The planet is ... The qualities and system of the planets

Any knowledge passes a number of stages of itsformation. Together with the change of theories and the accumulation of data, there is also a sharpening, clarification of terminology. This process did not pass astronomy. The definition of the concept of "planet" evolved over many centuries and even millennia. The word itself is of Greek origin. The planet is, in the understanding of the ancient inhabitants of the Peloponnese, any moving object in the sky. In translation, the word means "wandering wanderer". The Greeks referred to them and some of the stars, and the moon. According to this understanding, the Sun is a planet. Since then, our knowledge of the cosmos has greatly expanded, and therefore such an application of the term would introduce confusion into voluminous works on the universe. The opening of a number of new facilities led to the need to revise and consolidate the definition of the planet, which was done in 2006.

A bit of history

Before turning to the modern concept,let us briefly discuss the evolution of the term's semantic load in accordance with the world's pictures adopted in this or that epoch. The scientific minds of all ancient civilizations, from the Sumerian-Akkadian to the Greek and Roman, did not bypass the night sky. They noticed that some of the objects are relatively immovable, while others are constantly moving. They were named in ancient Greece by the planets. And for the astronomy of Antiquity is characterized by the fact that the Earth in the list of "wandering wanderers" was not included. During the heyday of the first civilizations there was an opinion that our house is motionless, and the planets "ply" around it.

"Almagest"

Knowledge of the Babylonians, picked up and processedancient Greeks, poured into a harmonious geocentric picture of the world. It was recorded in the work of Ptolemy, created in the second century of our era. "Almagest" (the so-called treatise) contained knowledge from various fields, including astronomy. It stated that around the Earth there is a system of planets that are constantly moving in circular orbits. They were the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. This idea of ​​the structure of the universe was central for the whole 13 centuries.

Heliocentric model

The sun and the moon were deprived of the status of "planet"only in the XVI century. The revival brought a lot of changes in the scientific views of Europeans. There was a heliocentric model, according to which planets, including the Earth, moved around the Sun. Our house has ceased to be the center of the universe.

After about a century, satellites of Jupiter and Saturn were discovered. For some time they were called planets, but in the end, behind them and behind the moon, the title of satellites was fixed.

Approximately until the middle of the XIX century the planets wereany body moving around the Sun. At this time, a large number of objects were opened that occupied the area between Mars and Jupiter, and by the early 1950s scientists had come to the conclusion that they all possessed the characteristics that allowed them to be singled out as a separate class. So asteroids appeared on the map of outer space. In the literature from that time, the expression "a small planet" began to occur - this is another designation of the asteroid. Planets in the usual sense began to be called only large enough objects, whose orbit passes around the Sun.

XX century

The past century was marked by the opening of the ninthplanet, Pluto. The found object was first considered to be larger than the Earth in size. Then it was discovered that its parameters are inferior to those of our planet. It was then that disagreements began among scientists about the location of Pluto in the classification of space objects. Some astronomers referred it to comets, others believed that it was a satellite of Neptune, for some reason left it. Pluto does not have the properties typical of standard asteroids, but in comparison with other "wandering wanderers" of the solar system is too small. The answer to the question of whether the planet is or not, scientists found for themselves only at the beginning of the XXI century.

Definition of 2006

Astronomers came to the conclusion that for furtherdevelopment of science, it is necessary to accurately identify the concept of "planet". This was done in 2006 at the meeting of the International Astronomical Union. Urgent need was determined not only by the controversial position of Pluto, but also by the numerous discoveries of the last century. Exoplanets (bodies circulating around other "suns") were found in systems of distant stars, some of them many times larger than Jupiter by mass. Meanwhile, such a characteristic is possessed by the most "modest" of the stars, brown dwarfs. Thus, the boundary of the concepts "planet" and "star" has become blurred.

And after long discussions at the meeting of the IAU in 2006 it was decided to consider that the planet is an object with the following characteristics:

• it turns around the Sun;

• has such a mass, which is sufficient to take the form of hydrostatic equilibrium (approximate to circular);

• cleared its orbit from other objects.

A little earlier, in 2003, a temporarydefinition of an exoplanet. According to him, this is an object with a mass that does not reach the level at which a thermonuclear reaction of deuterium is possible. In this case, the lower mass threshold for exoplanets coincides with the threshold fixed in the definition of the planet. Objects that have a mass sufficient for the course of the thermonuclear reaction of deuterium are considered to be a special type of stars, brown dwarfs.

Minus one

As a result of the adoption of the definition, our systemthe planets became smaller. Pluto does not correspond to all points: its orbit is "clogged" by other cosmic bodies, the total mass of which considerably exceeds the given parameter of the former ninth planet. MAS classified Pluto as a small planet and at the same time a prototype for trans-Neptunian objects, cosmic bodies, whose average distance to the Sun exceeds this parameter in Neptune.

Disputes about the situation of Pluto do not cease until now. However, officially the solar system today has only eight planets.

Brothers smaller

Together with Pluto in the number of small, or pygmyplanets, such objects of the solar system as Eris, Haumea, Ceres, and Makemake were also included. The first is part of the Broken Disc. Pluto, Makemake and Haumea enter the Kuiper belt, and Ceres is the object of the Asteroid belt. All of them have the first two qualities of the planets, fixed in the new definition, but do not correspond to the third point.

Thus, the solar system consists of 5Dwarf and 8 "high-grade" planets. There are more than 50 objects of the Asteroid belt and the Kuiper belt, which in the near future can get the status of small ones. In addition, further study of the latter, perhaps, will increase the list by another 200 space bodies.

Main characteristics

All planets revolve around the stars, andmainly in the same direction as the luminary itself. Today, only one exoplanet is known, which moves in the opposite direction to the star's direction.

The trajectory of the motion of the planet, its orbit, neverdoes not represent a perfect circle. Turning around the star, the cosmic body either approaches it, then moves away from it. And during the rapprochement, the planet begins to move faster, while removing - to slow down.

The planets also rotate around their axis. And all of them have a different angle of inclination of the axis relative to the plane of the equator of the star. For the Earth, it is 23º. Due to this slope, seasonal weather changes occur. The larger the angle, the sharper the differences in the hemispheric climate. For Jupiter, for example, is characterized by a slight slope. As a result, seasonal changes are almost invisible there. Uranium, you can say, lies on its side. Here, one hemisphere is always in the shade, the other in the light.

Road without obstacles

As already mentioned, the planet isThe cosmic body whose orbit has been cleared of all other objects. It has enough mass to either attract other objects and make them part of them or satellites, or push them out of orbit. This criterion in the definition of the planet today remains the most controversial.

Weight

Many of the characteristic features of the planets are a form,purity of the orbit, interaction with neighbors - depend on one determining quality. He is a mass. Its sufficient magnitude leads to the achievement by the cosmic body of hydrostatic equilibrium, it becomes round. An impressive mass allows the planet to clear its way from asteroids and other smaller objects. The mass threshold, below which the spherical shape can not be acquired, is determined individually and depends on the chemical composition of the object.

In the solar system, the largest planet isJupiter. Its mass is used as a certain measure. 13 masses of Jupiter - the upper limit of the mass of the planet. Then follows the stars, or rather, brown dwarfs. A mass exceeding this limit creates conditions for the beginning of thermonuclear fusion of deuterium. Scientists already know a few exoplanets, the mass of which is approaching this threshold.

In the solar system, the smallest planet isMercury, however, in the space of space, less massive bodies were discovered. The record holder in this sense is PSR B1257 + 12 b, which revolves around the pulsar.

Nearest neighbors

The planets of the solar system are divided into two groups: earth-like and gas giants. They differ in size, composition and some other characteristics. Earth-like are: Mercury, Venus, Earth and Mars - the fourth planet from the Sun. These are cosmic bodies, mostly consisting of rocks. The largest of them is Earth, the smallest, as already mentioned, Mercury. Its mass is 0.055 of the mass of our planet. The parameters of Venus are close to the terrestrial, and the fourth planet from the Sun is simultaneously the third largest among the earth-like.

Gas giants, as the name implies,considerably exceed in their parameters the previous type. These include Jupiter, Saturn, Uranus and Neptune. They are characterized by a lower average density compared to a similar parameter for earth-like planets. All gas giants of the solar system have rings. The most famous is Saturn. In addition, all are characterized by the presence of several satellites. It is interesting that most of the parameters decrease with distance from the Sun, that is, from Jupiter to Neptune.

Today, people have discovered a lot of exoplanets. However, the Earth among them still has one cardinal difference: it is located in the so-called zone of life, that is, at such a distance from the star, where conditions that are potentially suitable for the origin of life are created. Unfortunately, so far there are very few grounds for suggesting that somewhere there is as much a "cheerful" planet as ours, on which live creatures are able to think, create and even determine which cosmic bodies can be referred to the planets, and what are the titles not worthy.