Over the years, definitions have changed several times
The ancient Greeks first coined the name "planet." The word means "wandering star," according to David Weintraub. He's 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. These are the objects that today we call the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn. This view of planets would hold for the next 1,500 years, Weintraub notes.
"The seven planets according to the Greeks were the seven planets at the time of the Copernicus," he says. "And those seven included the sun and the moon."
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. By doing that, he removed the sun from the planet tally. Then, in 1610, Galileo Galilei pointed a telescope to the sky. This Italian mathematician saw, for the first time, those objects we now know as the four moons of Jupiter.
Later in that century, astronomers Christiann Huygens and Jean-Dominique Cassini spotted five additional objects orbiting around Saturn. We now know them as moon. But at the end of the 1600s, astronomers agreed to call them planets. That brought to 16 the grand total of objects called planets.
Between then and the early 1900s, the number of planets fluctuated. At one point it reached a high of 16. Then it went back to six, when the objects circling planets were reclassified as moons. With the discover of Uranus, the planet count bumped up to seven. Later, it jumped to 13 with the initial discovery of several objects lying between Mars and Jupiter — objects today known as asteroids.
Clearly, scientists have been naming, re-naming and categorizing parts of the solar system ever since people began documenting 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.
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 now referred to as exoplanets.