Definition of Planet

Although planets are the principal component of the solar system other than the sun, a definition of what a planet actually is, is surprisingly elusive. This article details the questions that may arise when trying to formulate a strict definition of the word.

History and etymology

What defines a planet has always been somewhat arbitrary. When the word was originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky; hence, "planetes", meaning "wanderers." This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon. Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and, after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. A "planet" could then be defined as "any object that orbited the Sun, rather than another object."

Asteroids: minor planets

The word did not need to be rethought again until 1801, when the first asteroid, Ceres, was discovered. Bode's Law, a mathematical function which generates the size of the semimajor axis of planetary orbits, predicts a body between Mars and Jupiter, and Ceres was found to lie at almost exactly the required distance.

Then in 1802, Heinrich Olbers discovered 2 Pallas, a second "planet" at roughly the same distance from the Sun as the recently discovered "planet" 1 Ceres. The idea that two planets could occupy the same orbit was an affront to thousands of years of thinking. Eventually these "planets" numbered in their thousands, and were given their own separate classification, asteroids, letting the concept of planet survive with little modification. Asteroids are often referred to as minor planets.

The Pluto controversy

The discovery of Pluto by Clyde W. Tombaugh in 1930 had little initial effect on the idea of what a planet should be, since, despite being smaller than Earth's Moon, and in a more eccentric orbit than any other planet, it still appeared to broadly fit the contemporary definition of a planet.

Then, beginning in 1992, astronomers began to notice large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-rumoured Kuiper Belt, the ring of icy debris that is the source of all short-period comets including Halley's comet. Pluto's planetary status was thrown into question, with many scientists claiming that it should be demoted to the largest object in the Kuiper Belt and that a "planet" should be redefined along the lines of "an object that orbits the Sun directly and in isolation, rather than as part of a larger group of objects." Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and Ceres (such as Pluto). There has been no small outcry at such suggestions, and the International Astronomical Union has not officially redefined Pluto's planetary status as of yet.

The continuing discovery of objects not much smaller than Pluto, such as 50000 Quaoar and 90377 Sedna, seems certain to re-ignite the debate over Pluto's planetary status in the near future.

Problems with strict definitions

However, even with the Pluto controversy excluded, there is no agreed consensus on what a strict definition of a planet should be. A definition such as "a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion" seems fairly unambiguous, but is actually frought with uncertainties. Do planets necessarily orbit stars? Many astronomers have claimed to have spotted "rogue planets" drifting in space unattended by any star. "Of considerable mass" should distinguish planets from minor solar system bodies such as comets and asteroids, but it is rather vague. What is "considerable mass?" Mercury has avoided any dispute to its planetary status for thousands of years, yet there are moons in the solar system that exceed it in size. And finally, "does not produce energy from nuclear fusion" certainly separates a planet such as the Earth from a star like our Sun, yet what about brown dwarfs, stars too small to commence fusion in their cores? Many of them are likely to orbit other stars. Does that make them planets? And what about objects that don't produce energy through nuclear fusion, but did at one time? Would a white dwarf orbiting a star be reclassified as a planet?

An arbitrary definition

Ultimately, there is probably no real need for a strict definition of what a planet is, with convention and history instead playing the major role in determining what is considered a planet and what is not. The situation is somewhat similar to the consideration of what is a continent - a consideration determined partly by geography and partly by history and culture, with no strict definition possible.