Any knowledge passes several stages of its formation.Along with the change of theories and data acquisition takes place and sharpening, clarifying terminology.This process is not passed, and astronomy.The definition of "planet" evolved over the centuries and even millennia.The word is of Greek origin.Earth - is in the understanding of the ancient inhabitants of the Peloponnese, any moving object in the sky.The translation of the word means "wandering stranger."The Greeks attributed to them, and some of the stars and the moon.According to this understanding, and the Sun - it is a planet.Since then, our knowledge of the cosmos expanded considerably, and because such use of the term confuses the voluminous writings of the universe.The opening of several new facilities led to the need to revise and consolidate the definition of a planet, which was done in 2006.
little history before we turn to the modern concept, briefly touch on the evolution of semantic meaning of the term in accordance with accepted in a particular era paintings in the world.Scientists minds of all ancient civilizations, from the Sumerian-Akkadian and ending with the Greek and Roman did not pass his attention the night sky.They have noticed that some of the objects are relatively immobile, while others are constantly moving.They were called in ancient Greece planets.And astronomy Antiquity characterized by the fact that the Earth is in the list of "wandering strangers" was not included.During the heyday of the first civilizations, it was thought that our house is stationary and the planet "cruising" around him.
"Almagest"
Knowledge Babylonians, caught up and processed by the ancient Greeks, resulted in a coherent picture of the geocentric world.It was recorded in Ptolemy's work, created in the second century AD."Almagest" (the so-called treaty) contained knowledge from different fields, including astronomy.It pointed out that the Earth is a system of planets that are constantly moving in circular orbits.It was the Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn.Such an idea of the structure of the universe was mostly over as much as 13 centuries.
heliocentric model
Sun and Moon were denied the status of "planet" only in the XVI century.The revival has brought a lot of changes in scientific views of Europeans.It developed the heliocentric model in which the planets, including the Earth moved around the sun.Our house has ceased to be the center of the universe.
After about a century were discovered the moons of Jupiter and Saturn.For a time they were called planets, but in the end for them and stuck the title of the Moon satellites.
about the middle of the XIX century to the planets include any bodies moving around the sun.At this time, it was discovered a large number of objects in the area between Mars and Jupiter, and by the early 50-ies of the last century, scientists have concluded that they all have the characteristics to distinguish them in a separate class.So on the map of outer space were asteroids.The literature from that time was to meet the term "minor planet" - this is another indication of the asteroid.Planets in the conventional sense were called just enough large objects whose orbit around the sun takes place.
XX century
The last century was marked by the discovery of the ninth planet, Pluto.The found object is first considered superior to the Earth in size.Then it was discovered that its parameters are inferior characteristics of our planet.And then it began disagreement among scientists about the position of Pluto in the classification of space objects.Some astronomers have attributed it to the comets, while others believe that it is a satellite of Neptune, for whatever reasons, had left him.Pluto does not have the properties characteristic of standard asteroids, but also in comparison with other "strangers wandering" of the solar system is too small.The answer to the question of whether or not the planet, scientists have found for themselves only at the beginning of the XXI century.
Definition 2006
Astronomers came to the conclusion that for the further development of science is necessary to precisely designate the concept of "planet".This was done in 2006 at a meeting of the International Astronomical Union.The urgent need was conditioned not only the controversial position of Pluto, but many discoveries of the last century.Were discovered exoplanets (body orbiting other "suns") systems in the distant stars, some of them many times the mass of Jupiter.Meanwhile, this characteristic has the most "moderate" of the stars, brown dwarfs.Thus, the boundary has become blurred concepts of "planet" and "star".
And after lengthy debate at the meeting of the MAC in 2006 it was decided to assume that the planet - is an object with the following characteristics:
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it revolves around the Sun;
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has the weight, which is enough to take the form of a hydrostatic equilibrium (approximate to the circular);
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cleared its orbit of other objects.
Earlier, in 2003, was made a temporary definition of exoplanets.According to him an object of mass does not reach the level at which the deuterium fusion reaction is possible.The lower threshold for the mass exoplanets coincides with the threshold laid down in the definition of a planet.Objects that have mass sufficient for the occurrence of a fusion reaction of deuterium, is considered to be a special type of stars, brown dwarfs.
Minus one
As a result of the definition of our system of planets became smaller.Pluto does not fit all counts: its orbit is "blocked" by other cosmic bodies, the total mass is much greater than the parameter of the former ninth planet.MAC classify Pluto as a minor planet, while the prototype for a trans-Neptunian objects, celestial bodies, whose average distance from the Sun is greater than this setting at the Neptune.
The debate about Pluto's status still continues to this day.However, officially the solar system today has only eight planets.
take less
With Pluto in the number of small, or dwarf planets, and were classified as such objects of the solar system, like Eris, Haumea, Ceres, Makemake.The first - a part of the scattered disc.Pluto, Makemake and Haumea are part of the Kuiper Belt, and Ceres - the object of the asteroid belt.All of them have the first two qualities of the planets, enshrined in the new definition, but does not comply with the third paragraph.
Thus, the solar system is composed of five dwarf and 8 "full" planets.There are more than 50 objects of the asteroid belt and the Kuiper belt, which may soon get the status of the small.In addition, further study of the latter is likely to increase by another 200 list of celestial bodies.
Key Features
All the planets revolve around stars, and preferably in the same direction as the star itself.Today only one known extrasolar planet, which moves in the opposite direction of the stars.
The trajectory of the planet, its orbit is never a perfect circle.Turning around a star, the cosmic body that approaches her, then removed from it.And during the Rapprochement of the planet begins to move faster when deleting - to slow down.
Planets rotate around its axis, and.Moreover, they all have different inclination axis relative to the equatorial plane of the star.For the Earth, it is 23º.With this slope there are seasonal changes in the weather.The greater the angle, the sharper the variations in climate hemispheres.For Jupiter, for example, is characterized by a slight slope.As a result, it is almost imperceptible seasonal changes.Uranium can be said, lying on its side.Where one hemisphere is always in the shade, the second - in the light.
road without obstacles
As already mentioned, the planet - it is a cosmic body, the orbit is cleared of all other objects.It has sufficient weight to pull or other objects and make their satellite or its part, or to push out of orbit.This criterion in the definition of a planet to date remains the most controversial.
Weight
Many characteristic signs of the planets - the form, the purity of the orbit, the interaction with the neighbors - depend on one defining quality.He is a mass.Sufficient its value leads to the achievement of the cosmic body of hydrostatic equilibrium, it becomes rounded.The impressive mass of the planet allows you to clear a path from asteroids and other smaller objects.Mass threshold below which it is impossible to acquire a spherical shape, and is determined individually depending on the chemical composition of the object.
in the solar system's largest planet - Jupiter is.Its mass is used as a specific measure.13 Jupiter masses - the upper limit of the mass of the planet.The following are already stars, or rather brown dwarfs.Weight exceeding this limit, creates the conditions for the beginning of the fusion of deuterium.Scientists already know a few extrasolar planets whose mass is close to this threshold.
in the solar system's smallest planet - it's Mercury, but on the spaces of the cosmos were found and less massive body.The champion in this sense - PSR B1257 + 12 b, orbiting the pulsar.
closest neighbors
planets of the solar system are divided into two groups: the Earth-and gas giants.They differ in size, composition, and some other characteristics.To Terrestrial include: Mercury, Venus, Earth and Mars - the fourth planet from the sun.This cosmic bodies, mostly composed of rocks.The largest of them, the earth, the smallest, as already mentioned, Mercury.Its weight is 0,055 from the mass of our planet.Parameters of Venus close to the earth, and the fourth planet from the Sun - is at once the third largest among the Earth-like.
gas giants, as the name implies, are far superior to the previous type of its parameters.They include Jupiter, Saturn, Uranus and Neptune.They are characterized by lower average density compared to the same parameter have Earth-like planets.All the gas giants of the solar system have rings.Saturn has the most famous.Furthermore, all characterized by the presence of several satellites.Interestingly, most of the parameters of the distance from the Sun, that is, from Jupiter to Neptune, falls.
Today, people had time to open the mass exoplanets.However, among them, the Earth still has one fundamental difference: it is situated in the so-called habitable zone, ie at a distance from the star where conditions are potentially suitable for life.Unfortunately, there is very little reason to believe that somewhere there is just as "cheerful" planet like ours, where the living creatures, the ability to think, create and even determine which space objects can be classified as planets and which of this titlenot worthy.
