The role of gravity in the formation of the planets, stars, and solar systems involves the condensation of stellar gases. The law of gravity states that all matter attracts a pull on other forms of matter, which is the force that draws gases together (Sagan 37). If enough material collects together, it becomes further condensed until it becomes superheated, eventually creating either a star. The gases and other material that orbits a new star can eventually come together through gravitational forces, forming planets that orbit the star. Once a star has identifiable planets in orbit, it becomes a solar system.
Gravity is also responsible for motion in a solar system. All of the planets orbit the sun, as the sun exerts the most gravitational force in each solar system because it has the most mass. The object with the highest mass will have the most gravitational pull. The sun is constantly pulling on each planet, but because the planets originated as part of the sun’s original disc, their motion causes them to orbit the sun rather than fall into it. Moons also have a similar pattern, although they orbit individual planets. The reason that moons are able to orbit planets instead of the sun, even though the sun has a much greater gravitational pull, is because a moon’s proximity to a planet means it feels gravity much stronger from a planet than the sun. However, the gravitational physics are the same as planets, in that planets orbit the sun while moons orbit planets.
Other objects and phenomena in a solar system, including asteroids, also rotate in the same pattern as planets; comets, however, have a much larger orbital range than planets and can have elliptical, rather than circular orbits (Sagan 117). All motion in the solar system is therefore caused by the sun’s gravitational pull, which is created by the amount of mass stored within the sun.
- Sagan, Carl. Cosmos. Random House, 2006.