The earth is moving with some initial momentum perpendicular to the sun. Newton’s first law states that an object will remain in motion unless a force acts on the object to change its motion.

In space there is no friction to slow down the moving earth, so it will keep moving. If there were no sun, the earth would travel in a straight line; however, the sun exerts a force of gravity on the earth that pulls the earth and keeps it from flying off into space.

If the earth was moving too fast, the sun’s gravity would not be enough to keep the earth in orbit, and it would fly off into space. If the earth was moving too slow, it would spiral into the sun due the sun’s gravitational pull. However, if a planet is moving at just the right velocity, then it can maintain a stable orbit around the sun and never fly off or crash into the sun.

Newton showed that a single planet’s orbit around the sun is stable, and forms an ellipse with the sun at one of the focus points. This orbit will continue indefinitely due to the constant force of gravity of the sun and law of conservation of momentum.

Using math to calculate a planet’s orbit is one of the crowning achievements of the Enlightenment age. However, this calculation only considers the earth + sun, and ignores the possible effect of the other planets and the moon, all of which exhibit a gravitation force on each other. Adding in the gravitational forces of all 8 planets into Newton’s equation becomes impossible to solve exactly. To study the stability of the complete solar system, scientists have used Newton’s laws to simulate the planetary motion on super computers.

For mathematical reasons, the exact long-time behavior of the planets is unpredictable, and so scientists have performed many computer simulations each starting from a slightly different set of parameters (typically shifting the position of each planet by a random amount of about one millimeter), and tracing the planetary orbits on the computer. What are the final results of these calculations? About 99% of the time, the computer simulations show that the current solar system is stable for around 8 billion years from now until the death of the sun. However, in 1% of the cases, Mercury collides with Venus at some point before the death of the sun. In some rare scenarios this could trigger a larger instability of the solar system and lead to a Venus-Earth or Earth-Mars collision, but this is highly unlikely.

For more information check out space.com ( here )
Or from the Paris Observatory ( here )