Any knowledge goes through a series of stages of its formation. Along with the change of theories and the accumulation of data, there is also a sharpening and clarification of terminology. This process has not bypassed astronomy either. The definition of the concept of "planet" has evolved over many centuries and even millennia. The word itself is of Greek origin. A planet is, in the understanding of the ancient inhabitants of the Peloponnese, any object moving in the sky. In translation, the word means "wandering wanderer." The Greeks referred to them both some stars and the Moon. According to this understanding, the Sun is also a planet. Since then, our knowledge of the cosmos has expanded significantly, and therefore such a use of the term would confuse the voluminous 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 we turn to the modern concept, let us briefly touch on the evolution of the semantic load of the term in accordance with the worldviews accepted in a particular era. The learned minds of all the ancientscivilizations, ranging from the Sumerian-Akkadian to the Greek and Roman, did not ignore the night sky. They noticed that some objects are relatively stationary, while others are constantly moving. They were called planets in Ancient Greece. Moreover, for the astronomy of Antiquity, it is characteristic that the Earth was not included in the list of “wandering wanderers”. During the heyday of the first civilizations, there was an opinion that our house is motionless, and the planets "cruise" around it.
Almagest
The knowledge of the Babylonians, picked up 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. "Almagest" (the so-called treatise) contained knowledge from various fields, including astronomy. It indicated that around the Earth is a system of planets that are constantly moving in circular orbits. These were the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. This idea of the structure of the universe was the main one for as many as 13 centuries.
Heliocentric model
The sun and the moon were deprived of the status of "planet" only in the XVI century. The Renaissance brought a lot of changes in the scientific views of Europeans. A heliocentric model developed, according to which the planets, including the Earth, moved around the Sun. Our home is no longer the center of the universe.
After about a century, the moons of Jupiter and Saturn were discovered. For some time they were called planets, but in the end they and the Moon were assigned the titlesatellites.
Until about the middle of the 19th century, any body moving around the Sun was considered a planet. At this time, a large number of objects were discovered that occupied the region between Mars and Jupiter, and by the beginning of the 50s of the century before last, scientists came to the conclusion that they all have characteristics that make it possible to distinguish them into a separate class. So asteroids appeared on the map of outer space. Since that time, the expression “minor planet” has become common in the literature - this is another designation for an asteroid. Planets in the usual sense began to be called only fairly large objects whose orbit passes around the Sun.
XX century
The last century was marked by the discovery of the ninth planet, Pluto. The found object was first considered to be larger than the Earth. Then it was found that its parameters are inferior to those of our planet. This is where 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, which for some reason left it. Pluto does not have the properties characteristic of standard asteroids, but in comparison with other "wandering wanderers" of the solar system, it is too small. The answer to the question of whether it is a planet 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 accurately define the concept of "planet". That was itmade 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 numerous discoveries of the last century. Exoplanets were discovered (bodies orbiting other "suns") in systems of distant stars, and some of them were many times larger than Jupiter in mass. Meanwhile, the most “modest” of stars, brown dwarfs, have a similar characteristic. Thus, the boundary between the concepts of "planet" and "star" has become blurred.
And after a long debate at the IAU meeting in 2006, it was decided to consider that the planet is an object with the following characteristics:
- it revolves around the Sun;
- has enough mass to take the form of hydrostatic equilibrium (approximately round);
- cleared its orbit from other objects.
Slightly earlier, in 2003, a temporary definition of an exoplanet was adopted. 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 with a mass sufficient for the deuterium thermonuclear reaction to proceed are considered to be a special type of star, brown dwarfs.
Minus one
As a result of the adoption of the definition, our system of planets has become smaller. Pluto does not meet all the points: its orbit is “clogged” with othercosmic bodies, the total mass of which significantly exceeds this parameter of the former ninth planet. The IAU has classified Pluto as a minor planet and at the same time a prototype for trans-Neptunian objects, cosmic bodies whose average distance from the Sun exceeds that of Neptune.
Disputes about the position of Pluto have not subsided so far. However, officially the solar system today has only eight planets.
Smaller brothers
Together with Pluto, such objects of the solar system as Eris, Haumea, Ceres, Makemake were included in the number of small or dwarf planets. The first is part of the Scattered Disc. Pluto, Makemake, and Haumea are part of the Kuiper Belt, while Ceres is an Asteroid Belt object. All of them have the first two qualities of the planets enshrined 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 Asteroid Belt and Kuiper Belt objects that may soon receive minor status. In addition, further study of the latter may increase the list by another 200 space bodies.
Key Features
All planets revolve around stars, mostly in the same direction as the star itself. Today, only one exoplanet is known to move in the opposite direction of the star.
The trajectory of a planet, its orbit, is never a perfect circle. Revolving around the star, the cosmic body either approaches it or moves away from it. Moreover, during the approach, the planet begins to move faster, while moving away, it slows down.
The planets also rotate around their axis. Moreover, 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 changes in the weather occur. The larger the angle, the sharper the differences in the climate of the hemispheres. Jupiter, for example, has a slight tilt. As a result, seasonal changes are almost imperceptible on it. Uranus, one might say, lies on its side. Here, one hemisphere is always in the shade, the second is 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 part of it or satellites, or push it out of orbit. This criterion in determining the planet today remains the most controversial.
Mass
Many characteristic features of the planets - shape, purity of orbit, interaction with neighbors - depend on one defining quality. They are the mass. Its sufficient value leads to the achievement of hydrostatic equilibrium by the cosmic body, it becomes rounded. The impressive mass allows the planet to clear its way from 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 compositionobject.
In the solar system, the largest planet is Jupiter. Its mass is used as a certain measure. 13 Jupiter masses is the upper limit of the planet's mass. This is followed by stars, or rather, brown dwarfs. A mass exceeding this limit creates the conditions for the start of thermonuclear fusion of deuterium. Scientists already know of several exoplanets whose mass approaches this threshold.
In the solar system, the smallest planet is Mercury, but less massive bodies have been discovered in space. The record holder in this sense is PSR B1257+12 b orbiting the pulsar.
Nearest neighbors
The planets of the solar system are divided into two groups: terrestrial and gas giants. They differ in size, composition and some other characteristics. Earth-like ones include: Mercury, Venus, Earth and Mars - the fourth planet from the Sun. These are cosmic bodies, mostly consisting 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. The parameters of Venus are close to those of the Earth, 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, are significantly superior in their parameters to the previous type. These include Jupiter, Saturn, Uranus and Neptune. They are characterized by a lower average density compared to that of Earth-like planets. All gas giants in the solar system have rings. Saturn is the most famous. In addition, all are characterized by the presence of several satellites. Interestingly, most of the parameters decrease with distance from the Sun, that is, from Jupiter to Neptune.
Today, people have managed to discover a lot of exoplanets. However, the Earth among them still has one fundamental 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, there are so far very few grounds for the assumption that somewhere there is a planet as "fun" as ours, on which creatures live who are able to think, create, and even determine which cosmic bodies can be classified as planets, and which of this title not worthy.
