun.

In order to study the astrometry of such objects researchers have studied several stars that are nearby the Solar System. They found a large number of stars within tens of light years. Out of these forty come within 6.4 light years of the Sun while fourteen come within three light years. The nearest star to the Sun is HIP 85605 a 16-light-year-distance star in the constellation Hercules.

The planets and comets orbit the Sun because of gravity. When a comet passes close to the Sun it will heat up and start to spew gas and dust into its giant head and tail which stretch millions of miles away from the Sun. In fact there are billions of comets orbiting the Sun. The Oort Cloud is the area where the comets reside.

In addition to the Sun’s gravitational pull planets and comets also pass by other nearby stars. Several other planetary systems may have planets in the neighborhood but they are not as common as we would think. One of the closest neighbor stars is Barnard’s star which is six light years away. The next closest is the Alpha Centauri system. Despite the many false claims about extrasolar planets more planets have been discovered in the last decade.

## Newton’s universal law of gravitation

The laws of physics show that planets and comets orbit the Sun because of the force of gravity. Newton first discovered that this force isn’t localized but instead is experienced throughout the entire universe. He then proposed his law of universal gravitation to explain the effects of gravity. In short gravity is an attractive force between objects of mass. This force is proportional to the mass of the object and the square of the distance between them. When two objects are at rest the force of gravity is equal to the mass of one object but it is stronger for the farther away object.

Because of the massive nature of the Sun the planets and comets orbit it tracing their paths within it. The law of gravitation implies that gravity never completely disappears with distance. Even at extreme distances gravity is still acting. For example the sun’s gravitational pull is much stronger at Mercury than it is at Pluto and it is felt far beyond that making planets and comets orbit the Sun.

The effect of gravitational force on the earth’s surface is shown graphically in the picture below. It is the product of mass and distance that causes the force of attraction between two objects. The force of gravity between two objects is equal to the product of their masses. It is possible to express the force of gravitation between objects using an equation using a constant of proportionality known as G. The unit of gravitation is Newtons.

Despite its simplicity this law of gravity is widely accepted and it helps explain many of the small perturbations in planetary orbits. For example Jupiter deviates from its smooth path when it approaches Saturn. The reason for this is the gravitational pull between Saturn and Jupiter. Essentially Newton’s law of universal gravitation explains why comets and planets orbit the sun.

A small steel ball placed on a spinning magnet gains momentum from the planet’s orbital speed. The magnetic field of a planet is analogous to the gravitational force on a small steel ball. A small steel ball that’s placed on a spinning magnet is pulled in and thrown out at a much faster rate than it would go in.