Planet definition Guide, Meaning , Facts, Information and Description
The word planet is currently very difficult to define. It has always been a rather fluid notion. When 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," or "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."The word did not need to be rethought again until 1802, when 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. Even the discovery of Pluto by Clyde W. Tombaugh in 1930 had little effect on the idea of what a planet should be, since, despite its size and eccentric orbit, it appeared to fit the concurrent definition.
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 redifined along the lines of "an object that orbits the Sun directly and in isolation, rather than as part of a larger group of objects." There has been no small outcry at this suggestion, and the International Astronomical Union has not officially done so as of yet.
However, even with the Pluto controversy excluded, there is no agreed consensus as to what a planet is. The definition of "planet" given in the Wikipedia is, "a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion", which seems fairly unambiguous; however it is 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" is obviously in place to differentiate a proper planet from a smaller object, such as an asteroid or a comet, 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 eclipse 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 alternative definition is "a body that orbits a star, never had nuclear fusion, and has sufficient mass to make itself spherical under its own gravity." This definition would:
- exclude brown dwarfs (which fuse hydrogen or deuterium),
- exclude white dwarfs and neutron stars (had fusion burning in their pasts),
- exclude moons (don't orbit stars),
- exclude asteroids (don't have sufficient mass to pull themselves into a spherical shape),
- include gas giants like Saturn,
- include rocky planets like Mars,
- include icy planets like Pluto,
- include Quaoar and Sedna (large enough to be spherical), and
- include 1 Ceres and 4 Vesta (large enough to be spherical).
The simple criteria in this definition remove many of the ambiguities and uncertainties in the previous definition. The "never had nuclear fusion" requirement sets a natural upper limit to planets at the lower end of brown dwarfs. The "sufficient mass to make itself spherical under its own gravity" requirement sets a lower limit to exclude irregular asteroids, comets, rubble piles, and burnt-out comet nucleii. This requirement also alleviates the need for defining planets by width - which would require setting an arbitrary lower limit.
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