It wasn't until the 16th century that the idea was revived by Nicolaus Copernicus.īy the 17th century, astronomers (aided by the invention of the telescope) realized that the Sun was the celestial object around which all the planets-including Earth-orbit, and that the moon is not a planet, but a satellite (moon) of Earth. The first known model that placed the Sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The ancient Greeks counted the Earth's Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Over historical time, objects categorized as planets have changed. An Evolving Definitionĭefining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them.

Our understanding about the universe and our place in it has changed over time.

Scientists assess and question each other's work in a critical process called peer review. Scientific ideas are developed through reasoning and tested against observations. Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned.

It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.ĭiscussion-and debate-will continue as our view of the cosmos continues to expand.

It must be big enough to have enough gravity to force it into a spherical shape.It must orbit a star (in our cosmic neighborhood, the Sun).The most recent definition of a planet was adopted by the International Astronomical Union in 2006. But both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them-a vigorous debate that continues to this day. Everyone knows that Earth, Mars and Jupiter are planets. This seemingly simple question doesn't have a simple answer.

Definition definition of flat character