Definition of planet

What exactly is a planet? The answer has changed many times throughout history.

The word itself comes from the ancient Greek term *planētes*. This translates to "wandering stars." Ancient astronomers used this to describe starlike objects that moved across the sky. They contrasted these with "fixed stars," which appeared to stay in place relative to one another. For much of history, the definition of a planet was simply based on this visible motion.

In antiquity, the definition was quite broad. Many Greek astronomers followed the geocentric model. This model placed a stationary Earth at the center of the universe. In this view, all celestial bodies revolved around the Earth. The Greeks identified five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.

However, many traditions also included the Sun and the Moon as planets. This grouping of seven "wandering" bodies was common in Graeco-Roman cosmology. Even later medieval writers accepted this seven-planet system.

A massive shift occurred with the rise of the heliocentric model. Nicolaus Copernicus proposed that the Sun, not the Earth, was the center. This required a complete reclassification of celestial bodies. Once the Earth was understood to orbit the Sun, Earth itself became a planet.

This era also changed how we viewed the Moon. While some early observers called the Moon a planet, it was eventually reclassified as a satellite. A satellite is a natural body that orbits a larger object. Galileo's discovery of Jupiter's moons helped prove that planets could have their own satellites.

Modern astronomy uses more precise scientific criteria to define a planet. There are two primary ways to approach this. The first is the geophysical definition. This focuses on the physical state of the object. A planet must be in hydrostatic equilibrium. This means it is massive enough for its own gravity to pull it into a rounded shape. The second is the dynamical dominance definition. This focuses on how a planet interacts with its surroundings. It means the object must control the gravitational fate of other small bodies in its vicinity.

The need for a formal rule became urgent in the early 2000s. In January 2005, astronomers discovered a trans-Neptunian object named Eris. Eris was found to be more massive than Pluto, which was then the smallest accepted planet. This discovery created a crisis in how we categorize objects in the outer Solar System. To resolve this, the International Astronomical Union (IAU) met in Prague in August 2006. The IAU is the international governing body for astronomical nomenclature. They released a specific definition that applies to our Solar System.

According to the IAU, a planet must meet three specific requirements. First, the body must orbit the Sun. Second, it must reach hydrostatic equilibrium to become round. Third, it must have "cleared its neighborhood."

Clearing the neighborhood means the planet's gravity has swept its orbital path clear of other significant objects. Pluto meets the first two criteria. However, it fails the third because it has not cleared its path of other objects. Therefore, under the current IAU definition, Pluto is not classified as a planet.

This decision remains a topic of debate among experts. While many astronomers follow the IAU rules, some planetary scientists disagree. They propose using a purely geophysical definition instead. This would focus more on the physical properties of the body rather than its orbital path. The history of discovery shows how our definitions must evolve.

In 1781, William Herschel discovered Uranus, adding a seventh planet to our known list. In 1846, Neptune was found by observing gravitational irregularities. Each new discovery forces us to refine our understanding of the cosmos.