Any knowledge goes through a number of stages of its formation. Along with the change of theories and the accumulation of data, honing and refinement of terminology also take place. This process has not passed astronomy. The definition of "planet" has 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 object moving in the sky. Translated, the word means "wandering wanderer." The Greeks attributed to them both some stars and the moon. According to this understanding, the Sun is a planet. Since then, our knowledge of the cosmos has expanded significantly, and therefore such a use of the term would have confused volumetric works on the Universe. The discovery of a number of new objects 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 concept that is contemporary to us, we briefly touch upon the evolution of the semantic load of the term in accordance with the pictures of the world accepted in a particular era. The learned minds of all ancient civilizations, from the Sumero-Akkadian to the Greek and Roman, did not bypass the night sky. They noticed that some of the objects are relatively motionless, while others are constantly moving. They were called planets in ancient Greece. Moreover, the astronomy of Antiquity is characterized by the fact that the Earth was not included in the list of “wandering wanderers”. During the heyday of the first civilizations, it was believed that our house was motionless, and the planets "ply" around it.
"Almagest"
The knowledge of the Babylonians, captured and processed by the ancient Greeks, resulted in a harmonious geocentric picture of the world. It was recorded in the work of Ptolemy, created in the second century AD. The Almagest (the name of the treatise) contained knowledge from various fields, including astronomy. It indicated that there is a system of planets around the Earth that are constantly moving in circular orbits. These were the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. Such an idea of the structure of the Universe was central for the whole 13 centuries.
Heliocentric model
The sun and moon were deprived of the status of "planet" only in the XVI century. The revival brought a lot of changes to 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, the satellites of Jupiter and Saturn were discovered. They were called planets for some time, but in the end, they and the moon were assigned the rank of satellites.
Until about the middle of the 19th century, any bodies moving around the sun were referred to planets. At this time, a large number of objects were discovered that occupied the area between Mars and Jupiter, and by the beginning of the 50s of the century before last, scientists had come to the conclusion that they all had characteristics that made it possible to separate them into a separate class. So on the map of outer space appeared asteroids. Since that time, the expression “small planet” has come across in literature - this is another designation of an 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 discovery of the ninth planet, Pluto. The found object was first considered to be superior in size to the Earth. Then it was discovered that its parameters are inferior to the characteristics of our planet. It was then that disagreements began among scientists about the location of Pluto in the classification of space objects. Some astronomers attributed it to comets, others believed that it was a satellite of Neptune, for some reason having left it. Pluto does not possess the properties characteristic of standard asteroids, but in comparison with other "wandering wanderers" of the solar system is too small. The answer to the question of whether this planet is or not, scientists found for themselves only at the beginning of the XXI century.
2006 definition
Astronomers have come to the conclusion that for the further development of science it is necessary to precisely define the concept of “planet”. This was done in 2006 at a meeting of the International Astronomical Union. The urgent need was determined not only by the controversial position of Pluto, but also by the numerous discoveries of the last century. Exoplanets (bodies orbiting other "suns") were discovered in systems of distant stars, some of which were many times larger than Jupiter in mass. Meanwhile, the most “modest" of the stars, brown dwarfs, have a similar characteristic. Thus, the boundary between the concepts of “planet” and “star” became blurred.
And after long disputes at the meeting of the IAU in 2006, it was decided to consider that the planet is an object with the following characteristics:
it revolves around the sun;
has such a mass that is enough to take the form of hydrostatic equilibrium (close to round);
cleared its orbit from other objects.
A little earlier, in 2003, a temporary definition of exoplanets was adopted. According to him, this is an object with a mass not reaching 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 having a mass sufficient for the thermonuclear reaction of deuterium to occur are considered to be a special type of star, brown dwarfs.
Minus one
As a result of the adoption of the definition, our planetary system has become smaller. Pluto does not correspond to all points: its orbit is “clogged” with other cosmic bodies, the total mass of which significantly exceeds this parameter of the former ninth planet. The MAC has classified Pluto as a small planet and at the same time a prototype for trans-Neptune objects, space bodies, whose average distance to the Sun exceeds this parameter for Neptune.
The debate about the situation of Pluto has not subsided so far. However, officially the Solar System today has only eight planets.
Little brothers
Together with Pluto, the objects of the solar system, such as Eris, Haumea, Ceres, Makemake, were also classified as small or dwarf planets. The first is part of the Scattered Disk. Pluto, Makemake and Haumea are included in the Kuiper belt, and Ceres is an 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 paragraph.
Thus, the solar system consists of 5 dwarf and 8 "full" planets. There are more than 50 objects of the Asteroid belt and Kuiper belt, which in the near future can get the status of small ones. In addition, further study of the latter may increase the list by another 200 cosmic bodies.
Main characteristics
All planets revolve around the stars, and mainly in the same direction as the luminary itself. Today, only one exoplanet is known, which moves in the opposite direction to the star.
The trajectory of the planet, its orbit, never represents an ideal circle. Turning around the star, the cosmic body is either approaching it or moving away from it. Moreover, during the approach of the planet begins to move faster, while moving away - to slow down.
Planets rotate around their axis. Moreover, they all have different angles of inclination of the axis relative to the plane of the equator of the star. For Earth, it is 23º. Thanks to this slope, seasonal weather changes occur. The larger the angle, the sharper the differences in the climate of the hemispheres. For Jupiter, for example, a slight slope is characteristic. As a result, seasonal changes are almost imperceptible on it. Uranus can be said to lie on its side. Here, one hemisphere is always in the shade, the second in the light.
Road without obstacles
As already mentioned, a planet is a cosmic body whose orbit is cleared of all other objects. It has enough mass to either attract other objects and make them their part or satellites, or push them out of orbit. This criterion in determining the planet today remains the most controversial.
Weight
Many of the characteristic features of planets - shape, purity of the orbit, interaction with neighbors - depend on one determining quality. It is the mass. Its sufficient value leads to the achievement by the cosmic body of hydrostatic equilibrium, it becomes rounded. The impressive mass allows the planet to clear its path of asteroids and other smaller objects. The mass threshold below which it is impossible to acquire a spherical shape is determined individually and depends on the chemical composition of the object.
In the solar system, the largest planet is Jupiter. Its mass is used as a specific measure. 13 masses of Jupiter - the upper limit of the mass of the planet. Next come the stars, or rather, brown dwarfs. A mass exceeding this limit creates the conditions for the start of thermonuclear fusion of deuterium. Scientists already know several exoplanets, the mass of which is approaching this threshold.
In the solar system, the smallest planet is Mercury, however, less massive bodies were discovered in the cosmos. The record holder in this sense is PSR B1257 + 12 b orbiting a 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. Earthlike ones include: Mercury, Venus, Earth and Mars - the fourth planet from the Sun. These are cosmic bodies, mostly composed of rocks. The largest of them is the Earth, the smallest, as already mentioned, Mercury. Its mass is 0.055 of the mass of our planet. Venus’s parameters are close to the Earth’s, and the fourth planet from the Sun is at the same time the third largest among the Earth-like ones.
Gas giants, as the name implies, significantly outperform the previous type. These include Jupiter, Saturn, Uranus and Neptune. They are characterized by a lower average density in comparison with a similar parameter in earth-like planets. All the gas giants of the solar system have rings. Saturn has the most famous. In addition, the presence of several satellites is characteristic of all. Interestingly, most of the parameters decrease with distance from the Sun, that is, from Jupiter to Neptune.
Today, people managed to discover a mass 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 are created that are potentially suitable for the emergence of life. Unfortunately, so far there is very little reason to believe that somewhere there is such a “funny" planet as ours, on which beings live, able to think, create and even determine which cosmic bodies can be attributed to planets and which ones not worthy.