Planetary Motion

The solar nebula theory is a popular model that explains the formation of our solar system.This eventually evolved into the theory of how the universe was formed as well. The theory states that a giant star exploded and created unstable gas clouds. The huge gas cloud then collapsed on itself and is supposed to create more stars, but planets were a result due to a natural result of star formation that lasted for at least 100 million years. During this time rocky planets formed towards the inner part of the cloud where the temperature is more hot than the more outer part of the cloud where gas planets were formed.

Over time scientists have developed theories about the solar system and the movement of planets within it. One famous scientist has come up with three laws describing planetary movement and is know as Kepler's Three Laws. His first law is known as "The Law Of Ellipses" which explains that the planets orbit the sun in a path that is called an ellipse. An ellipse is a special curve in which the sum of the distances from every point on the curve to two other points is a constant.The two other points are known as the foci of the ellipse. (The Physics Classroom) The sun is always located at one of the foci of each planets ellipse. This relates to Newtons first law stating an object will stay in motion unless acted upon by an unbalanced force. Because a planet is moving in an ellipse motion, this law states that there must be some force acting upon the planet to make it orbit the sun.

Kepler's second law, also referred to as the "Law Of Equal Areas" describes the speed of any planet while orbiting the sun. The speed of any planet that moves through space is constantly changing. Scientists can calculate the force that is required to keep the Earth on its circular path by using Newtons famous equation F=MA. A planet moves fastest when it is closer to the sun and slowest when it is further from the sun. Even with that said, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would equal the same area in each time period.

Kepler's third, and final law is also know as the law of harmonies. This law compares the orbital period and radius of orbit of a planet to those of other planets. (The Physics Classroom) This law is the only law made by Kepler that involves the characteristics of not just one planet, but multiple planets. There is a force that attracts all the planets towards the Sun, so there must be an equal and opposite force attracting the Sun towards the planets due to Newtons third law. The sun does give an opposite reaction, but not as extreme as the reaction is on the planets. An example of comparing two planets is:

Planet	Period (s)	Average Distance (m)	T2/R3 (s2/m3)
Earth	3.156 x 107 s	1.4957 x 1011	2.977 x 10-19
Mars	5.93 x 107 s	2.278 x 1011	2.975 x 10-19     (The Physics  Classroom)