A Look at the Big Picture - Our Solar System

The solar system is made up of the Sun, the 9 planets and their 137 known moons, asteroids, comets, dust and gas. The planets, asteroids, and comets travel around the Sun, the center of our solar system. Most of the bodies in the solar system travel around the Sun along nearly circular paths or orbits, and all the planets travel about the Sun in the anticlockwise direction (when viewed from above). Solar system formation began billions of years ago, when gases and dust began to come together to form the Sun, planets, and other bodies of the solar system.

Our solar system is located in the outer reaches of the Milky Way Galaxy, which is a spiral galaxy. The Milky Way Galaxy contains roughly 200 billion stars. Most of these stars are not visible from Earth. Almost everything that we can see in the sky belongs to the Milky Way Galaxy.

The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. All of the planets except for Venus orbit in the same direction (counter-clockwise looking down from above the Sun's north pole).

The nine planets are often classified in several ways:

The first method of classification is by composition. All planets are either rocky or gaseous. The terrestrial or rocky planets are Mercury, Venus, Earth, and Mars.The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

The Jovian or gas planetsare Jupiter, Saturn, Uranus, and Neptune.The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.

The planet are also categorized by size. The small planets are Mercury, Venus, Earth, Mars. A small planet is defined as one with a diameter less than 13000 km. The giant planets are Jupiter, Saturn, Uranus and Neptune. The giant planets are sometimes also referred to as gas giants. The giant planets have diameters greater than 48000 km.

Scientists also classify the planets by their position relative to the Sun. The inner planets are Mercury, Venus, Earth and Mars. The outer planets are Jupiter, Saturn, Uranus, Neptune. The asteroid belt between Mars and Jupiter forms the boundary breaking up the inner solar system and the outer solar system.

Further categorization is done by position relative to Earth. The inferior planets are Mercury and Venus. These planets are closer to the Sun than Earth. The superior planetsare Mars thru Neptune., and are farther from the Sun than Earth. It this type of classification system Earth is in its own category.

The last of the most common ways to classify the planets is by history. The classical planets are Mercury, Venus, Mars, Jupiter, and Saturn. These planets have been known since prehistorical times and are visible to the unaided eye. The modern planets are Uranus and Neptune. These planets were discovered in modern times and are visible only with optical aid.

Although people know a lot about the planets in our solar system, we have much more to learn. Scientists want to know more about the composition of the planets and their atmospheres.