The Far Planets - Three Centuries Of Discovery

It took three centuries for humans to uncover the far planets in our solar system. In the 1600s the telescope opened up new opportunities for observation. People learned that Jupiter and Saturn had moons and that Saturn had rings. The telescope also showed that wandering stars were not stars at all, because they did not generate their own light, but reflected light from the Sun.

No new planets were discovered during the 1600s. The far planets were still too distant and fuzzy to be recognized for what they were. Uranus was discovered in the late 1700s. Another century passed before the discovery of Neptune. Pluto, discovered in 1930, was the last planet found in the solar system.

Astronomers categorize planets based on geology and composition. Mercury, Venus, Earth, and Mars are called the terrestrial planets, because they are made of rock and metal. They have solid surfaces on which spacecraft could land. Jupiter, Saturn, Uranus, and Neptune are called the gas giants. Some scientists think they may have solid cores, but the exterior of these planets consists of huge clouds of gas. These planets are also known as the Jovian planets (after Jove or Jupiter). All of them have ring systems.

Pluto is in a class by itself. It is a small ice world. Astronomers argue whether it is even a planet at all. Some believe that it is a dormant comet. Nevertheless, Pluto has been classified as a planet for decades now, and will likely continue to be so.