China searches for "Earth 2.0" to open a new era of exoplanet exploration

Written by reporter Duan Ran (internship) Edited by Ding Lin

New Media Editor/Li Yunfeng

Interview experts

Gu Shenghong (Chief Researcher, Yunnan Astronomical Observatory, Chinese Academy of Sciences)

In the vast universe, are humans the only intelligent creatures? Is our home a lonely place? These philosophical questions probably arose when humans first looked up at the stars, and they continue to inspire the desire and enthusiasm of future generations to explore the universe.

After thousands of years of accumulation of scientific knowledge, construction of theoretical systems and iterative updating of concepts, our understanding of space has reached an unprecedented level. Especially in the past two decades, the astronomical community's exploration of extrasolar planets has continuously shown mankind that we may not be alone in the vast universe.

Recently, the British magazine Nature disclosed China's "Earth 2.0" plan. The project was initiated by the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. It plans to launch a new detection satellite into space in the next five years to conduct an in-depth "planetary census" of exoplanets in the vast deep space, contributing China's strength to the exploration of mankind's extraterrestrial home.

▲The Kepler Space Telescope aims at the Cygnus-Lyra direction in the Milky Way for observation (each rectangle in the picture represents a 2.25 million pixel CCD sensor). my country's "Earth 2.0" project plans to conduct a more in-depth exploration of the same area (original picture from NASA)

Judging from the information disclosed so far, "Earth 2.0" is definitely an ambitious plan: we must not only catch up with the advanced level of international exoplanet exploration, but also propose a Chinese version of the improvement plan for the problems and shortcomings exposed in advanced space exoplanet exploration projects such as Kepler.

Gu Shenghong, chief researcher at the Yunnan Astronomical Observatory of the Chinese Academy of Sciences, who has been committed to the research of extrasolar planetary systems for a long time and has presided over many related national scientific research projects, told reporters: "The 'Earth 2.0' project is quite exciting. It inherits the data legacy of the first four years of the Kepler project. With its own design, it can make another four years of observations, covering eight years of data, which greatly improves the detection capability of exoplanets."

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Exoplanets: The long road ahead

As the name implies, exoplanets are planets that are outside the solar system and orbit other stars. According to rough estimates by scientists, there are at least 400 billion exoplanets in the Milky Way alone, of which as many as 17 billion are rocky planets similar to Earth. Among such a large number of exoplanets, we have reason to believe that there is "another Earth" that orbits in the "habitable zone" and is suitable for nurturing life.

For a planet to be called "another Earth", it needs to meet relatively stringent conditions. Gu Shenghong explained to reporters: "First, just like the main star of our solar system is the sun, the main star of the exoplanet should be a sun-like star; second, the planet needs to be in the habitable zone of the planetary system, that is, the planet must be at a certain distance from the main star to ensure the existence of liquid water; third, the planet must have a rocky solid surface similar to that of the earth. Only when these conditions are met can Earth-like life exist."

Humans’ understanding of planets has gone through a long road of exploration. Starting from observing the planets in the solar system, modern celestial mechanics has guided astronomers to continuously expand the boundaries of planets. From considering Saturn’s orbit as the boundary of the solar system in the 18th century to kicking Pluto out of the list of planets in 2006, it took humans hundreds of years to understand the planets “at our doorstep”. The technical difficulty of exploring exoplanets is even greater.

▲Top view of the Milky Way, with the yellow arrow pointing from our solar system to the direction of Cygnus (original image from Sky & Telescope)

Just over 20 years ago, the question of whether there are planetary systems outside the solar system was still a matter of speculation among scientists. Since planets themselves do not emit light and are much smaller than their host stars, it is difficult to find planets obscured by the light of stars on a spatial scale measured in light years. The task of finding them has left even the best astronomers at a loss.

Until the 1980s, astronomers could only indirectly speculate on the possible existence of exoplanets through the dust clouds found around some stars. In 1992, astronomers discovered planets outside the solar system for the first time: two gas planets orbiting a pulsar 980 light-years away from Earth. With a high-speed pulsar as its parent star, the harsh environment of these two planets can be imagined, but this discovery opened the curtain for human exploration of exoplanets.

▲The history of human exploration of exoplanets is not long (original image from the European Space Agency)

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Nobel Prize-winning "Linear Velocity Method"

Most of the exoplanets are too far away from us. It is very difficult to detect these celestial bodies that do not emit light through the vast sea of ​​stars. Therefore, astronomers currently rely mainly on indirect detection methods to find exoplanets, that is, by analyzing the luminous stars, they can indirectly draw conclusions about whether the stars have planetary systems. Among the various indirect detection methods currently in use, the "radial velocity method" and "transit method" are the most common.

The basic principle of the "radial velocity method" (also known as the radial velocity method or Doppler spectroscopy method) is that when a planet moves around a star, the star will be affected by the planet's weak gravity, producing tiny periodic oscillations. Astronomers detect and capture this periodic oscillation by observing the Doppler effect of the stellar spectrum, and can indirectly infer the existence of a planetary system near the star.

▲The gravity of exoplanets causes stars to oscillate slightly, so we on Earth can observe the periodic redshift and blueshift of stars’ spectra (Image source: The Planetary Society)

In 1995, Swiss astronomers Michel Mayor and Didier Queloz used a high-precision spectrometer and the "radial velocity method" to discover a gas planet, 51 Pegasus b, near the star 51 Pegasus. This was the first time that humans discovered an exoplanet orbiting a main sequence star (a type of star including the sun), and the two won the 2019 Nobel Prize in Physics for this discovery.

After the discovery of 51 Pegasus b, the radial velocity method has been widely used in the detection and discovery of exoplanets. However, this measurement method also has obvious shortcomings - the planets that are suitable for this method are limited by the orbital inclination, and the mass of the observed planets is prone to large errors. Since this method can basically only accurately detect some massive planets, this greatly limits the scope of planetary detection. Therefore, in recent years' planetary exploration activities, another scheme, the "transit method", has gradually received attention.

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Kepler Mission: The Sky's "Fiery Eyes"

The so-called "transit method" (also called the transit method) uses the periodic changes in star brightness caused by the transit phenomenon of a planet passing through its host star to determine the existence of a planetary system. Compared with the radial velocity method, the transit method requires more stable observation conditions, so launching a space telescope into space orbit is the only choice for scientists who are fond of the "transit method".

▲When an exoplanet orbits a star, the brightness of the star changes periodically due to the planet (Image source: Astronomy Magazine)

In 2009, NASA launched the Kepler Space Telescope into its planned solar orbit. This is a groundbreaking project in the history of exoplanet exploration. Kepler carries a 0.95-meter Schmidt telescope with a mass of more than 1 ton, and its detection targets are located in the Cygnus and Lyra regions.

During its operation, the Kepler telescope used the "transit method" to multiply the number of known exoplanets. As of 2018, when the project was terminated, the Kepler project had discovered as many as 2,325 exoplanets (as of April 27 this year, a total of 5,014 exoplanets have been confirmed).

▲As of this year, astronomers have confirmed more than 5,000 exoplanets (original image from NASA)

The Kepler project has not only greatly expanded humanity's vision of extrasolar space, but has also refreshed the astronomical community's understanding of the concept of "planet". With the help of various data provided by the Kepler project, astronomers have discovered many phenomena that contradict traditional astrophysical knowledge: for example, planets orbiting two stars, or planets orbiting in a quadruple star system.

Regarding these bizarre phenomena, astronomer Michael Summers used the monarch butterfly to metaphorically represent the Earth in his book Exoplanets: assuming that people have only seen the monarch butterfly in their lives, they will naturally assume that the characteristics of the monarch butterfly are equivalent to the characteristics of butterflies. Once they encounter a butterfly with other characteristics, they will suffer a cognitive shock and have to re-examine their inherent ideas.

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Discovering Wangshu: my country's Journey into Exoplanet Exploration

In recent years, the United States' ground-based and space-based planetary observation systems have been flourishing. In comparison, my country's overall start in the field of exoplanet exploration was relatively late.

Due to the shortcomings in detector technology and observation data processing accuracy, the research samples used in my country's exoplanet exploration have always been highly dependent on observation data released by foreign detectors. This situation lasted for many years until 2008, when my country achieved a "zero breakthrough" in exoplanet exploration: a team led by Liu Yujuan and Zhao Gang, researchers at the National Astronomical Observatory of the Chinese Academy of Sciences, used the 2.16-meter telescope at the Xinglong Station of the National Astronomical Observatory and the 1.88-meter telescope at the Okayama Observatory in Japan to discover a planet in the direction of the Lyra constellation through the radial velocity method. This is a gas planet orbiting a red giant, 440 light-years away from the Earth, and has a mass 2.7 times that of Jupiter.

▲This exoplanet (HD173416 b) is located in the direction of the Lyra constellation. The matrix of dense green dots in the picture is the field of view of the Kepler telescope (Image source: Kyoto University)

Although it does not have the environmental conditions for the survival of life, this planet, as the first exoplanet discovered by Chinese astronomers, has become an important milestone in my country's space exploration.

At first, astronomers only gave this planet a monotonous code: HD173416 b. But more than 10 years after its discovery, it was given more "Chinese colors". In 2019, the "Exoplanet World Naming" global activity project organized by the International Astronomical Union (IAU) was launched. The project allocated 112 unnamed exoplanets according to countries, and the allocating countries held nomination and voting activities to officially name these planets.

HD173416 b was assigned to its discoverer, China. For a time, the activity of naming exoplanets set off a wave of enthusiasm among the domestic astronomy community and enthusiasts, and naming schemes from all over the country flew to the review experts like snowflakes. Finally, after multiple rounds of expert review and online voting, this exoplanet and its parent star were given the two poetic mythological names "Wangshu" and "Xihe", respectively, and were recorded in the history of space exploration in a way that was quite traditional Chinese cultural.

▲In December 2019, Xu Yipeng, the then president of the Astronomy Club of Guangzhou No. 6 Middle School, who named "Xihe" and "Wangshu", introduced the naming story at the Beijing Planetarium (Photo source: People's Daily client)

In the same year, my country's exoplanet exploration made another breakthrough. The team of Zhang Hui and Zhou Jilin from Nanjing University announced that with the help of the Antarctic Survey Telescope Array (AST3) built by my country at Kunlun Station in Antarctica, the team used the "transit method" to detect a total of 222 exoplanet candidates, of which 116 were high-confidence candidates. This is the first time that my country has used its own observation equipment to discover exoplanet candidates in batches. Such a large number of discoveries has not only greatly enriched the planetary sample library of human extraterritorial exploration, but also enhanced my country's voice in the field of exoplanets.

At present, my country's exploration of exoplanets is still far behind that of advanced European and American countries. Gu Shenghong believes that this gap is mainly manifested in two aspects. First, the number of exoplanets discovered and confirmed is too small - as of now, my country has only confirmed the existence of less than 20 exoplanets in total. Second, due to the lack of large optical telescopes, the research on discovered exoplanets is not in-depth.

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Earth 2.0: Standing on Kepler's shoulders

At present, my country's astronomical community mainly relies on ground-based equipment to promote exoplanet research. However, when looking up at the starry sky from the ground, the field of view and accuracy of observation are greatly limited by the complex climate conditions in the atmosphere. In the future, to get rid of dependence on foreign space-based detection methods, my country needs to deploy its own "eyes and ears" in space and extend the field of exploration to outer space. The planned "Earth 2.0" project is the product of this practical need.

In addition, the shortcomings of the transit observation method itself also affect the observation results. Gu Shenghong explained to reporters: Transit observation requires a relatively long period, and Kepler's own operating period is short, resulting in insufficient window period for transit observation. In addition, the occurrence of the transit phenomenon itself is a low-probability event, and the signal generated is very weak - all of these have greatly limited Kepler's observation results.

▲Folmalagh b (also known as "Dagon"), about 25 light-years from Earth, is one of the first exoplanets directly confirmed by images. But more recent evidence suggests that it may just be a dust cloud (Image source: NASA)

Although the Kepler project has made remarkable achievements, unfortunately, until the end of the mission, the mission has not yet achieved its core goal - to find another Earth-like planet suitable for life. This is closely related to the shortcomings of the Kepler telescope's hardware: for example, the telescope's field of view is too small and the instrument noise is too high.

Due to the huge cost of extrasolar space exploration projects and NASA's insufficient budget, the Kepler project was eventually taken over by the Transiting Exoplanet Survey Satellite (TESS) project contracted by SpaceX. Compared to the Kepler probe, which is only equipped with a Schmidt telescope, the TESS probe is equipped with four wide-angle telescopes, which gives it a larger observation field than the Kepler telescope.

▲Imaginary image of the TESS space probe (Image source: NASA)

In my country's "Earth 2.0" project, future domestic probes may be equipped with up to 7 telescopes, and through an unprecedented combination of the "transit method" and the "microlensing method", a comprehensive "census" of exoplanets will be completed. Among them, 6 telescopes with a diameter of 30 cm and an observation field of 500 square degrees will use the "transit method" for observation, and the detection area will also be locked in the area from Cygnus to Lyra, but compared with "predecessors" such as Kepler and TESS, its detection depth and clarity will be greatly improved. Another 4-square-degree telescope will aim the detection angle at the center of the Milky Way, using a new microlensing observation method to detect and capture those "wandering planets" that are free from the main star planetary system.

The leader of the "Earth 2.0" team is Ge Jian, a researcher from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. While teaching at the Department of Astronomy at the University of Florida in 2006, he led a research team to discover a gaseous exoplanet 100 light-years away. In 2018, he led the "Dharma Exoplanet Survey" project at the University of Florida and successfully discovered a massive Earth-like planet 16 light-years away, orbiting 40 Eridanus.

In 2020, Ge Jian returned to China and joined the Shanghai Astronomical Observatory. The "Earth 2.0" project he is in charge of has brought together more than 200 astronomers from more than 30 universities and research institutions at home and abroad. In the future, the observation data obtained by the project will also be shared globally - China's "Earth 2.0" has transcended national boundaries from the beginning, and it was born to promote the process of human exploration of the universe.

According to the plan, Earth 2.0 will complete satellite construction and launch by the end of 2026 and begin scientific observations in the summer of the following year. Scientists predict that after the project completes a cycle (4 years) of observation missions, it is expected to find 5,000 exoplanets and find those that are truly suitable for life. With the strong support of the country and the concerted efforts of the astronomical community, Earth 2.0 is expected to leave a shining Chinese mark in the exploration of exoplanets.

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