Definition of planet
From Biocrawler, the free encyclopedia.
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. The lexical definition has changed over time, initially being an extensional definition (a list), becoming an intensional definition (the necessary and sufficient conditions for belonging to the set), and finally today in scientific use a precising definition is used which fine tunes the dictionary definition (lexical definition) of the term for the current specific issue under discussion.
This article details the questions that may arise when trying to formulate a strict definition of the word.
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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, "planet", meaning "wanderer". 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, with the discovery of Ceres.
Bode's Law, a mathematical function which generates the size of the semimajor axis of planetary orbits, predicts a body between Mars and Jupiter. Ceres was found to lie at almost exactly the required distance, and was initially referred to as a new planet.
Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as 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 referred to as minor planets before being eventually given their own separate classification, asteroids, letting the concept of planet survive with little modification.
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 (sometimes it is closer to the Sun than Neptune), it still appeared to broadly fit the contemporary definition of a planet.
Then, beginning in 1992, astronomers began to detect 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.
Double Planets?
Pluto isn't the sole suspect in this case of planetary puzzlement. Pluto and its satellite Charon, are the only planet/moon pair in the solar system whose barycenter lies above the planet's surface. In addition, this is the only case where both the planet and its moon are tidally locked with respect to each other. If one were to stand on the surface of either Pluto or Charon, one would see its opposite partner hanging forever in the sky, perpetually still. As a result, it is common for astronomers to refer to Pluto/Charon as a double planet; two objects orbiting the Sun in tandem.
Even our own Moon, surprisingly, could be considered a partner in a double planet system, since, though it certainly orbits the Earth, the timing of its orbit round the Earth with the Earth's orbit round the Sun means that, looking down on the ecliptic, the Moon never actually loops back on itself, and in essence orbits the Sun in its own right see this link (http://www.nmm.ac.uk/site/request/setTemplate:singlecontent/contentTypeA/conWebDoc/contentId/8027/set_paginate/No/navId/00500300l005007002). However, most astronomers do not refer to the Earth/Moon system as a double planet, since the Moon has a much smaller mass in relation to the Earth than Charon has in relation to Pluto.
Problems with strict definitions
Even with the Pluto and lunar controversies excluded, there is no agreed consensus on what a strict definition of a planet should be. The criteria for most definitions generally include that it must orbit a star, be big, and yet not big enough to produce nuclear fusion in its core. However, none of these criteria are as clear-cut as they seem.
Do planets always orbit stars? Some astronomers have claimed to have spotted "rogue planets" drifting in space unattended by any star, while several moons in the Solar System, while they do not orbit the Sun, exhibit features in common with true planets. Jupiter's moon Ganymede and Saturn's moon Titan are both larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede even has a magnetic field, something among the terrestrial planets unique to Earth.
Assuming we limit our definition to objects in orbit round stars, that still leaves the question of size. How big is big? Pluto is a recognised planet, yet it is not much larger than many officially designated Kuiper Belt objects. Where does one draw the line? Some have argued that the cut-off point should be when an object can no longer remain spherical under its own gravity. Yet that would not only let Pluto into the planetary club, but also a number of asteroids, to say nothing of hundreds, possibly thousands, of Kuiper Belt and Oort Cloud objects. To refer to them both as planets and as members of thier own distinct populations would make the word "planet" essentially useless in practice.
And finally, not producing 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?
Extrasolar planets
The search for a dividing line between star and planet has been complicated still further since 1995, with the discovery to date of over 150 extrasolar planets (planet-sized objects in orbit around other stars). After all, apart from size and relative temperature, there is little to separate Jupiter from its host star, the Sun. Both have similar overall compositions; hydrogen and helium, with trace levels of heavier elements in their atmospheres.
The IAU has officially released a statement (http://www.dtm.ciw.edu/boss/IAU/div3/wgesp/definition.html) to define what constitutes an extrasolar planet and what constitutes an orbiting star:
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
- Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
The definition, like any definition of a planet, is also uncertain and ambiguous. By ignoring the process of formation, it is essentially saying that any star too small to commence nuclear fusion in its core is a planet, thus placing it in the same company as Earth, Mars and (at this time) Pluto. Are all planets therefore merely small stars?
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.
