On this most "flat" planet, summer lasts 42 years

Reviewer: Zhao Liangyu

Associate Professor, Flight Control Department, School of Aeronautics and Astronautics, Beijing Institute of Technology

The vast universe always attracts human curiosity and desire to explore. Is there life on that distant planet? How are celestial bodies formed and evolved? This time, let us go into the solar system and take a look at the strange sights on the planets. In addition to our familiar home Earth, how much do you know about other planets?

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Classification of planets

There are countless stardust scattered in the universe, including meteors, comets and planets, so which planets can be called planets? According to the standards of the international astronomical community, a planet must meet three conditions: it must be large enough to rely on its own gravity to make its appearance nearly spherical; it must orbit a star; and there must be no larger celestial body in or near its orbit. Based on this definition, eight planets have been discovered in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The eight planets in the solar system, displayed in proportion

Source: Wikipedia

Due to the different distances between the planets and the sun, the ways in which they are formed are different, so planets can be divided into two categories. One category is solid planets , whose cores are made up of solid dust particles bonded together, mainly containing silicon, iron, magnesium and other elements, and the cores have undergone a melting and condensation process. Mercury, Venus, Earth and Mars belong to this category, also known as terrestrial planets, which are small in size and high in density, with slow rotation speed and few satellites.

The other type is the gas planet , whose core is formed by a mixture of elemental particles and gases. Jupiter, Saturn, Uranus and Neptune are all gas planets, also known as Jovian planets. This type of planet is large in size and low in density, with a fast rotation speed and a large number of satellites.

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Planets with Different Personalities - Mercury - Planet without Water

Mercury has many "mosts": the closest to the sun, the smallest diameter and volume, the lightest mass, and the fastest orbital speed. Although it is the smallest planet in the solar system, its density ranks second, second only to the earth. Mercury's appearance is very similar to the moon, and its surface is covered with craters caused by comets or meteors. Mercury and Venus are the only two planets in the solar system without satellites.

Source: Wikipedia

Although the name has "water" in it, there is no water on Mercury. Since it is very light and has a weak gravitational pull, it cannot firmly absorb the atmosphere around it, so it does not contain water vapor and does not have the prerequisite for the formation of water. Moreover, it is too close to the sun, so the temperature of the side facing the sun reaches about 430℃. Under the high temperature, even if there is water on the surface, it will be evaporated. Due to the thin atmosphere, Mercury heats up quickly during the day and cools down quickly at night, so the temperature difference between day and night is about 600℃, which is the most dramatic temperature change in the solar system.

Venus - the brightest star in the night sky

If you see a very bright star next to the moon, then you may have seen the astronomical beauty of "Venus accompanying the moon". This star is Venus. Venus is the brightest star that we can see with our naked eyes except the sun and the moon. It was called "Taibai Star" in ancient times.

Source: Wikipedia

Venus is Earth's "good brother": it is the closest to Earth, with a radius only about 300 kilometers smaller than Earth, a mass 0.8 times that of Earth, and a density 5% lower than Earth. The two also have similar internal structures and atmospheric compositions. Perhaps because of these surface similarities, Venus became the first solar system planet to be explored by humans.

However, the probe revealed a very different side of Venus from Earth. It rotates from east to west, which is an "outlier" among the planets. Its rotation period is about 243 Earth days, so you can only see the sunrise once every 200 days. The revolution period of Venus is 225 Earth days, which means that a day on Venus is longer than its year!

Since Venus's atmosphere, which is thousands of meters thick, is mainly composed of carbon dioxide (97%), the greenhouse effect on Venus is particularly obvious. The average surface temperature is about 480°C, making it the hottest planet in the solar system. Some scientists believe that Venus once had a large amount of water, but due to high temperature evaporation, the water disappeared. There are also many large volcanoes on Venus, so that most of the surface is covered by lava flows.

Computer simulation of the origin of volcanoes on Venus | Wikipedia

The probe also observed the phenomenon of "magnetic field reconnection". When the solar wind (supersonic plasma charged particle flow ejected from the upper atmosphere of the sun) entered the magnetosphere of Venus, the electromagnetic field changed, and the magnetic field experienced a process of instantaneous disconnection and reconnection. Due to the loss of the protection of the magnetic field, the water-containing atmosphere of Venus escaped further.

Schematic diagram of planetary magnetic fields resisting solar wind Source: Internet

Mars – The Red Planet

Mars is the second planet closest to the Earth. The rotation periods and axial inclinations of the two planets are very similar. Therefore, humans have high hopes for Mars, hoping to find evidence of life on it.

Mars rover source: NASA

As its name suggests, Mars seems to have a blazing fire, making it a red fireball. How is the red glow formed? After analyzing the materials brought back by the Mars rover, scientists found that the rocks on the surface of Mars are easily decomposed by weathering, exposing the iron contained in them to the air, and then oxidized into reddish-brown iron oxide. Under the influence of wind, the sand and dust on Mars fly everywhere, and even develop into dust storms covering the entire planet. When the wind weakens, these sand and dust containing iron oxide settle on the surface of Mars, and appear red under the sunlight, so Mars looks red.

Source: Wikipedia

Mars has a complex and varied terrain, such as canyons and peaks. The largest peak is named Olympus Mons, which is three times higher than Mount Everest. Scientists have been actively looking for hidden water sources on Mars. Some people believe that there were oceans and lakes on Mars in the past, but the water disappeared and Mars became dry and barren today.

Ancient lake discovered on Mars (left) Source: NASA

Jupiter - Big Boss among Planets

If you want to describe Jupiter in one word, it is big! It has the largest diameter , about 11 times that of the Earth; the largest volume , equivalent to more than 1,300 Earths; and the largest mass , 2.5 times the sum of the masses of other planets in the solar system. Jupiter also has the most powerful planetary magnetosphere in the solar system, which will be affected within a range of 1.4 million to 7 million kilometers from Jupiter. Therefore, Jupiter is called the "king of planets" in the solar system.

Size comparison of the Sun, Jupiter, and Earth

Source: NASA

Jupiter's large mass gives it a strong gravitational pull, attracting many satellites. Humans have discovered 79 satellites so far. Galileo discovered Jupiter's four satellites as early as 1610, namely Io, Europa, Ganymede and Callisto. Even these four satellites are "big guys", and the largest Ganymede is larger than Mercury.

From left to right: Io to Callisto

Source: NASA

Jupiter's appearance is also eye-catching, with circles of colorful stripes around its surface , like an oil painting. What are these stripes? The answer is clouds. Jupiter is a gaseous planet with the largest planetary atmosphere in the solar system. The clouds in different latitudes interact with each other to form circulation, so vortices, storms and lightning are produced. People speculate that the wind direction in Jupiter's atmosphere is parallel to Jupiter's equator, forming 12 belt-shaped clouds parallel to the equator. The clouds contain a lot of ammonia, and due to differences in temperature and altitude, they appear red, brown, yellow, white and other colors.

Source: Wikipedia

Jupiter also has a notable feature - the Great Red Spot . Located in the southern hemisphere of Jupiter, the Great Red Spot is a mass of violently rising air currents, mainly composed of ammonia and methane, rotating in a counterclockwise direction. It is about 40,000 kilometers long from east to west and about 13,000 kilometers wide from north to south, much larger than the Earth.

Saturn - Beautiful Rings

Rings are a common feature of gas planets. Saturn's rings are the largest and most spectacular . The diameter of the rings is 300 times that of Saturn, the width is about 11.9 million kilometers, and the inner edge is about 5.95 million kilometers away from the surface of Saturn.

Saturn looks like it is wearing a straw hat, so it is also called the "Straw Hat Planet"

Source: Wikipedia

How are the rings formed? Astronomers have found that the rings are mainly composed of comets, asteroids, broken ice, a small amount of rock, dust and particles. The broken ice is likely to come from the "ice shell satellite". When it collided with Saturn, under the action of gravity, the outer ice formed a ring and moved around Saturn. The dust that makes up the rings has a very low temperature (about -193℃), but it emits thermal radiation, which is why the rings were discovered.

The material that makes up the halo

Source: ineplanets.org

In the solar system, Saturn has the largest number of satellites, 82, and can be called the "king of satellites" . The first satellite discovered was Titan, which is the largest satellite of Saturn and the second largest satellite in the solar system after Ganymede. Titan is almost 50% larger than the moon and 1.8 times more massive than the moon. There is an ocean under Titan's crust, and in this ocean, there may be an environment suitable for the survival of microorganisms, so humans will conduct detailed research on Titan in the future.

Titan (lower left) compared to the Moon and Earth

Source: NASA

Uranus and Neptune – the blue sisters

Due to the extremely low surface temperature, Uranus and Neptune are "ice giants", they are blue, which is related to the composition of the atmosphere. In addition to hydrogen and helium, their atmospheres also contain methane and ammonia. Methane can absorb part of the red spectrum in sunlight, thus reflecting blue-green.

Uranus

Source: NASA

Neptune

Source: Wikipedia

Although Uranus is not the farthest from the sun, it is the coldest, with an average surface temperature of only -220℃. Like Venus, Uranus rotates from east to west. Uranus is the most "lying flat" planet in the solar system . Since its obliquity (the angle between the ecliptic plane and the equatorial plane) is about 98°, it almost orbits lying down. Therefore, the sun's rays can almost directly hit the pole. When the sun hits the South Pole directly, the southern hemisphere is in summer and it is always daytime. The revolution period is 84 years, so there will be a change of seasons every 42 years.

Uranus's rotation axis is roughly parallel to the ecliptic plane

Source: ineplanets.org

Neptune is the farthest planet from the Sun and the only planet discovered through mathematical prediction. Neptune has the longest orbital period, about 165 years. The weather on Neptune is very active, often forming storms, and has the fastest winds in the solar system.

Well, today’s “star-chasing journey” ends here, but our exploration of space will not stop, and we look forward to everyone discovering more secrets of the stars.