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What is a Solar System?

A solar system is defined as a central sun with its associated planets, asteroids, meteors, satellites (i.e.,moons), and comets that are "captured" in its orbit. These various celestial bodies are trapped in a constant orbit around the sun by its tremendous gravitational pull. The paths that the planets take as they travel around the sun in the same direction - from west to east - is not truly circular, but more of an ellipse, or egg-shape path. Our solar system is nestled inside a very large galaxy of stars called the Milky Way. The outer limit of our solar system extends six billion kilometers from the sun.

Over 4.6 billion years ago our solar system was born when a nebula consisting of a dense nucleus, or protosun, surrounded by a thin shell of a gaseous matter and dust began to collapse in on itself. As the dense matter in the center of the solar system further condensed the extreme heat that was generated in the center began to burn the abundant hydrogen atoms in its core, becoming a self-sustaining nuclear-fusion reaction that grew to be our sun. As the dust in the nebula circulated the newly forming sun, it collapsed and clumped together to form larger chunks of space debris. Larger and larger pieces of space debris collided with each other to form the solid planets, and the gaseous matter condensed to form the gas planets.

The Nine Planets

We have nine major planets, with several of them having their own moons. How do we define the difference between a planet and a moon? A planet orbits the sun, and a moon orbits a planet. Technically, the moon also orbits the sun as it spins around its planet, but because it has its own suborbit of a planet we define it as a moon. Some of our planets have several moons. Scientists are still debating about whether our ninth and furthest planet, Pluto, is actually a planet, or a moon from a more distant planet that got caught in our solar system.

Here are the nine planets in our solar system, listed in order of their appearance from the sun. Mercury is the closest to the sun and Pluto is the furthest.