The Evolution of Defining Planets: From Ancient Greeks to Modern Astronomy

Human fascination with planets has spanned thousands of years, from the era of ancient Greeks to today's modern astronomers. The intrigue lies in uncovering what exactly a planet is and how it is defined. This article delves into the historical journey of defining planets, from the early days of astronomy to contemporary discoveries.

In antiquity, the definition of a planet was rooted in its observable attributes. The term "planet" originates from the Greek word "planētēs", translating to "wanderer". This came about as ancient Greeks observed certain stars shifting in the sky, contrary to the static stars. They named these shifting stars "planets".

The ancient Greeks recognized five planets: Mercury, Venus, Mars, Jupiter, and Saturn. These were the only planets visible to the naked eye, hence were termed as the "classical planets". They held the belief that these planets, along with the Sun and the Moon, revolved around the Earth.

The 16th and 17th centuries witnessed astronomers questioning the geocentric perspective of the universe. Nicolaus Copernicus introduced a heliocentric model where planets revolved around the Sun, and Earth was simply another planet. Galileo Galilei supported this model by using his telescope to observe planets, discovering they had moons and weren't perfect spheres. His observations of Venus’ phases also substantiated the heliocentric model.

With advancements in telescopes, astronomers started identifying more planets. Uranus was discovered by William Herschel in 1781, while Neptune was discovered by Johann Galle and Heinrich d’Arrest in 1846. These discoveries sparked questions about the precise definition of a planet and how they were distinct from other celestial bodies.

In 1930, Clyde Tombaugh discovered Pluto, which was initially categorized as a planet but was later reclassified as a dwarf planet. This decision was made in 2006 by the International Astronomical Union (IAU), which defined a planet as a celestial body that orbits the Sun, is spherical, and has cleared its orbit of other debris.

This definition has stirred controversy as it excludes Pluto and other dwarf planets from being classified as planets. Some scientists propose expanding the definition to include any spherical object that orbits a star. Others believe the definition should be based on physical attributes such as size and composition.

In recent times, astronomers have discovered numerous exoplanets, planets that orbit stars beyond our solar system. These discoveries have prompted further questions about the definition and classification of planets. As our exploration of the universe continues, our understanding of planets will likely evolve, potentially altering the definition once again.

In conclusion, the definition of planets has seen a significant evolution, from ancient Greeks' wandering stars to modern discoveries of exoplanets. Today, planets are defined as celestial bodies that orbit the Sun, are spherical, and have cleared their orbits of other debris. However, this definition invites controversy and may undergo changes as our understanding of the universe expands. The intrigue surrounding planets endures, ensuring their study remains a crucial part of astronomy for years to come.

* The International Astronomical Union (IAU), a global body of astronomers, has been engaged in a two-year project to establish a clear definition for a planet – distinguishing it from other 'solar system bodies'. During the IAU General Assembly in Prague, held from August 14-25, 2006, a proposal for this definition will be presented. If accepted, it will expand our Solar System to include 12 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, and 2003 UB313. The proposed additions are Ceres, Charon (associated with Pluto), and 2003 UB313. This redefinition wouldn't alter the planetary status of Pluto. An artist's interpretation presents these planets to scale, but without precise relative distances.