I think that the first people to discover that planets were not just stars were Arab astronomers who lived roughly a thousand years ago. They kept careful track of the positions of all the stars in the sky, and noticed that nearly all of them moved in very predictable circular paths, rising in the east and setting in the west each night. There were a few objects, however, which didn't follow such simple behavior. They rose in the east and set in the west, but they wandered around in the sky in much more complicated paths than all the other stars. They called these funny objects "planets", which I believe means "wanderers" in arabic. The ones which they could see (they had no telescopes) were Mercury, Venus, Mars, Jupiter, and Saturn. When people invented telescopes (Galilleo was the first to study the sky with telescopes) the could see other, fainter planets: Uranus, Neptune, and Pluto. In addition, they could see that nearly all of these planets had moons of their own. You could find out more information about the planets and the telescopes used to study them in your library, or on the NASA webpages.

For many centuries people have looked carefully at the night skies and all the object visible there.They were able to track how these objects move, and noticed that some of them move in very different paths than the majority of the objects. Some of these erratically-moving objects are the planets in our solar system. What are some of the other objects that these ancient people discovered? I'll give you a hint on one of the type - when was the first recorded sighting of the object now known as Haley's Comet?

The word planet means "wanderer" Greek. Ancient peoples who studied the sky could see that most of the stars in the sky moved across the sky through the night together in a specific way. But there were a few bright stars that didn't follow this pattern and "wandered" across the sky in different paths. These stars were seen as special because of their brightness and unusual paths and were often associated with gods or events (the names we have for the planets come from the names the romans gave them which were based on roman gods). But most of these ancient people thought that the planets were just unusual stars and didn't think they could be rocky objects like the Earth that were just reflecting the light of the Sun. It wasn't until the 17th century that people looked closely at the planets through telescopes and realized that they weren't just points of light and had features on their surface (Galileo is credited with first doing this). Galileo was the first to write about planets as objects like the Earth or Moon and since then, our understanding of the planets has become much greater with further observation by telescope, and more recently, with space probes sent to look closely at the planets. What do you think people thought when Galileo first wrote about there being other planets like ours?

Planets have been known since the beginning of recorded history as special objects in the heavens-- They being the only objects that moved. From a visual point of view, planets get brighter, dimmer, move across the sky -- stop and turn around, but always stay close to the Zodiac or the ecliptic or the path of the sun across the sky. From that point of view, the planets: Mercury, Venus, Mars, Jupiter and Saturn have all been known since the most ancient times. However, It was not until the work of Gallileo and Kopernicus that scientists developed a sun-centered model for the solar system, and realized that planets must be massive bodies in orbits. Galileo was first to use a telescope to observe the moons of Jupiter -- and published his views in defiance of the church. This was late in the 16th century.

Uranus was the first planet discovered in modern times -- by observations of William Herschel in 1781. Many earlier people had seen it, but because they didn't make systematic maps, they missed its motion across the sky.

After Uranus, there was a flurry of interest to discover yet more planets... However, by careful analysis of the measured orbit of Uranus, astronomers John Adams and Urbain Le'Verrier predicted there must be a more distant planet -- it was finally found by Galle and D'Arrest in 1846.

Today both planets are easy binocular objects, and are in the summer sky -- pick up a copy of Astronomy or Sky and Telescope for easy instructions to find them...

Pluto was discovered in 1930 by Clyde Tombaugh -- (who died in 1997) it is much smaller and harder to find in the sky, even with a telescope. It was thought that there had to be a larger planet because of discrepencies in the calculations of the orbits of Uranus and Neptune -- but this was later shown to be an error. It is unlikely that there is another planet in our solar system -- unless it is very far out from the sun. It would be detected by its gravitational effects on the known planets.

Please check out the web site:
http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html

Also, if you have a pair of binoculars, take a look at Jupiter and the other planets... It is easy to pick out the four moons of Jupiter and fun to draw the positions on a few consecutive nights... They really move!