Section 3: Solar System

Early Greeks thought planets, the sun, the moon, and stars all rotated around the Earth. This model is called the geocentric or Earth-centered model.  However, Nicholas Copernicus and Galileo Galilei observed that the moon revolved around the Earth and that Earth and the other planets revolved around the sun.  This model is called the heliocentric or sun-centered model.

Astronomers believe the solar system began 4.6 billion years ago in a cloud of gas and dust.  First, shockwaves from a possible supernova, or exploding star, caused clouds to compress, which resulted in a flat, spinning disc of heated material.  Next, heated material from the cloud triggered nuclear fusion, which formed the sun.  Next, material that was not used to create the sun began to cluster together to form asteroids, comets, planets, and more.  Eventually, only rocky materials could survive close to the sun, so gaseous and icy objects successfully clustered further away, forming the solar system we live in today.

Planets, dwarf planets, asteroids, comets, and meteoroids are all objects that orbit the sun.  A planet orbits the sun, has a nearly spherical shape, and has a much larger mass than the total mass of all other objects in nearby orbits.  A dwarf planet is a spherical-shaped object that orbits the sun but does not have more mass than the objects in nearby orbits.  An asteroid is composed of millions of small and rocky objects that orbit the sun in an asteroid belt.  They range in size from less than a meter to several hundred km.  A comet is made of gas, dust, and ice and moves around the sun in an oval-shaped orbit.  A meteoroid is made of debris left by colliding asteroids or dispersing comets.


  1. Compare the geocentric model to the heliocentric model.
  2. What is the difference between an asteroid and a meteoroid?
  3. What is a comet made up of?

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