Searching for Earth 2.0

  From ancient times, our ancestors gazed at the sky every night and were familiar with the work of the stars and the sun revolving around the earth. But 500 years ago Copernicus said to the world, no, it is not the sun revolving around the earth, but the earth revolving around the sun. After that, the sacred position of the earth dropped again and again. Later, astronomers discovered that the solar system is just an extremely ordinary planetary system in the great Milky Way galaxy, and the Milky Way is just an extremely ordinary galaxy in the universe. This makes us humans wonder if we are unique in this universe. Is life and intelligence on Earth, the technology and progress we are proud of, just an ordinary thing in this universe? Alien intelligence may be only a “river” away from us?
  Such an idea is just a conjecture. At present, not only is there no trace of extraterrestrial intelligence, but even the famous Drake formula for estimating interstellar civilization, we only know about the first two items: the number of stars in the Milky Way and the proportion of planets in the stars. So far, we know nothing about the third term in Drake’s formula, the proportion of planets that are terrestrial in the habitable zone (the so-called η-Earth). The following items of Drake’s formula are all related to life, let alone those of us who have not understood the mysteries of life.
  With current technology, we are expected to find Earth 2.0 (some planets similar in size, orbit, and host star to Earth) and measure ηEarth within a few years. What is even more exciting is that China’s space technology may help humans find this answer.

drake formula
The coincidence of Earth 2.0 and life?

  It seems logical to say that life appeared on the earth and then gradually produced wisdom, but philosophy tells us that existence is reasonable, but not necessarily inevitable. The current scientific research makes us realize that the emergence of life on earth may be the result of the accumulation of some accidental events.
  Let us first look at the Earth’s orbit. The distance between the earth and the sun is neither too far nor too close, and it happens to be in the habitable zone of the star. If the earth is closer to the sun than it is now, the surface temperature is slightly higher, and the greenhouse effect causes more and more water vapor in the atmosphere to accumulate, and the temperature continues to rise, then the earth will embark on the path of no return to Venus: the surface will become extremely hot. If the earth were a little farther from the sun than it is now, the surface temperature of the earth would drop below zero. The water vapor condenses into ice, freezing the earth, and the sunlight is reflected back to outer space by the ice, then the earth will become like Mars, which remains frozen all year round.
  The mass of the earth is neither too big nor too small, just right. If the mass of the earth was larger, the mountains on the earth would not be as high as they are now (because stronger gravity would make the mountains easier to be crushed by themselves), and the vertical niche space left for living things would be smaller than it is now. Diversity will not be as abundant as it is now. At the same time, increased gravity would make it difficult for chemical rockets to launch satellites. Unless atomic energy is used, aerospace will have no way to develop, and astronomy can only stay on the surface. Assuming that the mass of the earth is a little smaller and the gravity is reduced, the water molecules in the atmosphere are easily blown away by the solar wind. Our atmosphere is getting thinner, and the environment will become as harsh as Mars.

Habitable zone

  The Earth’s atmosphere and oceans are also perfectly regulated. If the concentration of carbon dioxide in the atmosphere is too high, the whole world will become hotter than the desert of the Arab Emirates; if the concentration is too low, the earth will become a white planet covered in ice; Composed of nitrogen and oxygen, sunlight cannot pass through the surface, and there will be no photosynthesis. The ocean occupies 70% of the earth’s surface area, and life evolved successfully in the ocean before landing on land. But if the ocean is too big, there will be no land, and there will be no wood for fire and cooking. The evolution of human intelligence will be much slower, and technologies such as steam engines and integrated circuits may never be born…
  These accidental events that create human intelligence , will it appear again on exoplanets? Nobody knows. But if we can find a second Earth, scientific verification could be possible. Due to the limitations of current technology, we have not been able to find planets exactly like the earth. Therefore, we can only take a step back and look for an “Earth 2.0” similar to the Earth. Of all the exoplanets, the environment of “Earth 2.0” is the most likely to harbor life.
Kepler Project: 2009—2013

  Even those who don’t know much about astronomy probably know that the search for exoplanets is nothing new. We now have more than 4,000 confirmed exoplanets. However, none of them looks like the earth!
  This story begins in 1995. This year, Swiss scientists Meyer and Queloz used a 2-meter small-aperture telescope in southern France to detect the first exoplanet. Once this discovery was released, it was immediately confirmed by other scientists. In 2019, Meyer and Queloz won the Nobel Prize in Physics for their discovery of exoplanets.
  So far, nearly a thousand exoplanets have been found by ground-based telescopes, most of them by the radial velocity method (the method used by Meyer and Queloz), which is based on observing the faint planetary motion produced by stars under the gravity of planets. swinging back and forth. However, this method can only find planets as massive as Jupiter, which is far from the “Earth 2.0” we are looking for.

Ways to detect exoplanets

  Fortunately, this is not the only “magic” that humans have. Back in 1984, when we knew nothing about exoplanets, Borucki, a NASA engineer at the time, had presciently planned another way to find them. His idea is simple: whenever Venus transits the sun (the sun-Venus-earth are on the same straight line), Venus blocks a very small part of the sun, and the brightness of the sun observed from the earth will weaken a little. By the same token, a well-placed (and smart enough) alien could also spot our planet by observing transits. Of course, the probability of a transit event is low. To find “Earth 2.0”, it is necessary to observe a large number of stars and stare for a long time (even years) without blinking. In addition, our light measuring instruments must be accurate enough: when the earth transits the sun, the earth only covers one ten-thousandth of the sun’s disc, and the difference in brightness caused by it is very small. So although this approach is easy to implement, NASA is not buying it. Since Borutsky first proposed it in 1992, this project has been rejected by NASA five times in a row on the grounds that the technology is not mature enough. Fortunately, Borutsky is not only foresight, but also has perseverance beyond ordinary people. He was not discouraged, patiently developed detectors and detection technologies on the ground, overcame technical difficulties one by one, and finally persuaded NASA to launch the detection telescope into space in 2009. This is NASA’s Kepler project.

  The Kepler telescope opened a large window to observe the universe. Within a few years, it single-handedly discovered nearly 3,000 exoplanets, which not only increased the number of exoplanets known to mankind several times, but also, more importantly, Yes, the Kepler telescope has opened the eyes of mankind and upended our understanding of the world of exoplanets.
  While traditional radial velocity methods tell us mostly giant planets (similar to Jupiter and Saturn, with some exceptions), Kepler reveals a previously unknown but extremely common population of planets in the universe (super Earth and sub-Neptune). These less massive planets orbit about a third of the stars in the Milky Way at close quarters, and there are four times as many giant planets. What excites scientists most is that these small planets are not much different in size from Earth.
  The Kepler mission marked a stellar decade for the entire field of planetary science. But amidst these brilliance, there is another huge regret hidden. The original intention of the Kepler project was to find planets like Earth. But among the thousands of planets it found, none of them looked like the earth!
  Someone asked, how do you know if those planets look like the earth?
  In fact, whether it is the giant planets detected by the radial velocity technique or the sub-Neptunes discovered by the Kepler mission, they can all be called “a generation of planets” (Saturn, Jupiter, Uranus and Neptune in the solar system are examples). Their surfaces are shrouded in thick hydrogen gas, suggesting they formed early in the stars’ lives. At this time, the star is still surrounded by a disk of hydrogen gas, which is the waste material of the star-making project. Even the so-called super-Earths, although they have a solid surface, are now theoretically believed to be sub-Neptunes, but because they are too close to the host star, their innate properties are stripped away by the star’s strong X-rays. hydrogen atmosphere, leaving a bare solid planetary core. In contrast, Earth and its little partner planets should be called “second-generation planets.” They may be formed by the collision of residue fragments from unknown sources after the hydrogen disk disappeared.

Some numbers about the Kepler telescope

Exoplanets detected so far

  A generation of planets is unlikely to harbor life. Suppose you come to such a planet, you will slowly drift down to a soft ground with the wind (thick hydrogen gas) – the greenhouse effect of hydrogen keeps the temperature of the planet at several thousand degrees Celsius, enough to melt steel , the cores of these generation planets are also melted. Even those super-Earths are not much better, being so close to their host stars that their rocky surfaces are not far from melting, if not melting into molten lava. Instead, the environments of second-generation planets are more hospitable to life formation and evolution, as we have witnessed on Earth. Their subsurface and atmosphere may contain elements needed for life such as carbon, oxygen, nitrogen, etc., and their atmosphere may be relatively thin to produce a suitable greenhouse effect.
  What is even more regrettable is that not only did the Kepler project fail to find Earth 2.0, but even few second-generation planets seemed to be found. Could it be that something unexpected happened in the solar system that gave birth to such a alien species as the earth?

  One possible explanation for the failure of the Kepler project is that the Earth is really alone in the universe. However, now is not the time to give up exploring. There are several other reasons why the Kepler project failed, enough to keep us looking.
  First, stars are not “constant”. The surface climate of stars is complex and changeable, and their brightness is constantly changing. After the Kepler telescope went to the sky, it made an unpleasant discovery: the flickering of stars is much worse than we expected. Under this noisy change, it is difficult for the Kepler telescope to capture weak changes less than one ten-thousandth of a star’s brightness.
  Second, the manufacturer’s quality control is not enough. After 4 years of operation of the Kepler telescope, the two reactionary rotating wheels failed continuously. Therefore, this sky-high price telescope could not adjust its direction and continue to observe exoplanets. This is a great pity. However, among the tens of thousands of stars stared at by the Kepler telescope, there may already be traces of “Earth 2.0”, but the transit effect produced by them is too weak, buried in the noise of the stellar signal, and cannot be interpreted . The premature death of the Kepler telescope made scientists miss “Earth 2.0”. Can humans make up for this regret?

How Space Telescopes Are Finding Exoplanets
“Earth 2.0” and China’s Space Program

  On December 3, 2019, a defense meeting was going on intensely at the China Space Center. Shanghai Astronomical Observatory and Nanjing Purple Mountain Astronomical Observatory each proposed a space exploration plan to search for “Earth 2.0”. The judges are listening to the reports, asking questions frequently, and judging which one is the winner.
  The Shanghai Astronomical Observatory’s plan is referred to as ET (Earth Two, meaning “Earth 2”). It is to build and launch a space telescope consisting of seven medium-sized wide-field telescopes. These telescopes will aim at several sky areas observed by the Kepler telescope and nearby sky areas, and monitor more than 200,000 bright stars for 4 years in order to capture the weak signals produced by planets transiting stars. It is currently estimated that there is about one Earth 2.0 in every ten sun-like stars (that is, η-Earth is about 10%). If correct, ET could find a dozen Earth 2.0s in 4 years. In addition, ET can also find nearly a thousand terrestrial planets outside the habitable zone, and conduct the first “planetary census” of these second-generation planets.

  Beyond that, ET can find thousands of different kinds of exoplanets, including Earth’s distant relatives. Although ET is smaller and less expensive than Kepler, its scientific output is expected to be comparable to that of Kepler. Chinese astronomers also believe that this plan is even more certain to discover Earth 2.0 than the PLATO spacecraft that ESA is building.
  Why expect a telescope smaller than the Kepler telescope to complete the unfinished business of the Kepler telescope in just four years? In addition to the technological progress of the past decade, it can also be explained as follows: imagine that you are chatting with someone in a noisy bar. Your voice is too low for the other party to hear. You have to speak several times to be effective. The Earth 2.0 we are looking for may actually have been traced in the vast detection data of the Kepler telescope, and we may only need to listen carefully a few more times to find it.
  Zijinshan station’s satellite program is even more unique. This bold plan calls for the development of another planet-finding technique called “celestial observation”: observing the subtle dance of stars across the sky under the influence of planetary gravity. To detect Earth, the technology would need to achieve an unprecedented precision: milliarcseconds — the equivalent of resolving two toothpicks placed side by side on the Moon. However, the advantages of this technology are indeed worthy of great efforts by scientists.

  The process of finding Earth 2.0 has greatly challenged human creativity. The two programs of Shanghai Observatory and Nanjing Zijinshan Observatory complement each other, sprinting from different directions and using different steps. The question of whether the earth is lonely is likely to be answered proudly by Chinese space scientists.
  To find Earth 2.0 is not to expect to immigrate there. These exotic worlds are often tens or even hundreds of light-years away from us. However, humans on Earth are building various space and ground-based giant telescopes, and are preparing to stare patiently at these planets (especially those transiting Earth-like ones), and investigate whether they can give birth to life from a distance. The next good show will be even more exciting!