Over the years, definitions have changed several times
The ancient Greeks first coined the name "planet," a word that means "wandering star," according to David Weintraub, an astronomer at Vanderbilt University in Nashville, Tenn. Aristotle, the Greek natural philosopher who lived over 2,000 years ago, identified seven "planets" in the sky — the objects that today we call the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn. It was a view of planets that held for the next 1,500 years, Weintraub says.
"The seven planets according to the Greeks were the seven planets at the time of the Copernicus, and those seven included the sun and the moon," he says.
Nicolaus Copernicus was the Polish astronomer who suggested in the early 1500s that the sun, and not the Earth, was at the center of what we today call the solar system. He removed the sun from the planet tally. Then, in 1610, the Italian mathematician Galileo Galilei pointed a telescope to the sky. He saw, for the first time, the objects we know today as the four moons of Jupiter.
Later that century, the astronomers Christiann Huygens and Jean-Dominique Cassini spotted five additional objects orbiting around Saturn. At the end of the 1600s, astronomers agreed that the objects orbiting Jupiter and Saturn, along with those two planets, Mercury, Venus, Earth, Earth's moon and Mars, should all be called planets. This brought the grand total of objects called planets to sixteen.
Between that time and the early 1900s, the objects astronomers called planets fluctuated from a high of 16, back to six when the objects circling planets were reclassified as moons, up to seven when Uranus was discovered, and back up to 13 after the initial discovery of several objects lying between Mars and Jupiter — objects we know today as asteroids.
Clearly, scientists have been naming, re-naming and categorizing the various parts of the solar system ever since people began looking at and documenting the objects in the night sky thousands of years ago. In 2006, the International Astronomical Union defined "planet" in a way that kicked Pluto out of the planet tribe.
And since then, astronomers have been logging stars throughout our galaxy that also appear to host their own planets. To differentiate these from planets in our solar system, those around other stars are called exoplanets.