As far as is known, Earth is the only planet with life. In a universe full of dangers and barren lands, it is a miracle that such a beautiful and vibrant oasis of all things can flourish.
In fact, it is extremely difficult for a planet to produce life. If nothing else, the planet itself alone must meet the following three conditions:
First, it must have a suitable quality, not too small or too large. If the mass is too small, it cannot rely on its own gravity to prevent the escape of the surrounding gases, thus becoming a rocky planet without an atmosphere around; if the mass is too large (such as more than 10 times the mass of the Earth), it will attract too much gas and become a gas planet. In both cases, life cannot survive.
Second, it must be in a proper position, not too close or too far from the star. If you get too close, the surface temperature of the planet will be very high, so that all the water will become water vapor; if it is too far away, the surface temperature of the planet will be very low, so that all the water will become ice. In other words, to ensure the existence of liquid water, the planet must be located in a narrow circular area, which is called the habitable zone. In fact, if the Earth were 5% farther away from the Sun, or 15% closer, it would fall out of this habitable zone.
Third, it must have a proper level of internal activity, not too calm or too intense. If the internal activity is too calm, there will be no geological activity, nor can it form an atmosphere and magnetic field; if the internal activity is too intense, earthquakes and volcanoes will continue to erupt, turning the planet into a living hell.
Before I go on to talk about why the earth is an oasis of life, I would like to talk about a more theoretical and cutting-edge topic.
In fact, no single scientific theory can determine the initial state of a planet. In other words, a planet's mass, position, and internal activity should be completely random. If so, why does the earth perfectly meet these three conditions?
The first person to answer this question was the Australian-American physicist Brandon Carter. If you've seen Hawking's A Brief History of Time before, you should have an impression of the name. In fact, Carter was a disciple of the British physicist Hawking, and together with Hawking came up with the famous black hole hairless theorem.
In 1973, Carter attended an academic symposium to commemorate Copernicus' 500th birthday. At this seminar, he proposed a far-reaching theory, that is, the famous anthropic principle. The anthropic principle says that if a natural phenomenon cannot produce any observer, then the phenomenon cannot be observed.
This means that it is meaningless to ask why phenomena that cannot be observed do not exist. With the anthropic principle, the problem mentioned earlier is immediately solved. If the earth does not meet these three conditions, there will be no life on the earth, and there will be no intelligent life like us to ask this question. In other words, we can ask the question, which in itself is the answer to the question. (Dude, is this a matryoshka doll?) Manual dog head)
It is precisely because of the right mass, location and internal activity that the Earth has long been able to possess the three elements that make it an oasis of life: the ocean, the atmosphere and the magnetic field.
Let's start with the oceans of the earth. As far as we know, Earth is a unique planet with liquid water on its surface. Scientists estimate that the total amount of water the Earth has is about 1.376 billion cubic kilometers, of which the total amount of seawater is about 1.335 billion cubic kilometers, accounting for about 97% of the earth's total water volume. In general, seawater absorbs longer wavelengths of red, orange, and yellow light, while reflecting or scattering shorter wavelengths of green and blue light can make the sea look blue. Since the oceans cover about 75% of the Earth's total surface area, they are often referred to as blue planets.
As we all know, water is the source of life. There are many creatures that can live without oxygen and sunlight; but no one creature has been found that can survive without water. In fact, the earliest life on Earth was born in the sea. The reason is simple. For organic macromolecules such as proteins, water is the best solvent. It is in the environment of liquid water that these organic macromolecules can mix with each other, and then form complex structures through various chemical reactions, and eventually evolve into real life. In addition, water is also the basis for many metabolic activities of life. For example, without water, photosynthesis and respiration could no longer be carried out. In this sense, liquid water is an indispensable stage for life activities. Therefore, the earth can have an ocean of liquid water, which is really a blessing for life on earth.
But just owning the ocean is not enough. Because at any temperature, water can evaporate. Let me give you an example. If you put a basin of water on the ground for a long time, you will find that the basin of water becomes less and less, and finally even disappears completely. This is because liquid water spontaneously becomes gaseous water vapor. In the same way, the water in the ocean spontaneously becomes water vapor. If nothing else stops it, this water vapor escapes into space; over time, All of Earth's oceans will dry up.
Fortunately, the Earth has a mechanism to prevent water vapor from escaping, and that is the Earth's atmosphere.
Composition of the atmosphere
As we all know, the Earth has an atmosphere, that is, a layer of mixed gases that are attracted by the Earth's gravity and orbit the Earth. Scientists estimate that the total mass of this gas mixture is about 5.15 × 1018 kg, which contains about 78% nitrogen and 21% oxygen by mass fraction, in addition to a small amount of other gases including argon and carbon dioxide.
In fact, it is this atmosphere that prevents the escape of water vapor. Due to the low temperature of the atmosphere, after entering the atmosphere, water vapor will freeze into small water droplets in the face of cold, and then return to the surface in the form of rainfall. If the Earth did not have an atmosphere, this water cycle would be broken; over time, the water on the Earth's surface would evaporate.
In fact, the Earth's atmosphere has many other important functions. For example, the Earth's atmosphere also contains some greenhouse gases, including carbon dioxide, water vapor, methane and ozone. The biggest feature of greenhouse gases is that they have little effect on electromagnetic waves with shorter wavelengths, and have a strong absorption capacity for electromagnetic waves with longer wavelengths.
The temperature on the surface of the sun is very high, and the electromagnetic wave energy emitted by the radiation is high in wavelength and short, so it will not be hindered by greenhouse gases and can successfully reach the surface of the earth. The temperature of the Earth's surface is low, and the electromagnetic wave energy emitted by the radiation is low and long, so a large part will be intercepted by greenhouse gases. Therefore, the presence of greenhouse gases can increase the temperature of the planet's surface, which is called the greenhouse effect. Scientists estimate that without the greenhouse effect, the average temperature on the Earth's surface would probably drop by 32°C.
In addition, there is an ozone layer in the Earth's atmosphere, which is roughly distributed in a circular area 20 to 30 kilometers away from the Earth's surface. The ozone layer absorbs 97% of the ultraviolet rays in sunlight. Without this ozone layer, these high-energy ultraviolet rays would have hit the Earth's surface directly, posing a serious threat to life on Earth.
Not all celestial bodies can have an atmosphere. For example, the moon we are familiar with does not. So what kind of celestial bodies can have their own atmosphere? It depends on its mass and its distance from the sun.
On the one hand, the atmosphere is attracted by the gravitational pull of this celestial body and thus bound to the surroundings of this celestial body. The strength of the gravitational bondage depends on the mass of the object: the greater the mass, the stronger the gravitational bondage.
On the other hand, the atmosphere in turn gets heat from the sun, creating an incentive to escape. The magnitude of the escape power depends on the distance between the celestial body and the Sun: the closer it is, the stronger the atmospheric escape power. These two forces have been fighting each other. If gravitational bondage takes precedence, the planet can have its own atmosphere; if the power to escape takes precedence, the planet becomes bare. Our Earth's ability to have an atmosphere is the result of the peak of the Earth's gravitational pull. However, just gravity is at its peak, and it's not enough for a celestial body to maintain an atmosphere all the time. The Earth has always maintained an atmosphere because it also has a strong magnetic field.
The Earth's magnetic field is actually a huge magnetic field that extends from the Earth's interior to space. The compass, one of the four great inventions of ancient China, can distinguish the direction because of the existence of the earth's magnetic field. However, the magnetic axis of the Earth's magnetic field does not coincide with the Earth's axis of rotation, and there is an angle of 11 degrees between the two.
In addition, the north and south poles of the Earth's magnetic field are actually the opposite of the geographical north and south poles.
The Earth's magnetic field
The main role of the Earth's magnetic field is to resist the solar wind. The solar wind is a stream of high-energy charged particles emitted by the Sun, consisting mainly of hydrogen and helium (that is, hydrogen nuclei and helium nuclei that remove outer electrons) in the ionized state. Because these streams of charged particles are so energetic, Earth's atmosphere cannot prevent the solar wind from driving straight in. So, if there's nothing else to stop it, the solar wind will blow all the way to the surface, posing a serious threat to life on Earth.
Fortunately, the Earth has a strong magnetic field. It is well known that in a magnetic field, the orbit of motion of charged particles is deflected. As a result, the Earth's magnetic field forces high-energy charged particles in the solar wind to deflect in the direction of the Earth's poles. At the poles, these charged particles collide with atmospheric molecules located in the upper layers, forming beautiful auroras.
aurora
What's more, the Earth's magnetic field prevents the solar wind from stripping away the Earth's atmosphere. If solar wind energy bombards the Earth's atmosphere directly, these high-energy charged particles will transfer their energy to earth's atmospheric molecules, allowing them to break free of Earth's gravity and escape into space. Over time, the Earth's atmosphere will be gradually stripped away by the solar wind. Fortunately, the Earth's magnetic field forms a natural barrier that effectively prevents the solar wind from coming into direct contact with the Earth's atmosphere. In this way, the Earth's gravity can firmly bind the atmospheric molecules, so that the Earth has its own atmosphere.
Let's summarize. As far as is known, Earth is the only planet with life. With the right mass, location, and level of internal activity, the Earth has been able to have oceans, atmospheres, and magnetic fields for a long time. The ocean provides the stage for the birth of life, the atmosphere prevents the evaporation of the ocean, and the magnetic field ensures that the atmosphere is not stripped away by the solar wind. It is because of these three elements that the earth has become a beautiful oasis of life.
So the earth with life is a miracle, and the birth of intelligent human beings on the earth is a miracle within a miracle, and each unique you in human beings is of course a unique miracle in the universe!
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
If you need to reprint, please contact the original public account
Original title: Earth Day | There are so many planets in the universe, why is only the earth suitable for human survival?
Source: Origin Reading
Edit: Garrett