Solar Systems

For several thousands of years, humanity at large did not recognize the existence of the Solar System. They believed the Earth to be a stationary planet fixed at the center of the universe and categorically different from the astronomical objects that moved through the sky.

An ancient mathematician-astronomer in India named Aryabhata, and the Greek philosopher Aristarchus of Samos both speculated on a heliocentric reordering of the cosmos; but Nicolas Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors Galileo, Johannes Kepler and Isaac Newton developed the science of physics which led to the gradual acceptance of the idea that the Earth moves around the sun and that the Earth and other planets are governed by the same physical laws.

Now, of course we have refined technology to the point where we even use unmanned spacecrafts to travel throughout our solar system, gathering data about various planets and other phenomena in space.

The solar system consists of the Sun and the celestial bodies bound to it by gravity, all of which formed approximately 4.6 billion years ago. These celestial bodies circle the sun in what is called an ecliptic plane, most of the mass of which is contained within eight solitary planets. The four smaller planets, Mercury, Venus, Earth and Mars, also known as the terrestrial planets, are comprised primarily of rock and metal.

The four outer planets, Jupiter, Saturn, Uranus and Neptune, also known as the "gas giants", are comprised primarily of hydrogen and helium and are much larger than the terrestrial planets in our solar system.

Now, you may ask - what about Pluto? Good question. Although it has been traditionally said that there are nine planets in our solar system, Pluto being one of them, Pluto is now categorized as a "dwarf planet".

You see, Pluto is a small rocky object that lies at the very edge of the solar system. It is only about two-thirds the size of our moon. This dwarf planet is so far out it takes light from the sun about five and a half hours to reach Pluto in contrast to the 8 minutes it takes to reach Earth. Its orbit of about 248 years sometimes takes it inside Neptune's orbit.

The International Astronomical Union recently decided on a definition of the word "planet". The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that:

• Orbits around the Sun
• Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
• Has cleared the neighborhood around its orbit.

As you can see, Pluto does not fit all three of these criteria and so a new label was born, the "dwarf planet" of which Pluto is the proto-type for what are suspected to be many dwarf planets within our solar system.