The solar system is a group of planets revolving in certain orbits around a bright star - the Sun. This luminary is the main source of heat and light in the solar system.
It is believed that our system of planets was formed as a result of the explosion of one or more stars and this happened about 4.5 billion years ago. At first, the solar system was a collection of gas and dust particles, however, over time and under the influence of its own mass, the Sun and other planets arose.
Planets of the solar system
In the center of the solar system is the Sun, around which eight planets move in their orbits: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Until 2006, Pluto also belongs to this group of planets, it was considered the 9th planet from the Sun, however, due to its considerable distance from the Sun and its small size, it was excluded from this list and called a dwarf planet. Rather, it is one of several dwarf planets in the Kuiper belt.
All the above planets are usually divided into two large groups: the terrestrial group and the gas giants.
The terrestrial group includes such planets as: Mercury, Venus, Earth, Mars. They are distinguished by their small size and rocky surface, and in addition, they are located closer than the others to the Sun.
The gas giants include: Jupiter, Saturn, Uranus, Neptune. They are characterized by large sizes and the presence of rings, which are ice dust and rocky pieces. These planets are made up mostly of gas.
The sun
The sun is the star around which all the planets and moons in the solar system revolve. It is made up of hydrogen and helium. The Sun is 4.5 billion years old, only in the middle of its life cycle, gradually increasing in size. Now the diameter of the Sun is 1,391,400 km. In the same number of years, this star will expand and reach the orbit of the Earth.
The sun is the source of heat and light for our planet. Its activity increases or becomes weaker every 11 years.
Due to the extremely high temperatures on its surface, a detailed study of the Sun is extremely difficult, but attempts to launch a special apparatus as close as possible to the star continue.
Terrestrial group of planets
Mercury
This planet is one of the smallest in the solar system, its diameter is 4,879 km. In addition, it is closest to the Sun. This neighborhood predetermined a significant temperature difference. The average temperature on Mercury during the day is +350 degrees Celsius, and at night it is -170 degrees.
If we focus on the earth's year, then Mercury makes a complete revolution around the sun in 88 days, and one day there lasts 59 earth days. It was noticed that this planet can periodically change the speed of its rotation around the Sun, its distance from it and its position.
There is no atmosphere on Mercury, in connection with this, asteroids often attack it and leave behind a lot of craters on its surface. Sodium, helium, argon, hydrogen, oxygen were discovered on this planet.
A detailed study of Mercury presents great difficulties due to its close proximity to the Sun. Mercury can sometimes be seen from Earth with the naked eye.
According to one theory, it is believed that Mercury was previously a satellite of Venus, however, this assumption has not yet been proven. Mercury has no satellite.
Venus
This planet is the second from the Sun. In size, it is close to the diameter of the Earth, the diameter is 12,104 km. In all other respects, Venus is significantly different from our planet. A day here lasts 243 Earth days, and a year - 255 days. The atmosphere of Venus is 95% carbon dioxide, which creates a greenhouse effect on its surface. This leads to the fact that the average temperature on the planet is 475 degrees Celsius. The atmosphere also includes 5% nitrogen and 0.1% oxygen.
Unlike the Earth, most of whose surface is covered with water, there is no liquid on Venus, and almost the entire surface is occupied by solidified basaltic lava. According to one theory, there used to be oceans on this planet, however, as a result of internal heating, they evaporated, and the vapors were carried away by the solar wind into outer space. Near the surface of Venus, weak winds blow, however, at an altitude of 50 km, their speed increases significantly and amounts to 300 meters per second.
There are many craters and hills on Venus, reminiscent of terrestrial continents. The formation of craters is associated with the fact that earlier the planet had a less dense atmosphere.
A distinctive feature of Venus is that, unlike the other planets, its movement does not occur from west to east, but from east to west. It can be seen from Earth even without the help of a telescope after sunset or before sunrise. This is due to the ability of its atmosphere to reflect light well.
Venus has no satellite.
Earth
Our planet is located at a distance of 150 million km from the Sun, and this allows us to create on its surface a temperature suitable for the existence of water in liquid form, and, therefore, for the emergence of life.
Its surface is 70% covered with water, and it is the only one of the planets that has such an amount of liquid. It is believed that many thousands of years ago, the steam contained in the atmosphere created the temperature on the Earth's surface necessary for the formation of water in liquid form, and solar radiation contributed to photosynthesis and the birth of life on the planet.
A feature of our planet is that under the earth's crust there are huge tectonic plates that, moving, collide with each other and lead to a change in the landscape.
The diameter of the Earth is 12,742 km. An Earth day lasts 23 hours 56 minutes 4 seconds, and a year - 365 days 6 hours 9 minutes 10 seconds. Its atmosphere is 77% nitrogen, 21% oxygen and a small percentage of other gases. None of the atmospheres of other planets in the solar system has such an amount of oxygen.
According to scientists, the age of the Earth is 4.5 billion years, about the same time its only satellite, the Moon, exists. It is always turned to our planet with only one side. There are many craters, mountains and plains on the surface of the Moon. It reflects sunlight very weakly, so it can be seen from Earth in a pale moonshine.
Mars
This planet is the fourth in a row from the Sun and is 1.5 times more distant from it than the Earth. The diameter of Mars is smaller than Earth's and is 6,779 km. The average air temperature on the planet ranges from -155 degrees to +20 degrees at the equator. The magnetic field on Mars is much weaker than that of the Earth, and the atmosphere is quite rarefied, which allows solar radiation to freely affect the surface. In this regard, if there is life on Mars, it is not on the surface.
When surveyed with the help of rovers, it was found that there are many mountains on Mars, as well as dried riverbeds and glaciers. The surface of the planet is covered with red sand. Iron oxide gives Mars its color.
One of the most frequent events on the planet are dust storms, which are voluminous and destructive. Geological activity on Mars could not be detected, however, it is reliably known that significant geological events took place on the planet earlier.
The atmosphere of Mars is 96% carbon dioxide, 2.7% nitrogen and 1.6% argon. Oxygen and water vapor are present in minimal quantities.
A day on Mars is similar in duration to that on Earth and is 24 hours 37 minutes 23 seconds. A year on the planet lasts twice as long as the earth - 687 days.
The planet has two moons Phobos and Deimos. They are small and uneven in shape, reminiscent of asteroids.
Sometimes Mars is also visible from Earth with the naked eye.
gas giants
Jupiter
This planet is the largest in the solar system and has a diameter of 139,822 km, which is 19 times larger than the earth. A day on Jupiter lasts 10 hours, and a year is approximately 12 Earth years. Jupiter is mainly composed of xenon, argon, and krypton. If it were 60 times larger, it could become a star due to a spontaneous thermonuclear reaction.
The average temperature on the planet is -150 degrees Celsius. The atmosphere is made up of hydrogen and helium. There is no oxygen or water on its surface. There is an assumption that there is ice in the atmosphere of Jupiter.
Jupiter has a huge number of satellites - 67. The largest of them are Io, Ganymede, Callisto and Europa. Ganymede is one of the largest moons in the solar system. Its diameter is 2634 km, which is approximately the size of Mercury. In addition, a thick layer of ice is visible on its surface, under which there may be water. Callisto is considered the oldest of the satellites, since it is its surface that has the largest number of craters.
Saturn
This planet is the second largest in the solar system. Its diameter is 116,464 km. It is most similar in composition to the Sun. A year on this planet lasts quite a long time, almost 30 Earth years, and a day is 10.5 hours. The average surface temperature is -180 degrees.
Its atmosphere consists mainly of hydrogen and a small amount of helium. Thunderstorms and auroras often occur in its upper layers.
Saturn is unique in that it has 65 moons and several rings. The rings are made up of small ice particles and rock formations. Ice dust perfectly reflects light, so the rings of Saturn are very clearly visible in a telescope. However, he is not the only planet to have a diadem, it is just less noticeable on other planets.
Uranus
Uranus is the third largest planet in the solar system and the seventh from the sun. It has a diameter of 50,724 km. It is also called the "ice planet", as the temperature on its surface is -224 degrees. A day on Uranus lasts 17 hours, and a year is 84 Earth years. At the same time, summer lasts as long as winter - 42 years. Such a natural phenomenon is due to the fact that the axis of that planet is located at an angle of 90 degrees to the orbit and it turns out that Uranus, as it were, "lies on its side."
Uranus has 27 moons. The most famous of them are: Oberon, Titania, Ariel, Miranda, Umbriel.
Neptune
Neptune is the eighth planet from the Sun. In its composition and size, it is similar to its neighbor Uranus. The diameter of this planet is 49,244 km. A day on Neptune lasts 16 hours, and a year is equal to 164 Earth years. Neptune belongs to the ice giants and for a long time it was believed that no weather events occur on its icy surface. However, it has recently been found that Neptune has raging eddies and wind speeds the highest of the planets in the solar system. It reaches 700 km / h.
Neptune has 14 moons, the most famous of which is Triton. It is known that it has its own atmosphere.
Neptune also has rings. This planet has 6.
Interesting facts about the planets of the solar system
Compared to Jupiter, Mercury appears to be a dot in the sky. These are actually the proportions in the solar system:
Venus is often called the Morning and Evening Star, since it is the first of the stars visible in the sky at sunset and the last to disappear from visibility at dawn.
An interesting fact about Mars is the fact that methane was found on it. Due to the rarefied atmosphere, it is constantly evaporating, which means that the planet has a constant source of this gas. Such a source can be living organisms inside the planet.
Jupiter has no seasons. The biggest mystery is the so-called "Great Red Spot". Its origin on the surface of the planet is still not fully understood. Scientists suggest that it was formed by a huge hurricane, which has been rotating at a very high speed for several centuries.
An interesting fact is that Uranus, like many planets in the solar system, has its own system of rings. Due to the fact that the particles that make up them reflect light poorly, the rings could not be detected immediately after the discovery of the planet.
Neptune has a rich blue color, so it was named after the ancient Roman god - the master of the seas. Due to its remote location, this planet was one of the last to be discovered. At the same time, its location was calculated mathematically, and over time it could be seen, and it was in the calculated place.
Light from the Sun reaches the surface of our planet in 8 minutes.
The solar system, despite its long and thorough study, is still fraught with many mysteries and mysteries that have yet to be revealed. One of the most fascinating hypotheses is the assumption of the presence of life on other planets, the search for which is actively continuing.
Even at the beginning of the 21st century, the answer to this question sounded extremely simple - nine. Today, not everyone can answer how many planets there are: since 2006, Pluto has ceased to meet the standards of a planet in the solar system.
Recently, any body of the Cosmos that rotates around a star, reflects its light and is larger than an asteroid could be considered a planet. At this time, it is customary to distinguish the following groups in the solar system: the inner - terrestrial planets, the outer - gas giants.
How many planets are in the solar system
Earth group
Mercury is 18 times smaller than Earth. The helium atmosphere is rarefied. The temperature fluctuates from -180 to +440оС.
Venus is a "hot planet" (up to +460), 0.8136 Earth masses. The atmosphere is made up of carbon dioxide, nitrogen and oxygen. Air pressure exceeds the earth's thirty-five times.
Mars - the mass of the planet is 11% of the earth's. Temperature per day, on average, minus 60oC. It has two satellites in orbit: Deimos and Phobos.
gas giants
Jupiter is the largest planet. The mass exceeds the earth's by 318 times, and all the planets of the system - by 2.5 times. It contains helium and hydrogen. It is surrounded by 63 satellites, among which one - Ganymede - is larger than Mercury.
Saturn is famous for its rings of dust and ice. 95 times heavier than Earth. Has sixty-two satellites. The wind speed on the surface can reach 1800 km/h.
Uranus is the coldest planet (-224oC). Has 27 satellites. It is 14.5 times heavier than the Earth, and 62.2 times larger in volume.
Neptune is the most distant planet. It has 13 satellites and the fastest winds - 2200 km / h. It is 17.2 times heavier from the Earth.
Plutoids
Pluto is much smaller than not only eight of these planets, but also seven (out of 170) of their satellites (including the Moon). Yes, and the chemical composition of Pluto did not come out, and the plane of the orbit did not please.
It is almost impossible to work out how many planets there are in our system. Next to the main planets, scientists have identified many so-called "minor planets" - asteroids. In 2003, the number of numbered asteroids was more than 50,000, and the total number was twice that. Moreover, large and small planets have a rather conditional border. Since 1992, beyond the orbit of Neptune, many icy bodies began to be discovered, the size of which is not inferior to Pluto. This cluster was called the Kuiper belt. Of the more than 1,000 objects known today (the total number may exceed seventy thousand), several are comparable to Pluto. They are called dwarf planets: Makemake, Eris, Haumea. Pluto itself, together with the satellite Charon, is recognized as a double dwarf. Together they make up a group of "plutoids". Now try to analyze for yourself the number of celestial bodies around the Sun. How many planets out of the total number of space objects could be called "planets" without the risk of being misunderstood by other audiences?
exoplanets
Since 1992, scientists began to discover planets of other star systems - exoplanets. More than 800 such planets are already known, the distance to which is measured in tens of light years. To answer the question of how many planets in the universe, humanity will not be able to soon.
Not so long ago, any educated person, when asked how many planets are in the solar system, would answer without hesitation - nine. And he would be right. If you do not particularly follow the events in the world of astronomy and are not a regular viewer of the Discovery Channel, then today you will answer the same question to the question posed. However, this time you will be wrong.
And here's the thing. In 2006, namely, on August 26, 2.5 thousand participants in the congress of the International Astronomical Union made a sensational decision and actually crossed out Pluto from the list of planets in the solar system, since 76 years after the discovery it ceased to meet the requirements set by scientists for the planets.
Let's first understand what a planet is, and also how many planets in the solar system astronomers have left us, and consider each of them separately.
A bit of history
Previously, a planet was considered to be any body that revolves around a star, glows with light reflected from it, and has a size larger than that of asteroids.
Even in ancient Greece, seven luminous bodies were mentioned that move across the sky against the background of fixed stars. These cosmic bodies were: Sun, Mercury, Venus, Moon, Mars, Jupiter and Saturn. 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 not the Earth, but the Sun should be in the center of the planetary system. Therefore, the Sun and the 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 next definition of a planet.
Planet- this 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 be spherical or close to it;
the body should not have other large bodies near its orbit;
The body does not have to be a star.
In its turn star- This 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- This is a planetary system that consists of a central star - the Sun - and all natural space objects revolving around it.
So, today the solar system consists of of the 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. The composition of gas giants consists mainly of hydrogen and helium.
The sizes of the planets in the solar system vary both within groups and between groups. So, the gas giants are much larger and more massive than the terrestrial planets.
Closest to the Sun is Mercury, then as far as the distance: 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 
The sun 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 over 5,000 degrees Kelvin, and the core temperature is over 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 complete revolution around it in about 230-250 million years! For comparison, the Earth makes a complete revolution around the Sun in 1 year.
Mercury
Mercury is the smallest planet in the system and is closest to the Sun. Mercury has no satellites.
The surface of the planet is covered with craters that arose about 3.5 billion years ago as a result of massive bombardments by meteorites. The diameter of the craters can range from a few meters to more than 1000 km.
The atmosphere of Mercury is highly rarefied, consists mainly of helium and is blown by the solar wind. Since the planet is located very close to the Sun and does not have an atmosphere that would keep warm at night, the temperature on the surface ranges from -180 to +440 degrees Celsius.
By earthly standards, Mercury makes a complete revolution around the Sun in 88 days. On the other hand, a Mercury day is equal to 176 Earth days.
Venus
Venus is the second closest planet to the Sun in the solar system. Venus is only slightly smaller than Earth, which is why it is sometimes referred to as "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 the earth.
Carbon dioxide and, as a result, the greenhouse effect, a dense atmosphere, as well as proximity to the Sun, allow Venus to carry 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.
Earth 
Earth is the only known planet in the universe today that has life on it. 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 atmosphere of the Earth is fundamentally different from the atmospheres of other planets due to the presence of life. Most of the atmosphere is nitrogen, but it also contains oxygen, argon, carbon dioxide and water vapor. The ozone layer and the Earth's magnetic field, in turn, weaken the life-threatening effects of solar and cosmic radiation.
Due to the carbon dioxide contained in the atmosphere, the greenhouse effect also takes place on Earth. It does not appear as strongly as on Venus, but without it, the air temperature would be approximately 40 ° C lower. Without the 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 oceans, the remaining 29% are continents and islands.
Mars
Mars is the seventh largest planet in the solar system. The "Red Planet", as it is also called due to the presence of a large amount of iron oxide in the soil. Mars has two moons: Deimos and Phobos.
The atmosphere of Mars is highly rarefied, 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 drops in some places reach 40 degrees during the day.
Distinctive features of the surface of Mars are impact craters and volcanoes, valleys and deserts, ice polar caps like those on Earth. The highest mountain in the solar system is located on Mars: the extinct volcano Olympus, whose height is 27 km! As well as the largest canyon: the Valley of the Mariner, the depth of which reaches 11 km, and the length is 4500 km.
Jupiter
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 radiates a huge amount of heat, equal to 4 * 1017 watts. However, in order to become a star like the Sun, Jupiter must be another 70-80 times heavier.
Jupiter has as many as 63 satellites, of which it makes sense to list only the largest ones - Callisto, Ganymede, Io and Europa. Ganymede is the largest moon in the solar system, larger than even Mercury.
Due to certain processes in the inner atmosphere of Jupiter, many vortex structures appear in its outer atmosphere, for example, stripes of clouds of brown-red hues, as well as the Great Red Spot, a giant storm known since the 17th century.
Saturn
Saturn is the second largest planet in the solar system. The hallmark of Saturn is, of course, its ring system, which consists mainly of ice 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 outer atmosphere of the planet looks calm and homogeneous, which is explained by a very dense layer of fog. However, the wind speed in some places can reach 1800 km/h.
Uranus
Uranus is the first planet to be discovered with a telescope, and also the only planet in the solar system that wraps around the sun, "lying on its side."
Uranus has 27 moons named after Shakespearean heroes. The largest of them are Oberon, Titania and Umbriel.
The composition of the planet differs from the gas giants in the presence of a large number of high-temperature modifications of ice. Therefore, along with Neptune, scientists have identified Uranus in the category of "ice giants". And if Venus has the title of the “hottest planet” in the solar system, then Uranus is the coldest planet with a minimum temperature of about -224 ° C.
Neptune
Neptune is the most distant planet from the center of the solar system. The history of its discovery is interesting: before observing the planet through a telescope, scientists calculated its position in the sky using mathematical calculations. This happened after the discovery of inexplicable changes in the movement of Uranus in its own orbit.
To date, 13 satellites of Neptune are known to science. The largest of them - Triton - is the only satellite that moves in the opposite direction 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.
The composition of 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 radiates 2.5 times more energy than it receives from the Sun.
The planet's blue color comes from traces of methane in the outer atmosphere.
Conclusion
Pluto, unfortunately, did not have time to get into our parade of planets in the solar system. But it is absolutely not worth worrying about this, because all the planets remain in their places, despite changes in scientific views and concepts.
So, we answered the question of how many planets are there in the solar system. There are only 8 .
The solar system is a system of planets, which includes its center - the Sun, as well as other objects of the Cosmos. They revolve around the sun. More recently, 9 objects of the Cosmos that revolve around the Sun were called “planet”. Now scientists have established that beyond the boundaries of the solar system there are planets that revolve around stars.
In 2006, the Union of Astronomers declared that the planets of the solar system are spherical cosmic objects revolving around the sun. On the scale of the solar system, the Earth appears to be extremely small. In addition to the Earth, eight planets revolve around the Sun in their individual orbits. All of them are larger than the Earth. They rotate in the plane of the ecliptic.
Planets in the solar system: types
Location of the terrestrial group in relation to the Sun
The first planet is Mercury, followed by Venus; next comes our Earth and finally Mars.
The terrestrial planets do not have many satellites or moons. Of these four planets, only Earth and Mars have moons.
The planets that belong to the terrestrial group are highly dense, composed of metal or stone. Basically, they are small and rotate around their own axis. Their rotation speed is also low.
gas giants
These are the four space objects that are at the greatest distance from the Sun: Jupiter is at number 5, followed by Saturn, then Uranus and Neptune.
Jupiter and Saturn are impressive planets, made up of compounds of hydrogen and helium. The density of gas planets is low. They rotate at high speed, have satellites and are surrounded by asteroid rings.
“Ice giants”, which include Uranus and Neptune, are smaller, their atmospheres contain methane, carbon monoxide.
Gas giants have a strong gravitational field, so they can attract many space objects, unlike the terrestrial group.
According to scientists, asteroid rings are the remains of moons altered by the gravitational field of the planets. 
dwarf planet
Dwarfs are space objects, the size of which does not reach the planet, but exceeds the dimensions of the asteroid. There are many such objects in the solar system. They are concentrated in the Kuiper belt region. The satellites of the gas giants are dwarf planets that have left their orbit. 
Planets of the solar system: the process of emergence
According to the hypothesis of cosmic nebulae, stars are born in clouds of dust and gas, in nebulae.
Due to the force of attraction, substances are combined. Under the influence of the concentrated force of gravity, the center of the nebula is compressed and stars are formed. Dust and gases are transformed into rings. The rings rotate under the influence of gravity, and planetasimals form in whirlpools, which increase and attract cosmetic objects to themselves.
Under the influence of the force of gravity, the planetazimals are compressed and acquire a spherical shape. Spheres can combine and gradually turn into protoplanets. 
There are eight planets within the solar system. They revolve around the sun. Their location is:
The nearest “neighbor” of the Sun is Mercury, followed by Venus, then the Earth, then Mars and Jupiter, further from the Sun are Saturn, Uranus and the last one, Neptune.
On a clear night, when light interference is not a serious factor, the sky looks breathtaking: a huge number of stars open up to view. But, of course, we can see only a small fraction of the stars that actually exist in our Galaxy. Even more amazingly, most of them have their own system of planets. The question arises, how many exoplanets are there? In our Galaxy alone, billions of extraterrestrial worlds must exist!
So let's assume that the eight planets that exist within the solar system represent the average. The next step is to multiply this number by the number of stars that exist within the Milky Way. The actual number of stars in our Galaxy is the subject of some dispute. Essentially, astronomers are forced to make rough estimates because we cannot view the Milky Way from the outside. And given that it is in the form of a barred spiral, the galactic disk is the most difficult to study due to the interference of light from its many stars. As a result, the estimate is based on calculations of the mass of our Galaxy, as well as the mass fraction of stars in it. Based on these data, scientists have calculated that the Milky Way contains between 100 and 400 billion stars.
Thus, the Milky Way galaxy could have between 800 billion and 3.2 trillion planets. However, in order to determine how many of them are habitable, we must take into account the number of exoplanets studied so far.
As of October 13, 2016, astronomers have confirmed the presence of 3,397 exoplanets out of 4,696 potential candidates that were discovered between 2009 and 2015. Some of these planets have been observed directly during direct imaging. However, the vast majority have been discovered indirectly through transit and radial velocity methods.
The histogram shows the dynamics of exoplanet discovery over the years. Credit: NASA Ames/W. Stenzel, Princeton/T. Morton
During the initial 4-year mission, the Kepler space telescope observed about 150,000 stars, which were mostly M-class, also known as red dwarfs. When Kepler entered a new phase of the K2 mission in November 2013, it shifted its focus to studying K- and G-class stars, which are nearly as bright and hot as the Sun.
According to a recent study by the Ames Research Center (NASA), Kepler found that about 24% of M-class stars could have potentially habitable planets comparable in size to Earth (those that are no more than 1 6 times the radius of the Earth). Based on the number of M-class stars in our Galaxy, there could be about 10 billion potentially habitable, Earth-like worlds.
In addition, an analysis of the K2 results suggests that about one quarter of large stars may also have Earth-like planets orbiting within habitable zones. Thus, it can be estimated that there are literally tens of billions of planets potentially suitable for the development of life in the Milky Way alone.
In the coming years, the James Webb and TESS space telescope missions will be able to detect smaller planets orbiting dim stars, and possibly even determine if any of them have life. Once these new missions go live, we will have better estimates of the size and number of planets that exist in our galaxy. Until then, their estimated number is encouraging: the chances of extraterrestrial intelligence are very high!
