A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals. The term "planet" originally came from the Greek word for "wanderer," reflecting their motion relative to the fixed stars.
The concept of planets has evolved over time. Initially, ancient astronomers like Ptolemy classified all celestial bodies that moved in the sky as planets, including the Sun and the Moon. With the advent of heliocentrism by Nicolaus Copernicus in the 16th century, the understanding shifted to planets as bodies orbiting the Sun, excluding the Sun and Moon.
The current definition of a planet was established by the International Astronomical Union (IAU) in 2006:
Planets can be categorized into several types:
Since the discovery of the first exoplanet around a main sequence star in 1995, thousands of exoplanets have been found, expanding our understanding of planetary formation and diversity. These planets orbit stars other than the Sun and provide insights into the potential for life elsewhere in the universe.
Human exploration of planets has been limited to our Solar System. Missions like Viking, Voyager, and Mars Rovers have provided detailed information about our planetary neighbors.