Introductory Astronomy: The Nine Planets

Our Solar System is a diverse collection of planets, asteroids, moons, comets, and meteoroids which all revolve around the central star, our Sun. These objects represent a wide range of sizes, compositions, and temperatures. Some of these objects are active, with volcanos or huge weather patterns; others are dead, dusty, cratered chunks of rock. They are united by a common center of gravity: the Sun.

We live on the third planet from the Sun: the Earth. It is one of the "terrestrial planets". Planets are generally divided into two groups: the terrestrialand the giantplanets. The terrestrial planets are the four inner planets: Mercury, Venus, Earth, and Mars. They are generally small in size (about the size of the Earth) and are predominantly rocky in composition. The giant planets are the next four: Jupiter, Saturn, Uranus, and Neptune. These four are the largest planets in the Solar System. They are rich in hydrogen ices and gases, generally have more satellites than the terrestrial planets, and have rings. The final planet, Pluto, is the exception: it falls into neither category!

Most of the planets have satellite moons which orbit about them like a miniature solar system. Our Earth, for example, has one moon. Jupiter, on the other hand, has 16! Also, most planets orbit the Sun in the same plane. Pluto's orbit is the most inclined to the plane of the Earth's orbit.

Asteroids, meteoroids, and comets are generally smaller bodies which orbit the Sun. Asteroids and Meteors are usually clustered in specific regions in the Solar System ("belts") while comets travel alone on often highly eccentric orbits. They are known collectively as "Space Debris."

This page will give you a general overview of the different planets that make up our Solar System. You will also find many helpful links to sites with more information about the planets. So, without further ado, here are the nine planets (in order):

To browse a really neat site, which contains information, pictures, and links to lots of other web sites on the Nine Planets, click here.

Now that you know all about the worlds of our Solar System, click here to learn about planets discovered in other solar systems.

After you review these sections, try a few sample questions to test your understanding. These questions are typical of questions given in introductory astronomy course exams. They are meant only to give you an idea of what kinds of questions MIGHT be on your exam. Just because these questions are here does NOT mean that you will have questions like them on your exam, NOR does it even mean that you will have questions on these topics on your exam. They are just PRACTICE questions!

GOOD LUCK!!