Planets of the solar system - a little history
Previously, a planet was considered to be any body that orbits a star, glows with light reflected from it, and is larger than an asteroid.
Even in Ancient Greece, they mentioned seven luminous bodies that move across the sky against the backdrop of fixed stars. These cosmic bodies were: the Sun, Mercury, Venus, Moon, Mars, Jupiter and Saturn. The earth was not included in this list, since the ancient Greeks considered the earth to be the center of all things.
And only in the 16th century, Nicolaus Copernicus, in his scientific work entitled “On the Revolution of the Celestial Spheres,” came to the conclusion that it was not the Earth, but the Sun that should be at the center of the planetary system. Therefore, the Sun and Moon were removed from the list, and the Earth was added to it. And after the advent of telescopes, Uranus and Neptune were added, in 1781 and 1846, respectively.
Pluto was considered the last discovered planet in the solar system from 1930 until recently.
And now, almost 400 years after Galileo Galilei created the world's first telescope for observing stars, astronomers have come to the following definition of a planet.
Planet is a celestial body that must satisfy four conditions:
the body must revolve around a star (for example, around the Sun);
the body must have sufficient gravity to have a spherical or close to it shape;
the body should not have other large bodies near its orbit;
the body should not be a star.
In turn, the polar star is a cosmic body that emits light and is a powerful source of energy. This is explained, firstly, by the thermonuclear reactions occurring in it, and secondly, by the processes of gravitational compression, as a result of which a huge amount of energy is released.
Planets of the Solar System today
solar system is a planetary system that consists of a central star - the Sun - and all the natural space objects orbiting around it.
So, today the solar system consists of eight planets: four inner, so-called terrestrial planets, and four outer planets, called gas giants.
The terrestrial planets include Earth, Mercury, Venus and Mars. All of them consist mainly of silicates and metals.
The outer planets are Jupiter, Saturn, Uranus and Neptune. Gas giants are composed mainly of hydrogen and helium.
The sizes of the planets of the Solar System vary both within groups and between groups. Thus, gas giants are much larger and more massive than terrestrial planets.
Mercury is closest to the Sun, then as it moves away: Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
It would be wrong to consider the characteristics of the planets of the Solar System without paying attention to its main component: the Sun itself. Therefore, we will start with it. 
The sun planet is the star that gave rise to all life in the solar system. Planets, dwarf planets and their satellites, asteroids, comets, meteorites and cosmic dust revolve around it.
The Sun arose about 5 billion years ago, is a spherical, hot plasma ball and has a mass that is more than 300 thousand times the mass of the Earth. The surface temperature is more than 5000 degrees Kelvin, and the core temperature is more than 13 million K.
The Sun is one of the largest and brightest stars in our galaxy, which is called the Milky Way galaxy. The Sun is located at a distance of about 26 thousand light years from the center of the Galaxy and makes a full revolution around it in about 230-250 million years! For comparison, the Earth makes a full revolution around the Sun in 1 year.
Mercury planet
Mercury is the smallest planet in the system, which is closest to the Sun. Mercury has no satellites.
The surface of the planet is covered with craters that appeared about 3.5 billion years ago as a result of massive bombardment by meteorites. The diameter of the craters can range from a few meters to more than 1000 km.
The atmosphere of Mercury is very thin, consists mainly of helium and is inflated by the solar wind. Since the planet is located very close to the Sun and does not have an atmosphere that would retain heat at night, the surface temperature ranges from -180 to +440 degrees Celsius.
By earthly standards, Mercury completes a full revolution around the Sun in 88 days. But a Mercury day is equal to 176 Earth days.
Venus planet
Venus is the second planet closest to the Sun in the solar system. Venus is only slightly smaller in size than Earth, which is why it is sometimes called “Earth’s sister.” Has no satellites.
The atmosphere consists of carbon dioxide mixed with nitrogen and oxygen. The air pressure on the planet is more than 90 atmospheres, which is 35 times more than on Earth.
Carbon dioxide and the resulting greenhouse effect, dense atmosphere, and proximity to the Sun allow Venus to bear the title of “hottest planet.” The temperature on its surface can reach 460°C.
Venus is one of the brightest objects in the earth's sky after the Sun and Moon.
Planet Earth 
Earth is the only planet known today in the Universe on which there is life. The Earth has the largest size, mass and density among the so-called inner planets of the Solar System.
The age of the Earth is about 4.5 billion years, and life appeared on the planet about 3.5 billion years ago. The Moon is a natural satellite, the largest of the satellites of the terrestrial planets.
The Earth's atmosphere is fundamentally different from the atmospheres of other planets due to the presence of life. Most of the atmosphere consists of nitrogen, but also includes oxygen, argon, carbon dioxide and water vapor. The ozone layer and the Earth's magnetic field, in turn, weaken the life-threatening influence of solar and cosmic radiation.
Due to the carbon dioxide contained in the atmosphere, the greenhouse effect also occurs on Earth. It is not as pronounced as on Venus, but without it the air temperature would be about 40°C lower. Without an atmosphere, temperature fluctuations would be very significant: according to scientists, from -100°C at night to +160°C during the day.
About 71% of the Earth's surface is occupied by the world's oceans, the remaining 29% are continents and islands.
Mars planet
Mars is the seventh largest planet in the solar system. “Red Planet”, as it is also called due to the presence of large amounts of iron oxide in the soil. Mars has two satellites: Deimos and Phobos.
The atmosphere of Mars is very thin, and the distance to the Sun is almost one and a half times greater than that of the Earth. Therefore, the average annual temperature on the planet is -60°C, and temperature changes in some places reach 40 degrees during the day.
Distinctive features of the surface of Mars are impact craters and volcanoes, valleys and deserts, and polar ice caps similar to those on Earth. The highest mountain in the solar system is located on Mars: the extinct volcano Olympus, whose height is 27 km! And also the largest canyon: Valles Marineris, whose depth reaches 11 km and length – 4500 km 
Jupiter planet
Jupiter is the largest planet in the solar system. It is 318 times heavier than the Earth, and almost 2.5 times more massive than all the planets in our system combined. In its composition, Jupiter resembles the Sun - it consists mainly of helium and hydrogen - and emits a huge amount of heat equal to 4 * 1017 W. However, in order to become a star like the Sun, Jupiter must be 70-80 times heavier.
Jupiter has as many as 63 satellites, of which it makes sense to list only the largest - Callisto, Ganymede, Io and Europa. Ganymede is the largest moon in the solar system, even larger than Mercury.
Due to certain processes in the inner atmosphere of Jupiter, many vortex structures appear in its outer atmosphere, for example, bands of clouds in brown-red shades, as well as the Great Red Spot, a giant storm known since the 17th century.
Saturn planet
Saturn is the second largest planet in the solar system. Saturn's calling card is, of course, its ring system, which consists mainly of icy particles of various sizes (from tenths of a millimeter to several meters), as well as rocks and dust.
Saturn has 62 moons, the largest of which are Titan and Enceladus.
In its composition, Saturn resembles Jupiter, but in density it is inferior even to ordinary water.
The planet's outer atmosphere appears calm and uniform, which is explained by a very dense layer of fog. However, wind speeds in some places can reach 1800 km/h.
Uranus planet
Uranus is the first planet discovered by telescope, and the only planet in the Solar System that orbits the Sun on its side.
Uranus has 27 moons, which are named after Shakespearean heroes. The largest of them are Oberon, Titania and Umbriel.
The composition of the planet differs from gas giants in the presence of a large number of high-temperature modifications of ice. Therefore, along with Neptune, scientists have classified Uranus as an “ice giant.” And if Venus has the title of “hottest planet” in the solar system, then Uranus is the coldest planet with a minimum temperature of about -224°C.
Neptune planet
Neptune is the most distant planet in the solar system from the center. The story of its discovery is interesting: before observing the planet through a telescope, scientists used mathematical calculations to calculate its position in the sky. This happened after the discovery of inexplicable changes in the movement of Uranus in its own orbit.
Today, 13 satellites of Neptune are known to science. The largest of them, Triton, is the only satellite that moves in the direction opposite to the rotation of the planet. The fastest winds in the solar system also blow against the rotation of the planet: their speed reaches 2200 km/h.
In composition, Neptune is very similar to Uranus, therefore it is the second “ice giant”. However, like Jupiter and Saturn, Neptune has an internal source of heat and emits 2.5 times more energy than it receives from the Sun.
The blue color of the planet is given by traces of methane in the outer layers of the atmosphere.
Conclusion
Pluto, unfortunately, did not manage to get into our parade of planets in the solar system. But there is absolutely no need to worry about this, because all the planets remain in their places, despite changes in scientific views and concepts.
So, we answered the question how many planets are there in the solar system. There are only 8 .
Planets of the Solar System
According to the official position of the International Astronomical Union (IAU), the organization that assigns names to astronomical objects, there are only 8 planets.
Pluto was removed from the planet category in 2006. because There are objects in the Kuiper belt that are larger/equal in size to Pluto. Therefore, even if we take it as a full-fledged celestial body, then it is necessary to add Eris to this category, which has almost the same size as Pluto.
By MAC definition, there are 8 known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
All planets are divided into two categories depending on their physical characteristics: terrestrial planets and gas giants.
Schematic representation of the location of the planets
Terrestrial planets
Mercury
The smallest planet in the solar system has a radius of only 2440 km. The period of revolution around the Sun, equated to an earthly year for ease of understanding, is 88 days, while Mercury manages to rotate around its own axis only one and a half times. Thus, his day lasts approximately 59 Earth days. For a long time it was believed that this planet always turned the same side to the Sun, since periods of its visibility from Earth were repeated with a frequency approximately equal to four Mercury days. This misconception was dispelled with the advent of the ability to use radar research and conduct continuous observations using space stations. The orbit of Mercury is one of the most unstable; not only the speed of movement and its distance from the Sun change, but also the position itself. Anyone interested can observe this effect.
Mercury in color, image from the MESSENGER spacecraft
Its proximity to the Sun is the reason why Mercury is subject to the largest temperature changes among the planets in our system. The average daytime temperature is about 350 degrees Celsius, and the nighttime temperature is -170 °C. Sodium, oxygen, helium, potassium, hydrogen and argon were detected in the atmosphere. There is a theory that it was previously a satellite of Venus, but so far this remains unproven. It does not have its own satellites.
Venus
The second planet from the Sun, the atmosphere is almost entirely composed of carbon dioxide. It is often called the Morning Star and the Evening Star, because it is the first of the stars to become visible after sunset, just as before dawn it continues to be visible even when all the other stars have disappeared from view. The percentage of carbon dioxide in the atmosphere is 96%, there is relatively little nitrogen in it - almost 4%, and water vapor and oxygen are present in very small quantities.
Venus in the UV spectrum
Such an atmosphere creates a greenhouse effect; the temperature on the surface is even higher than that of Mercury and reaches 475 °C. Considered the slowest, a Venusian day lasts 243 Earth days, which is almost equal to a year on Venus - 225 Earth days. Many call it Earth's sister because of its mass and radius, the values of which are very close to those of Earth. The radius of Venus is 6052 km (0.85% of Earth's). Like Mercury, there are no satellites.
The third planet from the Sun and the only one in our system where there is liquid water on the surface, without which life on the planet could not have developed. At least life as we know it. The radius of the Earth is 6371 km and, unlike other celestial bodies in our system, more than 70% of its surface is covered with water. The rest of the space is occupied by continents. Another feature of the Earth is the tectonic plates hidden under the planet's mantle. At the same time, they are able to move, albeit at a very low speed, which over time causes changes in the landscape. The speed of the planet moving along it is 29-30 km/sec.
Our planet from space
One revolution around its axis takes almost 24 hours, and a complete passage through the orbit lasts 365 days, which is much longer in comparison with its closest neighboring planets. The Earth's day and year are also accepted as a standard, but this is done only for the convenience of perceiving time periods on other planets. The Earth has one natural satellite - the Moon.
Mars
The fourth planet from the Sun, known for its thin atmosphere. Since 1960, Mars has been actively explored by scientists from several countries, including the USSR and the USA. Not all exploration programs have been successful, but water found at some sites suggests that primitive life exists on Mars, or existed in the past.
The brightness of this planet allows it to be seen from Earth without any instruments. Moreover, once every 15-17 years, during the Confrontation, it becomes the brightest object in the sky, eclipsing even Jupiter and Venus.
The radius is almost half that of Earth and is 3390 km, but the year is much longer - 687 days. He has 2 satellites - Phobos and Deimos .
Visual model of the solar system
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Sun
The Sun is a star that is a hot ball of hot gases at the center of our Solar System. Its influence extends far beyond the orbits of Neptune and Pluto. Without the Sun and its intense energy and heat, there would be no life on Earth. There are billions of stars like our Sun scattered throughout the Milky Way galaxy.
Mercury
Sun-scorched Mercury is only slightly larger than Earth's satellite the Moon. Like the Moon, Mercury is practically devoid of an atmosphere and cannot smooth out the traces of impact from falling meteorites, so it, like the Moon, is covered with craters. The day side of Mercury gets very hot from the Sun, while on the night side the temperature drops hundreds of degrees below zero. There is ice in the craters of Mercury, which are located at the poles. Mercury completes one revolution around the Sun every 88 days.
Venus
Venus is a world of monstrous heat (even more than on Mercury) and volcanic activity. Similar in structure and size to Earth, Venus is covered by a thick and toxic atmosphere that creates a strong greenhouse effect. This scorched world is hot enough to melt lead. Radar images through the powerful atmosphere revealed volcanoes and deformed mountains. Venus rotates in the opposite direction from the rotation of most planets.
Earth is an ocean planet. Our home, with its abundance of water and life, makes it unique in our solar system. Other planets, including several moons, also have ice deposits, atmospheres, seasons and even weather, but only on Earth did all these components come together in a way that made life possible.
Mars
Although details of the surface of Mars are difficult to see from Earth, observations through a telescope indicate that Mars has seasons and white spots at the poles. For decades, people believed that the bright and dark areas on Mars were patches of vegetation, that Mars might be a suitable place for life, and that water existed in the polar ice caps. When the Mariner 4 spacecraft arrived at Mars in 1965, many scientists were shocked to see photographs of the murky, cratered planet. Mars turned out to be a dead planet. More recent missions, however, have revealed that Mars holds many mysteries that remain to be solved.
Jupiter
Jupiter is the most massive planet in our solar system, with four large moons and many small moons. Jupiter forms a kind of miniature solar system. To become a full-fledged star, Jupiter needed to become 80 times more massive.
Saturn
Saturn is the farthest of the five planets known before the invention of the telescope. Like Jupiter, Saturn is composed primarily of hydrogen and helium. Its volume is 755 times greater than that of the Earth. Winds in its atmosphere reach speeds of 500 meters per second. These fast winds, combined with heat rising from the planet's interior, cause the yellow and golden streaks we see in the atmosphere.
Uranus
The first planet found using a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet is so far from the Sun that one revolution around the Sun takes 84 years.
Neptune
Distant Neptune rotates almost 4.5 billion kilometers from the Sun. It takes him 165 years to complete one revolution around the Sun. It is invisible to the naked eye due to its vast distance from Earth. Interestingly, its unusual elliptical orbit intersects with the orbit of the dwarf planet Pluto, which is why Pluto is inside the orbit of Neptune for about 20 years out of 248 during which it makes one revolution around the Sun.
Pluto
Tiny, cold and incredibly distant, Pluto was discovered in 1930 and was long considered the ninth planet. But after discoveries of Pluto-like worlds that were even further away, Pluto was reclassified as a dwarf planet in 2006.
Planets are giants
There are four gas giants located beyond the orbit of Mars: Jupiter, Saturn, Uranus, Neptune. They are located in the outer solar system. They are distinguished by their massiveness and gas composition.
Planets of the solar system, not to scale
Jupiter
The fifth planet from the Sun and the largest planet in our system. Its radius is 69912 km, it is 19 times larger than the Earth and only 10 times smaller than the Sun. The year on Jupiter is not the longest in the solar system, lasting 4333 Earth days (less than 12 years). His own day has a duration of about 10 Earth hours. The exact composition of the planet's surface has not yet been determined, but it is known that krypton, argon and xenon are present on Jupiter in much larger quantities than on the Sun.
There is an opinion that one of the four gas giants is actually a failed star. This theory is also supported by the largest number of satellites, of which Jupiter has many - as many as 67. To imagine their behavior in the planet’s orbit, you need a fairly accurate and clear model of the solar system. The largest of them are Callisto, Ganymede, Io and Europa. Moreover, Ganymede is the largest satellite of the planets in the entire solar system, its radius is 2634 km, which is 8% greater than the size of Mercury, the smallest planet in our system. Io has the distinction of being one of only three moons with an atmosphere.
Saturn
The second largest planet and the sixth in the solar system. In comparison with other planets, it is most similar to the Sun in the composition of chemical elements. The radius of the surface is 57,350 km, the year is 10,759 days (almost 30 Earth years). A day here lasts a little longer than on Jupiter - 10.5 Earth hours. In terms of the number of satellites, it is not much behind its neighbor - 62 versus 67. The largest satellite of Saturn is Titan, just like Io, which is distinguished by the presence of an atmosphere. Slightly smaller in size, but no less famous are Enceladus, Rhea, Dione, Tethys, Iapetus and Mimas. It is these satellites that are the objects for the most frequent observation, and therefore we can say that they are the most studied in comparison with the others.
For a long time, the rings on Saturn were considered a unique phenomenon unique to it. Only recently it was established that all gas giants have rings, but in others they are not so clearly visible. Their origin has not yet been established, although there are several hypotheses about how they appeared. In addition, it was recently discovered that Rhea, one of the satellites of the sixth planet, also has some kind of rings.
How many planets are there in the solar system? Nine is the wrong answer. There are either eight, or ten, or maybe twenty-one. There are even those who will say: a couple of million. We probably won’t answer this question anyway – until the International Astronomical Union finally comes to some kind of solution with the long-overdue definition of “planet.”
Nobody counts anymore Pluto ninth planet. Even the most conservative astronomers have admitted that this is a planet based more on “cultural” than on scientific considerations (in fact, this means that they will not lower her status, so as not to upset the people).
The discoverers of Pluto in 1930 themselves were not entirely sure about this issue - which is why, in fact, they called it a “trans-Neptunian object”, or TNO - a kind of something on the outskirts of the solar system, somewhere out there, beyond Neptune.
Pluto is much smaller than the other eight planets; it is even smaller than their seven moons. And not much larger than its own main moon, Charon (two more, smaller ones, were discovered in 2005). Pluto's orbit is eccentric and lies in a different plane from the rest of the planets in the solar system, plus Pluto has a completely different chemical composition.
The four planets closest to the Sun are of medium size and rocky; the remaining four are gas giants. Pluto is a tiny ball of ice, one of at least 60,000 small comet-like objects that form the Kuiper belt at the very edge of the solar system. 
All of these planetoid objects (including asteroids, TNOs, and a host of other subclassifications) are collectively known as “minor planets.” To date, 330,795 such celestial bodies have been officially registered, and another 5,000 new ones are discovered every month. According to astronomers, there may be about two million such objects with a diameter of more than a kilometer. Most of them are too small to be called planets, but twelve would give Pluto a hundred points ahead.
One of these “minor planets,” discovered in 2005 and charmingly named 2003 UB313, is actually even larger than Pluto. The others, like Sedna, Orcus and Quaor, also left not far from him.
It may well happen that we will end up with two systems: the eight-planet(3) Solar system and the Kuiper Belt system, which includes Pluto and all the other new planets.
By the way, there has already been such a precedent. The largest of the asteroids, Ceres was considered the tenth planet of the solar system from its discovery in 1801 until the 1850s, when it was downgraded to an asteroid.
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In August 2006, at the XXVI Assembly of the International Astronomical Union (IAU), astronomers with all over the world adopted the Prague Planetary Protocol. According to the text of the document, Pluto is finally deprived of its status as a “classical planet” and transferred to “dwarf planets.” Now, according to the definition developed by the commission, a planet is considered to be only a celestial body revolving around the Sun, having sufficient mass so that its own gravity exceeds the cohesion forces of solid bodies and it takes on a shape close to a sphere, and occupying its orbit alone (that is, “neighbors” should not have comparable dimensions). Thus, there are eight planets in the Solar System - four terrestrial planets (Mercury, Venus, Earth and Mars) and four giant planets (Jupiter, Saturn, Uranus and Neptune). Dwarf planets include Pluto, Charon (formerly called a satellite of Pluto), the asteroid Ceres, orbiting between the orbits of Mars and Jupiter, as well as the Kuiper belt objects Eris (object 2003 UB313) and Sedna (object 90377). In addition, the IAU, in a Q&A, called the Pluto-Charon system a “double dwarf planet.”
Theories on how it arose , a great many. The first of these was the famous theory put forward by the German philosopher Immanuel Kant in 1755. He believed that the emergence solar system originated from some primary matter, before which it was freely dispersed in space.
One of the subsequent cosmogonic theories is the theory of “catastrophes”. According to it, our planet Earth was formed after some kind of external intervention, for example, a meeting of the Sun with some other star, this meeting could cause the eruption of a certain part of the solar substance. Due to incandescence, gaseous matter quickly cooled and became denser, while forming many small solid particles, their accumulations were a kind of embryos of planets.
Planets of the solar system
The central body in our system is the Sun. It belongs to the class of yellow dwarf stars. The Sun is the most massive object in our planetary system. The closest star to Earth, as well as the main body in our planetary system. In our system, the planets are more or less ordinary. No, for example, almost no reflection of light. Images of planets are often used in interior signs.
The very first planet from the Sun in our solar system is Mercury - it is also the smallest planet in the terrestrial group (in addition to Earth and Mercury, it includes Mars and Venus).
Next, second in line, comes Venus. Next comes the Earth - the shelter of all humanity. Our planet has a satellite - the Moon, which is almost 80 times lighter than the Earth. The Moon is the only satellite of the Earth orbiting the Earth. After the Sun, it is the brightest object in the sky. The fourth planet is Mars - this desert planet has two satellites. Next comes a large group of planets - the so-called giant planets.
The sun and other planets played a big role in different. There were many religions that worshiped the Sun. And astrology, which studies the effect of planets on humans, still influences many people. Astrology used to be considered a science, but nowadays many people consider it a science.
The largest and most massive of all the giants is Jupiter, which represents our solar system in miniature. Jupiter has more than 40 satellites, the largest of which are Ganymede, Io, Europa, and Callisto. These satellites have another name - Galilean, in honor of the man who discovered them - Galileo Galilei.
Next comes the giant planet Uranus - it is unusual in that it has a “lying on its side” position - which is why there is a rather sharp change in seasons on Uranus. It has 21 satellites and a distinctive feature in the form of rotation in the opposite direction.
The last giant planet is Neptune (Neptune’s largest satellite is Triton). All giant planets have a distinctive feature in the form of many satellites, as well as a system of rings.
But the farthest and last planet in the solar system is Pluto, which is also the smallest planet in our system. Pluto has one satellite, Charon, which is slightly smaller than the planet itself.
How many planets are there in the solar system and what are their characteristics? Over the history of the planet, these data have been clarified, supplemented, and sometimes distorted many times.
During the times of Ancient Greece, it was believed that there were only 7 planets in the solar system. And, by the way, the Earth was not included in the list of these seven planets, since ancient people considered the “green ball” to be the center of the entire universe.
And only in the sixteenth century, the greatest scientist of his time, Nicolaus Copernicus, came to the correct conclusion: the center of the universe is the Sun. But along with the Sun, the satellite, the Moon, was also removed from the list.
And at the end of the eighteenth century, when the telescope appeared, there were two more planets in the solar system: Neptune and Uranus were added.
And Pluto was considered the last discovered planet in the solar system. It was opened in 1930. But if, after counting, you answer nine to the question “how many planets are there in the solar system,” you will be wrong! The fact is that in 2006, Pluto, by the will of the International Economic Union, was deleted from the list of planets of our system!
Scientists have considered that Pluto does not correspond to the parameters of a planet, therefore it is not one!
According to the latest definition of astronomers, a planet is a celestial body with the following parameters:
- Rotates around a star (if it is the Sun, then the system is solar)
- Due to sufficient gravity, it has a spherical shape
- The body is not a star
- Does not orbitally intersect with another large body.
How many planets are there in the solar system today?
Today there are 8 planets in the solar system. Four of them are internal (they belong to the terrestrial planets), four are external. They are also called gas giants. Terrestrial group of planets: Earth, Venus, Mars, Mercury. Outer group of planets: Jupiter, Uranus, Saturn, Neptune. They consist mainly of gases: helium and hydrogen.