Can we find a second habitable planet

Since the Kepler Space Telescope, the planetary exploration spacecraft launched by the National Aeronautics and Space Administration (NASA), retired in 2018, its successor, the Transiting Exoplanet Survey Satellite (referred to as “Tess”), has been highly expected .

The main goal of “Tess” is to continue to search for “new worlds” outside the solar system based on Kepler’s discovery, and to scan more than 200,000 bright stars near the solar system to find planets orbiting them and their possibilities. Signs of life.

At the beginning of 2020, NASA issued a communiqué stating that “Tess” discovered an exoplanet close to the size of the Earth and located in the habitable zone-TOI 700 d.

According to Agence France-Presse, the European CHEOPS space telescope, which was sent into space at the end of 2019, was also used to measure the density, composition and size of exoplanets.

Mankind’s exploration of exoplanets and exoplanets has never stopped.

Seeing the shadow of the earth outside the solar system

Exoplanets, as the name suggests, refer to planets outside the solar system where humans live. In the past 100 years, mankind has almost seen the beginning of the explosion of the universe using various advanced telescopes and discovered countless amazing celestial bodies. However, the detection of exoplanets has only happened in the last 20 years.

Since human beings were born on this blue planet, they have started to observe the starry sky with both eyes. In long-term observations, people have discovered that in addition to the sun and the moon, some celestial bodies are special. Compared with many other celestial bodies, their positions in the sky will change. Therefore, the ancient Greeks called these celestial bodies whose positions changed greatly as planets, which literally means “rovers”. Celestial bodies in sharp contrast are stars with relatively fixed positions.

Probably many people have heard the story of the Han Dynasty astronomer Zhang Heng counting the stars at night. At that time, the number of stars he could see was only three or four thousand. The starry sky he saw at that time was not much different from the starry sky we see today nearly 2000 years later. Only because of the city lights, the number of stars visible to the naked eye is less.

However, with telescopes, mankind still sees a farther and wider universe. This is thanks to the Italian scientist Galileo 400 years ago. In 1609, Galileo carefully studied the working principle of the telescope and produced his first telescope with a diameter of 3.7 cm.

Since then, telescopes with larger calibers and more powerful capabilities have been continuously manufactured to help humans see more celestial bodies. It has been discovered that there are more planets and other small celestial bodies in the solar system. And outside the solar system, there are hundreds of millions of stars, which together constitute our Milky Way.

In the 1920s, the American astronomer Hubble discovered that there are a large number of celestial bodies outside the Milky Way that are similar to the Milky Way, that is, extragalactic galaxies. To commemorate this great astronomer, the space telescope launched by NASA in 1990 was named after him.

At the end of 1995, the Hubble telescope pointed to a seemingly innocent area the size of a pinpoint in Ursa Major, which became the Hubble deep field that was later known to the world. In this small picture, it contains thousands of galaxies of different sizes and similar to the Milky Way. For the first time, mankind knows that the universe contains so many galaxies, the number of stars in the universe is even more than the number of sand grains on the earth.

In November of this year, Michel Mayor and Didier Queiroz of Switzerland discovered the first exoplanet 51b Pegasus orbiting a sun-like star, which opened the extrasolar space. The prologue to the exploration of planets and extrasolar life. The pair of masters and apprentices also won the 2019 Nobel Prize in Physics for this discovery. It is them that let us see the shadow of the earth outside the solar system.

Looking for life outside the department

Why do astronomers keep searching for exoplanets? Perhaps it comes from human curiosity about the existence of extraterrestrial life. Since ancient times, human beings have very much wanted to know whether there are higher intelligent life similar to human beings or other simple life forms outside the earth.

There are many similarities between Mars and the Earth, so people on Earth have always thought that Mars has life. The discovery of the Martian “canal” in the 19th century led many people to believe that there were Martians on Mars. In the 1930s, radio broadcasting news of the Martian invasion of the earth caused many people to panic and believe it. However, in recent decades, direct observations of the high altitude and surface of Mars have found that Mars is a barren land with almost no grass growing, although it is likely that there were water and oceans billions of years ago.

In the solar system, the planet most similar to the earth is Venus. However, the thick atmosphere makes its surface temperature reach more than 400 degrees Celsius. As far as we know, there is no possibility of life.

Many countries search for traces of life on the farther planet by launching different detectors. Enceladus and Europa both show possible signs of life, however, humans have not detected direct signs of life.

As a result, human beings pin their hopes on a larger world. That is looking for extrasolar life in exoplanets.

Before the launch of the probe, the scientists theoretically discussed where to find life. Liquid water is considered to be an indispensable condition for the emergence of life on earth. Therefore, in the early 1950s, based on the understanding of life on Earth, scientists proposed the concept of a habitable zone. They believed that for a particular star, liquid water can only exist on a planet that is not far away from it and located in an appropriate location. It may exist, and life may exist. In the absence of a deeper understanding of life, this basis has become the starting point for people to explore life outside the department.

Since the livable area is located in a moderately located place, the method of selecting a livable area is also called the “Principle of Golden Phoenix”. The Golden Phoenix Girl is a fairy tale character in American tradition. She likes everything that is “just right” such as cold or hot porridge, neither soft nor hard chair. Therefore, Americans often use “Butterfly Girl” to describe “just right”.

In the search for life before, scientists did not focus on stars. Recently, some scientists have applied the “blooming flower principle” to stars. According to the number of stars, lifespan, and the frequency of high-energy explosions, they have proposed that planets with K-type stars are more suitable for gestating life. This type of star is also called “butterfly stars.” .

There are three main types of stars in the Milky Way: G-type sun-like stars; K-type stars with lower mass and lower temperature than the sun; M-type stars that are darker and lower in temperature than K-type stars.

Previously, scientists were looking for habitable zones around M-type stars, because such stars are darker and easier to observe their planets. But such planets are very close to the central star, so they are easily locked by their stellar tides. This means that half of these planets are always in the daytime and the temperature is very high, and the other half are always in the dark night and the temperature is extremely low, such as the planets around Proxima Centauri discovered in 2016. In such an extremely harsh environment, it is hard to imagine that there will be life.

However, K-type stars do not have an active magnetic field to produce strong X-rays, ultraviolet radiation, and high-energy explosions. Therefore, they have a very low frequency of flares. The deadly X-ray radiation received by their planets is about 100% of those in the habitable zone of M-type stars. One part.

However, the theory is the theory after all. Whether the K-type star called the “butterfly star” gives birth to life still needs to be verified by future observations.

Can humans realize interstellar migration?

So far, astronomers have found more than 10 Earth-like planets in the habitable zone through various methods. But we don’t know whether there is life or liquid water on these planets. Because they are still too far away, it is difficult for mankind’s current telescopes to directly see all the details on these planets. In the future, perhaps we can judge through some indirect methods. The powerful telescopes that may appear in the future, whether on the ground or in space, may help mankind.

In the American science fiction blockbuster “Interstellar”, when the earth is facing a disaster, astronauts use wormholes to search for a place suitable for human habitation in the distant universe, but they can’t find it. What astronomers are currently doing is to select targets in advance for future human migration. In the movie, humans can quickly reach somewhere in the distant universe from the solar system with the help of wormholes. But in reality, interstellar travel technology is more challenging than finding habitable exoplanets.

When Michel Mayor was asked “Whether humans can achieve interstellar migration in the future”, the answer was pessimistic. Because under his visible technological development, it is difficult for humans to fly out of the solar system and reach the nearest neighbor to us in a short period of time. According to estimates, it will take tens of thousands of years for the fastest detector to reach Proxima. Even at 5% of the speed of light in “The Wandering Earth”, it would take about 2500 years to arrive.

At the end of 2018, when Chinese science fiction writer Liu Cixin received the Clark Foundation’s “Imagination Serve Society” Award, he said that although science fiction cannot predict the future, it can only show the many possibilities of the future, but the future without “interstellar travel” is gloomy. . He encourages people to use their infinite imagination to promote universe exploration and interstellar migration.