Transcription
In the last segment of the previous episode, I discussed a question that appears simple but is profoundly deep upon closer examination: from a quantum perspective, why do scientists always prioritize looking for water on a planet when searching for extraterrestrial life? A portion of this discussion was dedicated to why life absolutely requires water, specifically the substance H2O. You might intuitively feel that scientists are too conservative and lack imagination. However, when we trace the necessary conditions for the formation of complex multicellular life from the microscopic world of interactions between atoms and electrons, you will realize that it is indeed H2O, a seemingly ordinary yet exceptionally special substance, that can support life.
If you haven't listened to the previous episode, I highly recommend you do. After discussing this topic, many friends expressed that they still had lingering questions and wanted me to discuss another topic: all life on Earth is carbon-based. Could silicon-based life exist? Is it possible for us to find silicon-based life in this universe governed by physical laws? My thoughts are as follows: Firstly, blind guessing is meaningless. If you say it might exist, and someone else says it absolutely cannot, no one can convince the other, right? Today, we will try to use the same deductive method, starting from the most fundamental physics level, and follow the logic step by step to see what answer we arrive at. The most important thing is not the final answer of whether silicon-based life exists, but rather that the process of deduction itself is a very enjoyable intellectual exercise.
Our story begins with a news item from a magazine. On September 21, 2023, an article was published in the journal Science. The protagonist of this article is Jupiter's moon, Europa, also commonly known as Jupiter's second moon. If there is a telescope that can give us a glimpse into the mysteries of the universe, then the James Webb Space Telescope is such a marvel. At that time, scientists used it to discover something very interesting: a large amount of carbon dioxide was being ejected from the surface of Europa. This evidence directly increased the possibility of life existing on Europa.
Let's discuss this discovery. Firstly, it was completed independently by two teams: one from Cornell University and the other from NASA. Both used the infrared cameras of the James Webb Space Telescope to capture this data. When two independent teams using different methods arrive at the same result, we can be more confident that the discovery is real. Both teams detected a large amount of carbon dioxide in a region on Europa called Tara Regio.
So, we can't help but ask, why is this discovery so important? Let's talk about carbon dioxide first. We all know that carbon dioxide is a colorless and odorless gas. It affects our entire biosphere in a very subtle way on Earth. Plants absorb carbon dioxide through photosynthesis and release oxygen. Humans and other animals need oxygen to breathe and release carbon dioxide. This forms a perfect cycle of the ecosystem. However, the situation on Europa might be different. Scientists have been speculating that a vast ocean of liquid water might be hidden beneath Europa's ice shell. If this ocean truly exists, it could become a cradle for life. And the carbon dioxide discovered on Europa's surface this time might have overflowed from this potential ocean. It's as if Europa is sending us a signal: "Hey, look here, there might be life here!"
You might ask, why is carbon dioxide a sign of life? This is because on Earth, carbon dioxide is an important component of biological activity. For example, some microorganisms can survive in environments without oxygen. They produce food through a process called chemosynthesis, using carbon dioxide and other chemical substances. If similar microorganisms exist on Europa, they might survive in a similar way, meaning they don't rely on photosynthesis but on chemosynthesis. Therefore, the carbon dioxide discovered on Europa this time is not just a simple chemical element; it could be an important clue to the existence of life, at least adding weight to the possibility. Of course, we need more evidence to confirm it.
Of course, it's not that scientists get excited just because they find carbon dioxide on a distant planet and declare there's life there. Of course not. Carbon dioxide is a very common substance. However, the speculation about life on Europa has been around for a long time. This discovery of carbon dioxide has made the signs of life more apparent. From a geological perspective, it is a relatively young region. Scientists speculate that it might be a large fissure on Europa, a geological hotspot. Since the launch of the Galileo probe in 1995, this region has been a focus of attention for astronomers. In the last decade or so, astronomers have basically confirmed Europa's geological state. Its geological structure can be described as a layered world. The uppermost layer is a thick ice shell, about twenty to thirty kilometers thick. Beneath this ice shell lies a liquid water ocean, up to 100 kilometers deep, filled with various salts. Further down is the rocky seafloor, where hydrothermal vents might exist, providing warmth to this cold world. This ocean is considered an ideal place for life to originate. Liquid water is one of the necessary conditions for life, as we discussed in the previous episode. Furthermore, the internal temperature of Europa might be sufficient to support hydrothermal vent activity, providing necessary energy and nutrients for life.
Originally, with our current observational capabilities, we couldn't see the composition inside. Fortunately, this ice shell is not completely sealed. In the Tara Regio area we mentioned, the ice shell has cracked, forming a gap for material exchange. In 2019, astronomers observed the presence of sodium chloride and magnesium sulfate in Europa's ocean using the Hubble Space Telescope, and this was discovered through this gap. This further confirms the uniqueness of this region. The reason we are confident that the carbon dioxide observed here is highly correlated with signs of life is because the carbon dioxide observed here is dynamic, meaning it was just produced a few minutes ago. Since the highest temperature on Europa's surface is also around minus 160-170 degrees Celsius, even if carbon dioxide gas erupts, it will immediately freeze into dry ice and solidify. If there are significant observational changes within a few minutes, there is only one possibility: this carbon dioxide was not left from the past but was just ejected with the underground seawater.
Of course, this evidence still does not prove that there is definitely life on Europa, as it could be generated by the reaction of carbonates with acids, escaping from the water. However, if there is life in the underground seawater, the process of respiration would produce carbon dioxide, which could also erupt. Therefore, the rigorous statement is that this observation increases the possibility of life existing on Europa. If life truly exists on Europa, it might have very similar characteristics to the ecosystems found near deep-sea hydrothermal vents on Earth. As we mentioned earlier, there, chemosynthetic bacteria that do not rely on sunlight obtain energy and sustain life by utilizing dissolved minerals.
In addition, NASA plans to launch the Europa Clipper probe in October 2024 to conduct more detailed investigations of Europa. The mission's objective is to confirm whether Europa's subsurface ocean is habitable and to search for possible signs of life.
Alright, that's our interpretation of this news. Have you noticed that just by observing the ejection of carbon dioxide from a small area of Europa on Earth, the possibility of life there increases? Why is that? If silicon dioxide were ejected, would the scientific community be so excited? Let's take this opportunity to mention that one of the important methods humans use to search for extraterrestrial life is spectral analysis. This involves detecting atmospheric components of exoplanets, such as carbon dioxide or methane, to indirectly detect signs of life. After all, not all planets are as close to us as Europa, allowing for direct observation. This method can reveal signs of atmospheric gases by analyzing the infrared and visible light radiation from a planet, thereby determining whether an environment suitable for life exists.
The above passage sounds quite normal, but if you are attentive, you might ask: why do we analyze atmospheric components and specifically look for gases like carbon dioxide or methane, which are carbon-centric, as signals of life? This brings us to today's topic: why do we talk about carbon-based life when we talk about life? Is silicon-based life really not viable?
First, let's clarify the definition. What is silicon-based life? All life on Earth is based on carbon as the fundamental element, combined with other elements to form the basic framework of life. Silicon-based life, on the other hand, would have silicon elements replace carbon elements, serving as the foundation with equal status and role to carbon, combining with other elements to form life. The reason for this definition is that there is a popular notion that artificial intelligence is the silicon-based life of the future. However, this is just a popular term. Since current computer chips are mainly made of silicon dioxide, people jokingly call artificial intelligence silicon-based life. However, whether artificial intelligence can be considered life is a separate issue. Currently, apart from its computational abilities, it shows no other signs of life, such as metabolism or reproduction. Therefore, this broad, somewhat internet-slang term "silicon-based life" is outside the scope of our discussion today. What we will discuss below is something that is primarily silicon-based internally, capable of a series of energy and material exchanges, and can reproduce. This is a rough definition. Of course, if some friends insist that artificial intelligence is life, we won't argue. If we talk about life in a broad sense, mountains, lakes, and even stars can be considered life, at least you cannot disprove this statement. However, if we define it this way, we cannot continue the discussion.
So, let's not discuss whether silicon-based life is feasible first, but rather ask: why do people talk about silicon-based life being able to replace carbon-based life? Why don't they talk about iron-based life or gold-based life? This has to do with the properties of various elements. Looking at the periodic table, the number of basic elements that can form life is actually very small. Most are discarded at the first step.
First, let's revisit the process of different atoms combining to form molecules, as discussed in the previous episode. Schrödinger's equation dictates the number of electrons each atom can accommodate. We can imagine the atomic nucleus as the center stage of a circular theater, and electrons as the audience sitting in surrounding seats. The higher the seat, the more energy is required for an electron to reach it. Generally, we call this the electron's energy level. Electrons have a peculiar habit: they only consider occupying higher seats after the lower seats are filled. This is one property.
Next, Schrödinger's equation also tells us how many electrons each shell can hold, like the general rules of a quantum theater. The first shell can hold a maximum of two electrons, the second shell a maximum of eight electrons, and the third shell a maximum of eighteen. Once filled, they move to the next shell. So, 2, 8, 18. These numbers are very important, and we will discuss them later.
Furthermore, atoms have a kind of compulsion: they dislike half-filled states. If an electron shell is completely empty or completely full, the atom feels very satisfied. But if an electron shell is only half-filled, the atom becomes anxious. Some atoms are very lucky and naturally have their electron shells perfectly filled. For example, helium has two electrons, which perfectly pair with the two protons in its nucleus. These two electrons occupy the first shell. Additionally, the neon atom has ten electrons. They first fill the two positions in the first shell, and the remaining eight electrons sit in the second shell. Earlier, we mentioned the rule: the second shell can hold a maximum of exactly eight electrons. Therefore, helium and neon atoms are relatively very stable and don't interact much with other atoms.
However, most other atoms are not so lucky. Their number of electrons cannot perfectly fill the electron shells. Thus, they seek help from their neighbors. If your outer shell is missing two electrons, and my outer shell has exactly two extra, I'll give my electrons to you, and we'll both be happy. However, in doing so, you gain a negative charge by accepting electrons, and I gain a positive charge by losing electrons. Opposite charges attract, and we naturally combine to form a neutral molecule. This is one way. Alternatively, if we both need one electron to fill our outer shells, we can achieve this by sharing an electron pair. This way, we not only satisfy each other's needs but also combine due to sharing electron pairs, forming a neutral molecule. This process of atoms combining to fill electron shells is what we call a chemical reaction. Everything that happens in the Earth's life system is based on this.
Finally, there is one more rule: regardless of how many shells are arranged, the electrons in the outermost shell have a strong desire to arrange themselves in a group of eight. Only when the number of electrons in the outermost shell is eight is it most stable. This is called the octet rule. For example, chlorine has seven electrons in its outermost shell, while sodium has only one. When chlorine and sodium encounter each other, there is a strong desire to react. Chlorine gains one electron, making its outermost shell have eight electrons, while sodium loses one electron, causing its next inner shell to be perfectly filled with eight electrons. Thus, sodium chloride is formed.
Of course, hearing this, you might wonder why. Why is there such a rule? This can be explained, but we can only go this far today. If we delve deeper into the underlying principles, we won't get to life today. If you are interested, you can leave a comment, and I will consider making it an episode.
So, based on this knowledge, in the previous episode, we deduced why life cannot exist without water. Next, we will build upon this knowledge and take another step forward. We need to eliminate elements that cannot form the main body of life. We just mentioned that the process of chemical reactions between different elements is the mutual gifting or sharing of electrons. Here, let me add another piece of knowledge: the relationship between different elements when they combine with other elements is not the same. In chemistry, we call this relationship a bond. Some bonds are called ionic bonds, some are called metallic bonds, and some are called covalent bonds. Let's explain them separately.
First, ionic bonds. Ionic bonds are formed when one or more atoms lose or gain electrons, forming positively charged cations and negatively charged anions. These ions with opposite charges attract each other through electrostatic forces and combine. This type of bond usually occurs between highly reactive metals and highly reactive nonmetals. Metal elements are willing to generously donate their outer electrons, while nonmetal elements are happy to accept these electrons. In this way, the metal loses negatively charged electrons, becoming a positively charged ion, which we call a cation. The nonmetal becomes a negatively charged ion, called an anion. They combine through the mutual attraction of positive and negative charges. This is called an ionic bond.
Next, let's talk about metallic bonds. This type of bonding generally exists between metal atoms, formed by the electrostatic force between free electrons and metal ions arranged in a crystal lattice. Metal atoms tend to lose negatively charged electrons, becoming cations, as we mentioned earlier. These cations are uniformly distributed within the crystal lattice of the metal. Meanwhile, the electrons in the metal can move freely throughout the entire metal. Metallic bonds do not have fixed directionality or saturation. Therefore, macroscopically, metals exhibit high elasticity and ductility. Because the electrons in metals can move freely, metals have strong electrical conductivity. We can use an analogy here: imagine a large party where everyone is constantly exchanging business cards and shaking hands. In metals, electrons are like these business cards; they flow freely between metal atoms rather than being fixed to a specific atom. Metal atoms are like the people at this party, forming a close social network through the sharing of electrons. This is a metallic bond.
Finally, let's talk about covalent bonds. These are chemical bonds formed when two or more atoms share one or more pairs of electrons. This method of sharing electrons allows each atom to achieve a stable electron configuration, thus forming a stable chemical structure. Covalent bonds generally occur between nonmetal elements. Each atom shares its outer electrons. They share electrons but do not completely give them to each other.
So, let's summarize here. Ionic bonds are like friends exchanging gifts: metals donate electrons, and nonmetals accept them. Metallic bonds are like people at a large party exchanging business cards and shaking hands, with electrons flowing freely between metal atoms. Covalent bonds, on the other hand, are like two ice hockey players constantly passing a puck back and forth; their perfect coordination allows atoms to form harmonious relationships by sharing electron pairs.
Once we understand these three types of relationships, we can return to the topic of life. Here, I will give you a direct conclusion: the elements that can form ionic bonds and metallic bonds cannot be selected as the primary constituent materials of life. Why? Because ionic bonds and metallic bonds are less stable compared to covalent bonds. For example, salt, which we mentioned earlier as sodium chloride, is a typical substance with ionic bonds. Chlorine is a nonmetal element, and sodium is a metal element. It is usually a solid, but once dissolved in water, it dissociates back into chloride ions and sodium ions. This point is precisely why life cannot exist without water, as discussed in the previous episode. However, sodium chloride can be a substance and a resource in our bodies, but it cannot be the main structural component of life. If it dissolves in water, it's not suitable.
Even for substances that can maintain a stable solid state, materials formed by metallic bonds and ionic bonds have another characteristic: high crystallinity. What is a crystal? It is a structural characteristic where the internal atoms, ions, or molecules are arranged in a periodic and regular manner in three-dimensional space. Crystals in nature can also grow, but if you look at them magnified, you will find that it is a process of purely mechanical, repetitive increase of matter. The internal structure is like rows of iron beads arranged in a regular pattern, with no changes. And matter without structural change, of course, cannot form complex life.
Therefore, only one type of chemical bond remains that can form life: the covalent bond. Earlier, we said that covalent bonds occur between nonmetal and nonmetal elements. You will find that most elements have a "metal" radical, meaning they are metal elements. Nonmetal elements that can form covalent bonds are actually a minority. They are concentrated in the upper right corner of the periodic table. On the far right is a column called inert elements, which we mentioned earlier, like helium and neon atoms, whose outer shells are already satisfied with electrons and are reluctant to interact with other atoms. Inert elements do not react with other elements, so they cannot be used as materials for life. They are excluded.
When we exclude all metals and inert elements, the remaining nonmetal elements available are very few. The first row is hydrogen. The second row includes boron, carbon, nitrogen, oxygen, and fluorine. The third row includes silicon, phosphorus, sulfur, and chlorine. The fourth row includes arsenic, selenium, and bromine. We can stop here because elements further down are unlikely to form life. We will explain why later. You will find that life on Earth uses elements from the first and second rows, primarily hydrogen, carbon, and oxygen, with carbon being the core element. The relationships they form are covalent bonds.
Here, let's first discuss why carbon can become the fundamental element of life in these two rows. Earlier, we said that the first shell of an atom requires two electrons, and the second shell has a strong tendency to arrange itself with eight electrons. Different elements have different numbers of electrons in their outermost shells, and they choose different ways to combine with their neighbors. Elements with fewer than four electrons in their outermost shell are relatively more likely to lose electrons, while elements with more than four electrons in their outermost shell are relatively more likely to gain electrons. Elements with exactly four electrons in their outermost shell have similar abilities to gain or lose electrons.
Look at carbon, with atomic number 6. Its first shell has two electrons, and its second shell has four electrons. If, in covalent bonds, gaining or losing electrons is like one hand grasping another element, then carbon has four hands, each capable of grasping another atom, and it can even hold hands with other carbon atoms. Therefore, carbon is like a social butterfly, attracting various different atoms to combine with it. For example, carbon forms methane through covalent bonds. We can remove one hydrogen from two methane molecules and link them together to form ethane. Continuing this process can form longer alkane molecules. Replacing one hydrogen with a hydroxyl group turns it into ethanol.
First, let's add another piece of knowledge: two adjacent elements in the periodic table have similar properties in terms of combining with other substances. Why? The principle is actually very simple: they have the same number of electrons in their outermost shells. For example, phosphorus is element number 15. Let's calculate: the first shell has two electrons, the second shell has eight electrons, and the third shell has five electrons. Arsenic is directly below phosphorus; it is element number 33. Let's calculate again: the first shell has 2 electrons, the second shell has 8 electrons (total 10), the third shell has 18 electrons (total 28), and its fourth shell has exactly 5 electrons remaining. Elements directly above and below each other have similar chemical properties. This knowledge point is very important and will be used later.
Phosphorus and arsenic have similar properties when combining with other elements because they both have five electrons in their outermost shells. However, you know that phosphorus is one of the essential elements for life, while arsenic is highly toxic. Arsenic trioxide, also known as white arsenic, is a poison. The knowledge we are discussing now can explain at a microscopic level why it is toxic. When the elements in our cells encounter arsenic, because its outermost electrons are the same as phosphorus, it is very easy for arsenic to replace phosphorus. In other words, cells cannot distinguish between arsenic and phosphorus and use arsenic indiscriminately to build their structures. We mentioned earlier that because arsenic atoms are much larger than phosphorus atoms, with an extra 18 electrons in the third shell, the atomic nucleus of arsenic has much weaker control over its outer electrons than phosphorus. This instability leads to breakage in any place where arsenic is haphazardly used. If this breakage involves important life processes, the cell dies. If there are many such cells, the entire organism dies.
Hearing this, don't you feel a sense of wonder? An element, from a physical perspective, cannot inherently possess the property of being highly toxic. Its toxicity to life might simply be due to one of its physical characteristics. This physical characteristic might disrupt certain life structures. However, if we disregard life and only talk about physics and chemistry, there is no statement that one element is more dangerous than another between phosphorus and arsenic.
So, after supplementing this knowledge, you now understand why we have excluded the fourth row, where arsenic is located, and subsequent rows from the candidates for the main elements of life. After this process of elimination, we are left with only two rows to choose from: the second row, where carbon is located, and the third row, where silicon is located. As we mentioned earlier, the reason we are carbon-based is that carbon has four electrons in its outer shell, giving it four "hands" to grasp other elements and to hold hands with other carbon atoms, forming a wide variety of organic molecules by attracting various different atoms.
Next, we will discuss silicon-based life. Silicon is located directly below carbon in the periodic table. Let's calculate again: silicon has atomic number 14. The first shell has 2 electrons, the second shell has 8 electrons, and the third shell has 4 electrons. This is the same as carbon. This is why people always talk about silicon-based life when exploring extraterrestrial life.
Now, we come to the final question: can silicon replace carbon as the core substance of life? Let's give the answer upfront: no. While silicon can replace carbon in many organic molecules to form similar structures, as we mentioned earlier, when you replace phosphorus with arsenic to form a molecule, the molecule is prone to breakage because the arsenic atom is larger and less stable. Replacing carbon with silicon has a similar problem: the resulting molecules are not stable enough. The direct manifestation of this instability is that silicon compounds are easily decomposed by water; they dissolve upon contact with water. How can life survive then?
Under what circumstances can organic compounds with silicon replacing carbon be relatively stable? The answer is in very low-temperature environments, for example, around minus 160 degrees Celsius. However, in such an environment, water is frozen into ice, and other chemical reactions are difficult to occur in a frozen world, making it difficult to have life as we define it. Secondly, silicon cannot form as rich and diverse compounds as carbon. Carbon can form over 10 million known compounds, while silicon can only form a relatively limited number of compounds. This is because silicon atoms are larger, and their electrons are distributed in outer orbitals, making their ability to combine with other atoms relatively weaker and harder to approach another silicon atom. Therefore, compared to carbon, silicon's chemical diversity is greatly limited. For example, silicon cannot easily form alkene and alkyne structures equivalent to carbon, which are commonly referred to as ethylene and acetylene. Structures like aromatic rings are also not possible. These are fundamental structures in the chemistry of life.
Furthermore, there is a fatal problem: silicon is too sensitive to another element, which is oxygen. As soon as silicon encounters oxygen, it immediately forms silicon dioxide, and once formed, it is very difficult to separate. For example, there is a large amount of silicon on Earth, but when it encounters oxygen, it forms silicon dioxide, commonly known as quartz, which is essentially rock. It's as if silicon was locked by oxygen at the beginning of Earth's formation and could not enter the cycle of life substances, thus being unable to become a part of metabolism. In contrast, carbon is much more flexible. Although carbon also easily combines with oxygen to form carbon dioxide, it is not difficult for carbon to detach from oxygen. For example, photosynthesis in plants can separate oxygen and carbon, allowing them to enter subsequent metabolic cycles.
If silicon-based life also requires metabolism, then let's imagine it would need a silicon-based substance to enter the body and another silicon-based substance to leave the body. A more reasonable assumption would be to inhale monocrystalline silicon and exhale quartz to complete the metabolism. In nature, energy metabolic substances involved in circulation are preferably liquids or gases for easy flow. For carbon-based life, this is not a problem because oxygen and carbon dioxide meet these requirements. However, if you need monocrystalline silicon to become liquid, it requires a high temperature of 1,500 degrees Celsius, and for silicon dioxide to become liquid, it requires temperatures around 1,700 degrees Celsius. At such high temperatures, there are even more problems. Firstly, liquid water cannot exist, and even molecules that can maintain a solid state are few. In such an environment, the only elements that can form stable material forms are some superheavy elements. We can leave a little imagination for this super-high-temperature, super-heavy world. Perhaps such a world truly does not need water as a medium for life. Perhaps carbon elements cannot form solids at all, and instead, heavier elements react normally to form life.
In fact, the reason I leave a little imagination for this super-high-temperature world out of personal preference is that one of my favorite science fiction novels, "Project Hail Mary," describes aliens living in a super-high-temperature world. Our room-temperature world is like a world of minus several hundred degrees Celsius to them, very fatal. And their life is indeed silicon-based. However, this episode is about popular science, not science fiction.
Finally, we must say that even setting aside all the difficulties mentioned earlier, the probability of such a high-temperature world supporting life is extremely low. This is because there is a fundamental law of the universe behind it: the law of element formation. In the previous episode, we told you that only a few very light elements were formed in the early universe, such as hydrogen, helium, and lithium. Heavier elements are born in the nuclear fusion reactors of stars, but they can only synthesize up to iron. We won't elaborate on the reasons; you can listen to it again. To form even heavier elements, one can only rely on supernova explosions and collisions between neutron stars. There is a saying that we are all children of the stars, but it is more accurate to say that we are the legacy of those stars that died tragically.
Here, a problem arises: supernova explosions and neutron star collisions are not free. They are the process of massive stars dying, or their remnants wandering in the vast universe and colliding with extremely low probability. This requires a large number of dead massive stars to increase the probability of this event. However, our universe is currently about 13.8 billion years old, an age that has not yet seen many generations of stars die. According to our current data, the universe is mainly populated by second-generation and third-generation stars. First-generation stars, also known as Population III stars, were the earliest stars formed after the Big Bang. They are mainly composed of hydrogen and helium and do not contain heavy elements. Second-generation stars were formed from nebulae produced by the supernova explosions of first-generation stars, which then re-aggregated. These stars contain a certain amount of metal elements, so their chemical composition is more complex than first-generation stars. For example, our Sun is a typical second-generation star; it contains various elements, including iron, but not heavier elements. Third-generation stars evolved from the further death of second-generation stars. These stars generally have higher metallicity and more complex chemical compositions. However, there are still not enough heavy elements to construct the silicon-based life we imagined under super-high temperatures. It's not enough for these heavy elements to simply appear; they also need to be abundant and concentrated on the same planet. This requirement is even higher, and the probability is even lower.
In summary, to have enough superheavy elements and a relatively high probability of them gathering together, the universe might need to evolve to a stage where stars are predominantly fifth or even sixth generation. At the current stage, our universe is too young and has not yet developed to that extent.
So, by now, you know how harsh the conditions are for forming silicon-based life and how difficult it is to achieve. In contrast, carbon elements and the organic matter systems that can be built around carbon are more readily available in the universe. We have no reason to skip them and use silicon, which has more stringent conditions, right? Perhaps, I say perhaps, in a very strange corner of the universe, for various reasons, there was not much carbon element left, or it lacked carbon element partners. And for various reasons, there was a large amount of silicon present. And for various reasons, dozens of supernova explosions and hundreds of neutron star collisions occurred, ultimately forming the super-high-temperature world full of heavy elements with a very low probability of occurrence. Perhaps the silicon-based life described in the novel "Project Hail Mary" can exist. But if that is the case, we don't need to detect life on such a planet at all. The various strange reasons that led to the existence of this planet would be enough for us to detect anomalies in this region from afar, attracting our attention long ago. However, unfortunately, we have not yet discovered such anomalous regions.
If, after hearing this, you still say, "No, your definition of life above is too narrow. Life doesn't necessarily have to eat. It doesn't necessarily have to metabolize. Life doesn't necessarily have to have a stable solid state. Liquid life, gaseous life, or even plasma life, why can't they exist?" You are right. If it only exists in hypothetical worlds, and we are just chatting in the comment section of a small popular science column, then we can let our imaginations run wild and end all discussions with "everything is possible." In fact, if you insist that everything is possible, I can only say that you are right. But let's come to the real world. Now, imagine you are a scientist, and you hope to find life beyond Earth. The budget you apply for cannot allow you to observe all the stars in the universe, and your lifespan does not permit it. Your entire career can only be spent observing and analyzing a few hundred galaxies in the night sky. You cannot fly there; you can only observe their spectra from Earth and analyze their composition. If you are fortunate enough to find a planet with life, you will be recorded in history. However, the universe is too vast, and there are too many galaxies. There is a greater probability that you will spend your entire life without discovering any planet with life, simply because you were unlucky and chose the wrong few hundred galaxies to observe from the beginning. In such a situation, would you choose life forms with a higher probability? That is, life with a stable solid form, insoluble in water, but requiring liquids and gases for metabolism? If so, then carbon would be your 100% first choice. Or would you be willing to gamble your entire life's effort on an even smaller probability, on a planet with silicon-based life that has much harsher formation conditions? Or even more absurdly, gamble on some pure energy life form that we cannot understand or observe at all? This is precisely why we, as ordinary people, rarely hear about scientific miracles. Because scientists are also human, they need careers and want a relatively higher probability of success. They are unwilling to gamble their lives on believing in a miracle. That's all.
Alright, that's the story of silicon-based life we've discussed today. I hope you enjoyed it. We'll talk again next time. The region we just mentioned, Tara Regio, can form single, double, or even triple bonds with other carbon atoms, allowing it to form long chains, rings, or even three-dimensional network structures. Thus, carbon has the ability to form millions of different organic compounds, providing the basis for the diversity and complexity of life. Now you know why all life on Earth is carbon-based. Let's continue. Among the choices in these rows, we must also exclude the fourth row and below. Why? The reason is that the further down the elements, the larger the atoms, and the more electrons they have. Correspondingly, the atomic nucleus's constraint on the outermost electrons becomes weaker. Macro-scopically, this means the resulting substances are less stable. For the basic composition of life, instability means fatality, meaning extreme toxicity. Here's an example: