Above: The eight planets of the solar system, in order from the sun (far left). This illustration shows the planets' relative sizes, but the distance between planets is not to scale (International Astronomical Union/NASA)
Everything in the night sky looks more or less like a star. But the ancient Greeks noticed that five of these “stars” did not behave like the others. In fact, the word “planet” is derived from the Greek word for “wanderer,” because while the other stars rose and set in a fixed pattern, the five visible planets moved around. It wasn't until much later that astronomers realized these “stars” were actually bodies of rock and gas, orbiting the sun just like Earth. The five planets visible to the ancient Greeks (and which remain visible to anyone observing a dark sky) were Mercury, Venus, Mars, Jupiter, and Saturn.
Did you know? The only two planets that were first discovered using a telescope were Uranus and Neptune. While the first six planets were known to ancient cultures, Uranus moves too slowly for its motion to be easily detected and Neptune is not visible to the naked eye.There are a total of eight planets in our solar system. Uranus and Neptune, the farthest from the sun, cannot be seen without a telescope. No planet produces light, and we can only see planets because of light reflected from the sun. As a result, only the closest and the largest planets can be seen without a telescope. So even though Uranus and Neptune are very big, they're too far away and to be seen with the naked eye.
The four planets closest to the sun—Mercury, Venus, Earth, Mars—are called “terrestrial planets,” because they're made of rock. The other four planets—Jupiter, Saturn, Uranus, Neptune—are called the “gas giants,” because they're so much bigger than the terrestrial planets and they're made of gas.
Even though they're all a part of the solar system, the planets are pretty far from each other. If Earth were the size of a golf ball, Venus, our closest neighbour, would be at least 182 metres away. If that doesn't seem very far, consider that, on the same scale, Neptune would be at least 14.5 kilometres away! Distances between planets can vary greatly depending on where they are in their orbits and whether they are on the same side or different sides of the sun.
Earth seems to be the only planet capable of supporting life. However, scientists think Mars may have once had liquid water on its surface. If this were true, Mars may also have supported life! Venus, with a thick and dense atmosphere of carbon dioxide, couldn't support life. However, its enormous greenhouse effect gives climate scientists an excellent model to study when considering the consequences of global warming on Earth.
The gas giants have no solid surface. Made entirely of gas, they orbit far out from the sun, offering spectacular views through a telescope. Jupiter, for example, has many visible cloud bands that circle the planet about once every 10 hours. One region of Jupiter, called the Great Red Spot, is covered by a hurricane-like storm that is more than twice the size of Earth and that has been raging since at least the late 1600s!
Did you know? Venus, the second planet from the sun, rotates so slowly that it orbits the sun faster than it can complete a single rotation. In other words, a Venusian day is longer than a Venusian year!Saturn is famous for its enormous and beautiful ring system, also easily seen through a telescope. Even though these rings are huge, extending far away from the planet, they're average thickness is only about 100m. Some areas are as thin as 5 metres! When Saturn's rings are seen edge-on from Earth, they almost disappear entirely!
Uranus and Neptune are similar in many ways. Both are made of the same gases, like methane, and they’re also of comparable size and weight. However, each planet has some unique traits. Uranus is the only planet in the solar system whose north pole points at the sun. As a result, it rotates completely on its side! Neptune, the farthest planet from the sun, has the biggest difference between its year and day. A Neptunian year lasts 165 Earth years, while a Neptunian day lasts only 16 Earth hours!
Each planet has fascinating characteristics, and the more we study them, the more we learn about how each one is unique.
Our Solar System: Overview (NASA)
http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys Ask an Astronomer: "Why isn't Pluto a planet any more?" (Robert Hurt)
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ask_astronomer/video/transcripts/2006-003.pdf Nine Planets. A Multimedia Tour of the Solar System: one star, eight planets, and more
http://nineplanets.org/ How far is each planet from Earth (Astronomy Department, Cornell University)