Kim Jolin, age 11, of Sioux City, Iowa, for her question:

Which planet was discovered first?

Our ancient ancestors spotted the planets ages and ages before they knew what they were. For a long time, they must have mistaken them for stars. After all, unless we look very carefully even we can mistake a planet for a star. What's more, we still refer to the Morning Star and the Evening Star which really are planets.

Maybe the first planet to be discovered was Venus or Jupiter ¬or maybe it was Mars or Saturn. It was hardly Mercury because this little planet is so close to the sun that it is rarely seen. Certainly it was not Uranus or Neptune or little far away Pluto. We know that it was not one of these three because we cannot see them without a telescope, which was not invented until around 1608.

Many thousands of years ago, early astronomers began watching and checking the heavenly bodies as they moved over the sky. They made charts that showed how the starry constellations change with the seasons, year after year. In time they noticed that a few bright starry objects do not fit into this heavenly parade. They wander this way and that way against the orderly background of fixed stars.

These wanderers, of course, were the planets. Four and then five of them were named way back before the dawn of history and nobody knows which one was discovered first. For a long time, the early astronomers were confused by the planet we call Venus. Sometimes it appeared as the Morning Star and they named it Lucifer, which means light. Sometimes it played the role of the Evening Star and it was called Herperus, meaning the west. In those days, people seemed to think that Venus was two different planets.

In any case, Mars and Jupiter and Saturn were named for gods that went out of style thousands of years ago. Eventually the early astronomers named and recognized Mercury as a wandering planet. At last they had discovered all the visible planets. And so matters stood until the year 1781 A.D.

By this time, astronomers had fairly good telescopes for scanning the skies. In 1781, the English astronomer William Herschel spotted an unknown wanderer in the constellation Gemini, the Twins. After some debate, it was decided to name the newly discovered planet Uranus.

After this, astronomers began to suspect that perhaps there are other planets, which proved to be true. Neptune was discovered in 1846 and little Pluto, the last of the family, was discovered on January 31, 1930.

So all we know for sure is that five of the planets were discovered thousands of years ago and three were discovered during the last 200 years. We could have given more modern names to the late arrivals. But astronomers decided to give similar names to the entire planet family. So the last three also are named for ancient gods that went out of style long ages ago.