(c) Anthony K. Grafton 2003

Our Solar System, Part 7: The Outer Planets and Beyond

Humans have known for thousands of years about the six planets closest to the sun (although it has only been a few hundred years since people realized that Earth was one of them). All they had to do was watch the night skies and notice that the shining points of light called planets moved differently than all the other stars in the heavens.

However, it was not until 1781 that the first of the outer three planets, Uranus, was discovered by astronomer William Herschel. Careful observations of the orbit of Uranus showed that another planet must exist even further away, and in 1846 a pair of French astronomers discovered Neptune. Due to its small size and extreme distance, Pluto, usually the outermost planet in our solar system, was not discovered until 1930.

The biggest claim to fame for Uranus is its orientation. Most planets in our solar system orbit the sun with their axes (an imaginary line going through the planets from the north to the south poles) always nearly perpendicular (at a right angle) to their direction of orbit. Uranus, however, orbits on its side, so that for part of its 84-year orbit, its north pole points almost directly toward the sun, while on the other side of its orbit the south pole points at the sun.

Neptune, like Uranus, Jupiter, and Saturn, is classified as a gas giant, which mean that it is made up mostly of gases like hydrogen, helium, and methane. Neptune is so far away from the sun that it takes 165 years to complete one orbit, and, depending on the location of Pluto, Neptune is sometimes the outermost planet in our solar system.

Pluto is unusual for three reasons: it is very small (having only a tiny fraction of Earth’s mass), it is very far away (30 times farther from the sun than is Earth), and it orbits the sun at a very different angle than the other planets. In fact, all these unusual features have recently led some astronomers to suggest that Pluto should no longer be called a planet at all! NASA plans to launch a mission called “New Horizons” in 2004 that will send a probe to Pluto, but the great distance the probe must travel means it won’t arrive until 2015.

There are other parts of our solar system besides the planets and moons. What can you find out about the things called the Kuiper Belt, the Oort cloud, and the asteroid belt?