Another world in the depths of the universe: other "earth" and "sun"

 We were explorers a long time ago, even before humans had words. The permanent remains of our ancient ancestors, such as discarded stone tools, extinguished bonfires, and scattered fossil bones, are scattered all over the earth. Starting from central Africa, to Australia, to the Andes, and finally all over the earth, their footprints show that we humans have never been content to stay in the same place.

2021 Hermes - This innate desire to explore has been implanted in our human genes, and it appears from the moment we are born. But only when our line of sight flies over the horizon, our imagination is truly released, like a wild horse. Our ancestors created all kinds of wonderful myths when looking up at the sky, gazing at the sun and the moon. We imagine the sun as a flame chariot driven by the sun god Helios, the moon is the daughter of the Titans, and her sister is the dawn goddess Eos. Throughout the entire history of mankind, from ancient Egypt to Aztec to ancient Celtic, every civilization has imaginative myths about the sun and the moon, depicting the surprises we feel when we look up to them. And solemnity.

However, what completely liberates our imagination is the little stars in the night sky. Some people think that our world is surrounded by a firmament, a huge and complex dome that revolves around us. However, no matter how hard we try to imagine, it is difficult for us to appreciate how intricate, unruly, and vast that real sky is. When Galileo used the newly invented curved polished glass to build the first telescope in 1610, he suddenly saw a scene that the weak human eye would never see. He was surprised to find that the moon was not as smooth and bright as he had imagined, but in fact it was "bumps and bumps, rough, all over low valleys and mountains." He was also surprised to see the small moons of Jupiter and the wonderful and mysterious halo around Saturn. Galileo is like a toddler, taking his first step in understanding the universe. With each new discovery, his imagination is further sublimated, and his works challenge the fundamental beliefs of many people. Galileo opened a window for our solar system through which we can begin to understand galaxies outside the solar system.

　　 planetary system

　　The pace of exploration has not stopped, but until a few decades ago, the solar system was the only planetary system known to mankind. Today, we know that there are thousands of such systems, although they are still only a tiny part of the universe. Just like the solar system, the stars of other galaxies and the planets orbiting them all come from molecular clouds.

　　 There are hundreds of billions of exoplanets in the Milky Way. According to calculations, about 10 billion should have orbital characteristics similar to those of the earth, revolving around stars like the sun. Therefore, some of them may have bred some form of life. But considering that the interstellar space is so vast, it is difficult for humans to contact or communicate with them. Even if there is a civilization inhabiting the planet closest to us, it will take us decades to transmit information to each other.

The stars of the Milky Way and the planets orbiting them all originate from the same molecular cloud. Shock waves caused by supernova explosions may have triggered their formation of celestial bodies. When the shock wave arrives, the molecular cloud gradually becomes flat, and the rotation speed is increased to maintain the angular momentum, and the molecular cloud is generated.

The molecular cloud is the seed of the protoplanetary disk. The molecular cloud revolves around an axis, and the dust particles revolve around the center of mass of the molecular cloud. When such a large number of particles are in motion, collisions are inevitable. The collision slows down the molecular cloud, and at the same time their angle with respect to the orbital surface is getting smaller and smaller, so the particles gather together to form a disc.

The accretion disk revolves around a young star and will form a planet. The protoplanetary disk of β Pictoris, about 60 light-years away from the earth, its planetary system is in a period similar to the nascent stage of the solar system. There are signs that a huge planet is orbiting it, while rocky planets and comets Being formed in the disc.

The Trappist-1 system discovered in 2017 is a planetary system about 40 light years away from the earth. Trabister-1 is a red dwarf star with at least 7 planets orbiting it within a distance smaller than the distance between Mercury and the sun. The star is very small, with 3 planets in the habitable zone of the system, and their size is similar to that of the Earth.

another world

Although humans did not discover the first exoplanet until the end of the 20th century, advances in detection technology have allowed us to find thousands of celestial bodies orbiting stars.

No one knows exactly what life on other planets looks like, but one thing is certain: it must obey the laws of chemistry.

When dissolved in a liquid, those molecules that may be transformed into life are more likely to react with each other. Since water can dissolve so many compounds, it is an ideal condition for the formation of life. For this reason, astronomers believe that we may find life on the so-called habitable zone of stars or on a planet whose surface temperature allows liquid water (0 to 100 degrees Celsius).

The habitable zone refers to the range of distance beyond the star that allows liquid water to exist on the surface of the planet. In 1957, "Father of Space Medicine" Hubertus Stragerhold first proposed the concept of "Livable Range". It was not until 1993 that James Casting gave a more detailed model to describe the "stellar habitable zone" (or "circular stellar habitable zone"). This concept has developed in recent years. In 2000, several scientists put forward the concept of "galaxy habitable zone". In 2013, some scientists also proposed the "Ring Planet Habitable Zone".

　　 A suitable temperature range is essential for life, neither too hot nor too cold. Because although some substances can become liquids at higher and lower temperatures, many organic molecules lose their stability in hotter environments. In addition, in very cold conditions, chemical reactions tend to slow down, which also makes life difficult to form. The following icon shows a large number of exoplanets divided according to their positions in the habitable zone of their respective stars. The so-called "super-Earth" refers to an exoplanet that is much larger than the Earth but much smaller than Neptune.

The best area where life exists

Whether a planet can produce life depends not only on its position in the star system, but also on its position in the galaxy.

Recent studies have shown that the Milky Way has 200 to 400 billion stars. Although not all stars have orbiting planets around it, even the most conservative estimates indicate that there are at least 100 million such planets in our galaxy. However, not all planets are potentially habitable. Whether they are habitable depends on whether they are located in the habitable zone of the star and the position of the planetary system in the galaxy. Scientists believe that planetary systems close to the nucleus or globular clusters of the Milky Way are almost impossible to form life, because they are subject to intense radiation bombardment, and their close proximity to potential supernovae will obliterate all life possibilities.

The picture below shows the state of the Milky Way from its birth to the present, and the distance between the celestial bodies and the core of the galaxy. In the early stage of galaxy formation and very close to the center, too little metal content and too many supernovae, which hinder the formation of planets. On the other hand, there will not be enough heavy metals to form rocky planets too far from the center of the galaxy. Therefore, the optimal habitable zone cannot exist in the early days of the Milky Way, nor can it be too close to its center.

Near the galactic center, life is unlikely to be formed due to its extremely high energy, and it is unlikely to find a habitable planet outside of this ring, because there are too few elements heavier than helium. This picture outlines the main structure of the Milky Way and the theoretical ring-shaped habitable zone. The ring-shaped habitable zone extends from 15,000 light-years from the Galactic Center to 35,000 light-years away. Our sun is about 27,000 light-years from the center of galaxy. The solar system is in a favorable position, close to the center of this area.

Indirect detection method

　　 It is very difficult to observe planets outside the solar system directly, so we have to rely on indirect detection methods.

　　 The discovery of planets outside the solar system, that is, exoplanets, is an extremely complex task. Their own faint light is almost impossible to pass such a long distance to be seen by us; their dimensions are too small to be captured by a telescope; and the light of the stars is too strong, obscuring all traces of planets orbiting them . Nevertheless, some of the larger young exoplanets are brighter, which makes them easier to find.

　　Exoplanets are small spots of light (in the form of visible or infrared light) surrounding stars. Therefore, direct observation is very difficult, and it is usually impossible for scientists to directly observe their reflected light or infrared radiation. We need to apply the indirect detection method as shown in the figure below.

  1. Ling Xing Method

　　 We observe the brightness of the star to see if it will dim, because this may be a signal that a planet orbiting it obscures a part of it.

  2. Line-of-sight velocity

　　 When the light source approaches us, the light it emits becomes bluer; conversely, when the light source moves away from us, the light it emits becomes redder. If a star has planets orbiting it, we can learn the movement of the star around the center of mass of the system through the change of light.

　　3. Microgravity lens

　　 When a star passes in front of another star (from the perspective of the Earth), the gravity of the closer star will bend the light from the farther star and increase its brightness. If the star has a planet, there will be a strong change in brightness.

　　4. Astrometry

　　 When the star is far enough away from the center of mass of the entire planetary system, we can detect the existence of the planet by accurately measuring the minute changes in the position of the star.

　　 It is worth mentioning that the outline of an exoplanet cannot be directly observed, but if the light is strong enough, we can understand the size and atmosphere of the planet based on the light emitted by the star that passes through the planet’s atmosphere.

Stars with exoplanets

　　Exploring exoplanets is a difficult task, so false positives occasionally occur in this field. Nevertheless, in the past few decades, we have discovered thousands of planetary systems.

　　 After an exoplanet is discovered, it will have to wait for further observations to confirm it before it can officially become part of the known universe. In recent decades, humans have discovered thousands of planets outside the solar system, but the signals detected by the instruments are often turned out to be false positives. For example, the temporary decrease in brightness of a star may be due to a simple change on its surface or a solar eclipse in a binary star system. The observation of Gliese 667C (Gliese 667C) is a typical case. In the past, people thought that this star had 7 planets orbiting it, but later observations showed that 5 of them were actually interfering signals recorded during the measurement.

　　 Even so, we are still discovering some nearby stars with exoplanets, such as 55 Cancri (55 Cancri) and 47 Ursae Majoris (47 Ursae Majoris) visible to the naked eye.

　　 Generally speaking, exoplanets are classified into 5 types based on the comparison of masses with known celestial bodies in the solar system.

　　 In addition to mass, size and orbit, it is difficult for us to obtain more information about exoplanets, but this information can still tell us a lot about their characteristics. Based on the mass of an exoplanet, astronomers can speculate whether it is a rocky planet as small as the Earth or a gas giant planet as large as Jupiter.

    Classified by quality

Although the boundaries of each type are not clear, exoplanets can be roughly divided into five categories based on their masses: "Jupiter-like planets"-the most massive exoplanets, similar in size to Jupiter, or even larger; "Neptune-like planets"-exoplanets similar in size to Neptune; "super-Earths"-exoplanets larger than the earth and smaller than Neptune; "earth-like"-exoplanets with similar mass to the earth; "sub-Earth"- -Exoplanets with a mass less than Earth.

    Quality and composition

The nature of 　　 exoplanets may vary greatly depending on their composition. Astronomers must be very careful when classifying a planet into one type or another, especially those with masses between super-Earths and Neptune-like planets.

Potentially habitable planet

　　 To assess whether an exoplanet has the necessary conditions for the existence of life, we need to have accurate data about its composition, orbit, especially the atmosphere.

　　 Among the thousands of exoplanets we have discovered, some are similar to Earth and may be habitable. Given that we don't know much about their composition and atmosphere, we usually assess their habitability based on whether they are rocky planets and whether the distance from the central star allows liquid water to exist on their surface. However, these two factors cannot guarantee that these exoplanets can provide a favorable environment for life. For example, Venus is located in the habitable zone, but its composition is not conducive to the formation of life, while a planet located in a fairly cold area may reach a higher surface temperature due to the greenhouse effect of the atmosphere. Scientists believe that a considerable number of exoplanets are habitable, but once we measure their atmospheres, the number of habitable exoplanets will decrease.

　　 Among the planetary systems where life conditions may exist, TRAPPIST-1 is particularly promising and very representative. There are 7 rocky planets in this system, 3 of which are located in the habitable zone of their stars. Not long ago, it was thought that the Gliese 667C (Gliese 667C) system had 7 planets in its habitable zone, but further exploration reduced this number to 2. A planet in the Kepler-62 system is in its habitable zone.

Looking for extraterrestrial life

　　Only when organisms or fossils are discovered on another planet, the urgent scientific question of whether there is extraterrestrial life will be finally answered. However, before we can access other planetary systems, scientists can only use indirect methods to find life elsewhere in the universe. Life often leaves traces in its surroundings, so we can start looking for traces of life in the planet’s atmosphere.

　　 If we can find some chemical imbalances in the atmosphere of a certain planet that indicate the existence of organisms, maybe there are traces of life here. We can infer the composition of its atmosphere by analyzing the electromagnetic spectrum of an exoplanet. Oxygen and methane, as well as compounds such as methyl chloride and dimethyl sulfide, are important clues to the possibility of life. The figure below compares the emission spectra of Earth and Mars and Venus. The atmosphere of the former contains both oxygen and water molecules.

But it is worth noting that chemical imbalance cannot be used as conclusive evidence. An inert planet will reach a chemical imbalance in its atmosphere sooner or later, and very active elements like oxygen will combine with other materials until they are exhausted. In other words, when there is a large amount of oxygen on an exoplanet, it may also be the product of a simple chemical reaction, not because of life.

　　 The earth is an excellent sample at our fingertips. The organisms on earth show how life adapts to various environments, even very extreme environments. The microorganisms in the Grand Prismatic Hot Springs of Yellowstone National Park can live in environments with temperatures as high as 70 degrees Celsius. Some bacteria can survive in high-dose radiation or vacuum environments. The following table lists the extreme conditions under which organisms on earth can survive, which may represent the limit of life on earth.

Looking for a new earth

　　 In 1989, humans discovered HD 114762b. It was the first exoplanet discovered, but it was not confirmed until 3 years later. On October 6, 1995, Professor Michel Mayor of the University of Geneva, Switzerland, and Didier Queloz, a professor of the University of Geneva and Cambridge, discovered an exoplanet. This much larger planet orbits the small star 51 Pegasus 50 light-years away. This exoplanet orbits 51 Pegasus (a sun-like star in Pegasus) and is named 51 Pegasus b. Scientists believe that its mass is about half that of Jupiter. The two scientists therefore won the Nobel Prize in Physics in 2019. With the discovery of this extrasolar giant planet, which opened a window for mankind to the world outside the solar system, the search for exoplanets began to heat up.

The mass of the exoplanet 51b Pegasus is at least half that of Jupiter, and its volume may be similar to or even larger than that of the giant planets in the solar system.

　　 Since the discovery of the first exoplanet, scientists have been looking for planets where life might exist.

　　 In the study and observation of exoplanets, the "blue giant" HD189733b is the part that cannot be circumvented. HD189733b is one of the most studied giant planets and one of the largest exoplanets currently known. Its mass is slightly larger than Jupiter and it is closer to its central star. It was first observed in 2005 when it was passing in front of its central star. HD189733b is not an ordinary gas giant planet, because its mass is 13% larger than Jupiter, and its central star is only 4.5 million kilometers away. The operating speed of HD189733b is 152 kilometers per second, so the orbital period is only 2.2 days. HD189733b and some other Jupiter planets are quite close to their central star (by comparison, the Earth is 150 million kilometers away from the sun), which makes scientists reexamine the theory of planet formation. Before HD189733b, the scientific consensus was that gas giant planets were formed far away from stars, where the low temperature caused the compression of a large amount of gas surrounding the core of the rock.

　　 There are only two theories that can explain HD189733b and other gas giant planets orbiting their central star at close range. These gas giant planets are either formed very close to the central star, which is contrary to the original theory of planet formation; or formed at a greater distance, but migrates towards the central star over time. Currently, the second theory is more accepted by people.

　　 Thereafter, in 2007, the Spitzer Space Telescope detected water vapor in the atmosphere of HD189733b, and the Hubble Space Telescope confirmed this again a year later. When an exoplanet passes in front of its central star, that is, a "transit", the outer atmosphere of the planet will filter the light of the central star, thereby affecting the star's spectrum. The composition of the planet's atmosphere can be known by analyzing the spectral data. Astronomers have noticed that if viewed through an infrared filter, HD189733b will absorb light to different degrees in each band. This phenomenon can only be explained by the existence of a certain kind of molecule: water molecules.

　　 But water is not the main reason for the intense blue of this giant planet. By measuring the light of HD189733b passing behind its central star, scientists determined that HD189733b is the first exoplanet with life color discovered by humans. They inferred that its atmosphere contains tiny silicate particles, and the howling wind scatters them everywhere. The cobalt blue color of this planet comes from the glass body falling like raindrops and the misty clouds that scatter blue light.

　　 Scientists also tried to outline the atmospheric temperature distribution of HD189733b. Given the size of HD189733b and the fact that it is only 63.4 light-years away from us, astronomers can learn more about the atmosphere of HD189733b compared to other extracorporeal objects. They determined the planet’s atmospheric temperature profile through 33 consecutive hours of observations and drew the first atmospheric map of an exoplanet. The figure shows the temperature change of the atmosphere, and the lighter color corresponds to the higher temperature area.