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Knowing one's place in the universe is the core desire of human beings, and people have been thinking about this problem for thousands of years. In fact, hundreds of years before the birth of Christ, Greek philosophers such as anaximander and Epicurus speculated that the universe was full of planets, many of which might support the existence of life. However, for a long time, we couldn't go beyond pure speculation-until we invented the telescope and had a correct understanding of the scientific method. Humans achieved these two milestones centuries ago, and now scientists are doing their best to find extraterrestrial life. Well, it's a little exaggerated. But we have made considerable progress, especially in the past decade or so, and some important discoveries may come soon. Let's take a brief look at the direction and method of finding aliens.

In the early years, unfortunately, the "cosmic pluralism" advocated by anaximander, Epicurus and their followers did not succeed. Overturned by Plato, Aristotle and other heavyweights, the earth is unique. The latter view is very consistent with the Christian doctrine that ruled Europe throughout the Middle Ages and beyond. Therefore, for a long time, it was considered strange and usually completely dangerous to assume that there was a life-supporting world outside us. With the dawn of the Copernican revolution, it began to swing in another way in the16th century. In 2009, Steven Dick, then the chief historian of NASA, told the writer Michael Hilber: "Once you realize that all planets revolve around the sun, it is not hard to imagine that other planets will be like the Earth."

This momentum was established with the knowledge giants such as johannes kepler, galileo galilei and isaac newton constantly enriching the operation mode of our solar system, and these efforts reached an almost fanatical level during the Enlightenment. For example, in the17th century, Czech astronomer Anton Schyrleus considered what life might be like on Jupiter. /kloc-in the 0/8th century, William Herschel discovered the existence of Uranus and infrared light, and speculated that the solar system of the whole life was extensive-including the surface of the sun. (Herschel thinks the sun is a huge planet. Almost at the same time, scientists began to think about how to communicate with our supposed neighbors in the universe. C.F.Gauss, a famous German mathematician, is one of the pioneers in the emerging METI communication field. In the early 1800' s, Gauss proposed carving huge geometric figures in the Siberian forest to show the "Moon Man" who lives on the moon that we are here and know how to do some mathematical operations.

At the same time, the Austrian astronomer Joseph Johann von Litro suggested to dig a huge trench in the Sahara desert, which was filled with water and covered with kerosene. Then kerosene will be ignited, generating a hot signal, hoping to attract the attention of any alien who pays close attention to the earth. These special ideas have never been put into practice. But a century later, scientists did start to put some conversations about alien hunting into action.

The first real-life search project took place in August 1924, when a group of scientists led by astronomer David Peck Todd used an airship to put a radio receiver a few miles above the ground-people thought it was a good place to listen to biological information on Mars, which was very close to the earth at that time.

But it was not until 1960 that the search for extraterrestrial civilization really began. That year, frank derek, an astronomer at Cornell University, used a radio telescope in West Virginia to monitor the signals of Cepheus and Bojiang. This effort, called "Ozma Project", combines the ideas in the groundbreaking paper published by Giuseppe Cocconi and Philip Morrison 1959. Since then, scientists have been looking for it. At first, they focused almost exclusively on radio signals, but now they are also using flash. These are the goals of the increasingly popular "optical SETI" efforts.

SETI scientists must keep an open mind; After all, we don't know what kind of information advanced alien civilization will send. Therefore, astronomers in this field usually look for strange and artificial signals, which come from deep space and are not produced by any known natural astrophysical phenomena.

If the signal appears again, it is also good, and it can be studied in detail again and again. One-off can always remain mysterious and frustrating, just like the famous "Wow!" On 1977. Signal display. In that case, the radio antenna operated by Ohio State University received such interesting things that astronomer Jerry Ayman wrote, "Wow!" On the data printout. Researchers searched the same sky again and again, hoping to get another one, but they never got it.

It should be pointed out that SETI hunting has always been a small-scale operation in history. Finding enough money to keep the telescope running has always been a problem. Congress cancelled the planned SETI project of NASA in 1993. Since then, most alien hunters have had to turn to the private sector for cash. Without stable funds, progress has been slow for several years. But recently, private funds have flowed into SETI more freely. Most of them came from one person: yuri milner, a technology billionaire. Milner is keen on finding extraterrestrial life. In 20 15, he made an ambitious plan called "Breakthrough Plan" to find aliens.

Breakthrough's projects include the Breakthrough Listen SETI activity at a cost of 1 billion dollars and the Breakthrough Starshot at a cost of 1 billion dollars, aiming at developing the technology needed to send micro-robot probes to nearby exoplanet systems at a speed of about 20%. There is also breakthrough news, which aims to help mankind make the best news to send to the universe and encourage debate and dialogue about SETI. There are also many arguments about SETI in the scientific community. Some people, including the late physicist Stephen Hawking, believe that it is unwise to publicize our existence to aliens, and the nature and intentions of aliens are completely a mystery to us. After all, these creatures may plunder our planet after receiving our address. But other researchers believe that any creature advanced enough to enslave or eat us on earth will know that we are here anyway. Breakthrough promised that no SETI signal would actually be broadcast until the end of this debate. However, humans have also sent many messages, the most famous of which is the Arecibo message of 1974. And those are just intentional and directional letters; We are leaking radio signals in all directions at any time, providing cosmic information to anyone close to them.

Looking for life on Mars

Around the same time that SETI left the ground, planetary scientists began to observe the alien world carefully for the first time. 1964, mariner 4 flew over mars and sent back the first close-up photo of the red planet. These photos reveal a dry, potholed and seemingly desolate world, forcing many scientists to readjust their previous optimistic views on the livability of Mars. (At the turn of the19th century, the astronomer Percival Lowell ignited the hope that Mars could support life. He claimed that the passage on Mars was actually a canal built by intelligent creatures. But optimists got some good news at 1969. After Mariner 9 reached the orbit of Mars, it became the first spacecraft to orbit another planet in the process. This exploration found other evidence of liquid water activities on the surface of rivers and Mars in the past. These findings helped NASA to develop two ambitious Mars life search missions, Viking 1 and Viking 2, which were launched several weeks apart at 1975. Each of the same Viking lander carried out four biological experiments to look for signs of microbial life in red soil. One experiment named Label Release (LR) returned data consistent with the evidence of microbial life. In fact, Jill Levin, the principal researcher of LR, thinks (and continues to argue today) that the Vikings found evidence of life on Mars. However, most scientists who study data disagree with Levin's view that these data can be explained by abiotic (abiotic) chemical reactions. The result of Viking gave NASA and astrobiologists some valuable lessons-mainly because they didn't know enough about Mars to find life there. Therefore, NASA finally began a long-term "follow the water" exploration strategy, seeking to know more about the ancient environmental conditions on this red planet and how they changed with time. In the past decades, this war has provided us with many important mission to mars, including Mars Odyssey orbiter, Mars reconnaissance orbiter and Mars Atmosphere and Volatilization Evolution (MAVEN); Roamer spirit, opportunity and curiosity; And the Phoenix lander.

These robot explorers did a good job, found a lot of evidence that ancient Mars was very wet, and helped scientists better understand why, how and when this red planet became today's arid desert world. Curiosity took this work to the farthest, and found that its landing site, 96 miles wide (154 km), had a long-lived lake and river system billions of years ago, which may have supported life similar to Earth. At the same time, some scientists continue to search for life on Mars, focusing on aliens who may accidentally fall into the earth. For billions of years, the rocks of billions of red planets have gone into space after being hit by powerful asteroids or comets. A lot of earth materials have finally arrived on Mars, but the books are obviously unbalanced; The strong gravity of the sun pulls more things inward towards the earth. By the way, this extensive exchange of rocks led some scientists to assume that life actually first appeared on Mars and then reached the earth. )

1996, researchers announced that they had found potential signs of life in such a Martian meteorite, which was called Mount Allen 8400 1 (ALH8400 1). This is a great event. The research results were published in the famous Science magazine, and President Bill Clinton held a press conference on the White House lawn to discuss the news. The story of ALH8400 1 finally embarked on the Viking road. Other scientists chose this statement and agreed that the meteorite evidence was vague at best. However, like Levin, the ALH8400 1 team stuck to its findings and continues to do so today.

Ocean satellite

Of course, over the years, NASA and the wider exploration have not only focused on Mars. The Cassini-Huygens mission ended in September 20 17, which changed scientists' understanding of Saturn system and the potential of our solar system. The mission found that Titan, Saturn's largest satellite, has a hydrocarbon-based weather system, and there are lakes and oceans of liquid ethane and methane on the surface of this cold satellite. Life can swim in these oceans, although it is certainly very different from what we know on earth.

Cassini orbiter discovered another geyser erupted by Enceladus, a Saturn satellite. This discovery and other observations by Cassini show that there is a large area of salt water under Enceladus' shell. Geysers produce a lot of water ice and other substances. These clouds are so huge that Saturn's e-ring is formed. Cassini flew over the plume several times to collect samples, and scientists analyzed these samples to find clues about the underground environment of the moon. The researchers found carbon-containing organic compounds and free hydrogen, which indicates that there is a hydrothermal system in the ocean where Enceladus is buried. Submarine hydrothermal vents are a widely cited environment for the origin of life on earth. Cassini did not look for signs of life in this feather; Spaceships don't have the ability to do this, because no one knows about this feather before the mission begins. Scientists have realized that buried oceans are relatively common outside the solar system. Many ice-covered Jupiter moons seem to own these oceans-Ganymede, Callisto and, most interestingly, Europa. Europa's huge underground ocean seems to be in contact with the rock core of the moon, just like Enceladus' ocean, which makes a series of complex chemical reactions possible and may theoretically lead to life. Scientists think the oceans of Ganymede and Callisto are more boring, sandwiched between ice sheets. )

Titan also seems to be buried in a sea of salt water, which means that the moon may have two different potentially livable environments. Observations by NASA's New Horizon spacecraft show that liquid water may also exist under Pluto's surface. And the list goes on. In fact, the rich water world in the outer solar system shows that finding "Earth 2.0" may not be the best strategy for finding life. Most habitable real estate in the universe may be buried by ice.

Life on an exoplanet?

These discoveries about our celestial backyard coincide with important news about the whole universe. In the past ten years, as anaximander and Epicurus speculated many centuries ago, we have learned that our galaxy is full of potential life-supporting worlds. Most of this knowledge comes from NASA's groundbreaking Kepler mission, which was run from 2009 to 20 18, 1 1. So far, Kepler has accounted for nearly two-thirds of the 4,400 confirmed exoplanets. Mission data show that the number of planets exceeds the number of stars in our Milky Way.

Many of these planets may be more like the earth than this. Kepler found that at least 20% of the stars in the Milky Way may have rocky planets in their livable zone, which is the orbital distance range where liquid water can persist on the earth's surface. Some of these potentially livable worlds are just a stone's throw away from everything in the universe. For example, the nearest star, proxima centauri, is about 4.2 light years away from the sun, and there is a planet about the size of the earth in the livable zone. This world called proxima b is the main goal of breaking through interstellar shooting. ) TRAPPIST- 1, 39 light years away, there are seven rocky worlds, three of which may support the existence of life as we know it. But proxima centauri and TRAPPIST- 1 are both red dwarfs, accounting for 70% of the stars in the Milky Way. Red dwarfs are very active small stars, and their intense flares may seriously affect the livability of planets.

Kepler's legacy is being inherited by other exoplanet missions, such as NASA's transit exoplanet survey satellite (TESS), which is expected to discover thousands of alien worlds around nearby stars, and ESA's CHEOPS detector, aiming to describe the characteristics of these neighboring worlds.

The perseverance of the rover, the discovery of many exoplanets such as james webb, and the discovery closer to home made astrobiology enter the mainstream from the edge of science. As some current and upcoming missions show, NASA has recently publicly made the search for extraterrestrial life a priority. For example, in July 2020, the agency launched the rover Perseverance, which landed in February 20021year, looking for signs of life on the ancient Mars, and collecting samples to finally return to Earth. Finding evidence of long-term microbial death is expected to be a very difficult task. Ideally, a group of scientists will conduct research in a well-equipped laboratory, and select the original Mars fragments with life-saving potential. (The European Space Agency also plans to launch its own Mars probe named rosalind franklin in July 2020, but technical problems have delayed the launch to the next window, that is, the autumn of 2024. )

In 2024, NASA's Europa Clipper is scheduled to be launched into Jupiter system. The clippers will orbit this gas giant planet, but will fly over Europa dozens of times to describe the underground ocean of the moon, find a good landing site for the future life hunting lander, and other tasks.

In 2027, NASA plans to launch a dragonfly detector, which will fly over Titan's thick smoky sky. Dragonflies' main goals include studying complex chemicals, laying the foundation for the emergence of life, and evaluating the livability of Titan, but rotorcraft will also look for biological characteristics.

The agency will soon start looking for aliens farther away. NASA's $9.7 billion James Webb Space Telescope is scheduled to be launched on 2002110. It is the successor to the iconic Hubble Space Telescope, and it is often delayed.

One of the many things to do after the launch of the powerful new telescope is to detect the atmosphere of nearby exoplanets in order to find potential biological characteristics-gases such as oxygen and methane, which exist in the air of the world at the same time and will provide strong evidence for life.

If all goes according to plan, three highly anticipated giant telescopes will start similar work on the ground in the middle and late 1920s. If an agreement can be reached between the telescope team and the local community, the giant Magellan telescope and the super telescope will be observed in the mountains of Chile, and thirty meter telescope will be located at the top of Monaci. SETI activities may also increase quickly, not only because of breakthrough listening. The largest radio telescope in history, namely the China 500-meter spherical radio telescope (FAST), was fully launched in early 2020, and the search technology is one of its many expenses. Of course, this is only a partial list of upcoming activities to find life. As the cost of building and launching spacecraft continues to drop, the complete list may eventually become dazzling. This trend may eventually make astrobiology tasks feasible for all kinds of interested groups from university groups to ordinary citizens. In fact, Milner has considered launching a life hunting mission to Enceladus or Europa. Some alien searches will continue to be carried out in earth research, which involves not only the inspection of Martian meteorites. We have been looking for the "shadow biosphere" on the earth: a complete tree of life, separated from all other trees of life, including bacteria, bats, birds and things we think are alive at present.

Come to think of it, this pursuit is not so crazy. After all, life appeared on the earth about 4 billion years ago-considering that our planet was formed only 4.5 billion years ago and remained hot and uninhabitable for a long time after that, this is very fast. Therefore, the appearance of life does not seem magical, which in turn means that it may happen here more than once.

fermi paradox

In view of the amazing number of potential habitable real estate-this only applies to life similar to the earth, not to mention the environment that can support all kinds of "strange life"-why haven't we found ET yet? Enrico Fermi, a Nobel Prize-winning physicist, raised this question in 1950, especially referring to intelligent aliens. Seven years later, the answer to Fermi's paradox is still elusive. However, "answer" may be a better expression, because many factors may prevent us from finding intelligent aliens. The most important thing is the vast space, and it is difficult for the two civilizations to contact the base. Consider this: neighboring star B is only 4.2 light years away and is located in a galaxy 100000 light years wide. But 4.2 light years is about 26 trillion miles (42 trillion kilometers), and it will take tens of thousands of years for a human spacecraft to cross this range. Contact with intelligent aliens also requires time and temperament adjustment; Their civilization must keep pace with us, which is not easy in the universe of138.2 billion years. ET must want to help. This is not given; As pessimists in the Ministry of Economy, Trade and Industry have pointed out, there are many reasons why some aliens may want to remain silent. Or maybe wisdom is rare in the whole universe, even if life is not. After all, the earth has been inhabited for about 4 billion years, and we only launched radio waves for about a century, and we didn't launch the spacecraft until 1957. And it is difficult to find microorganisms in the distance. They may not have invented the radio. Our technical youth may be the most important factor: after all, we have just begun to look for our cosmic neighbors. This kind of search is mostly stopped and random, and is conducted by a small team of professional researchers, who have to raise funds to keep the light.

But this situation is changing, as shown by the exciting new tasks and tools being developed at present. So we may get some answers soon.