In search of "Earth 2.0", Chinese scientists propose a sky survey plan

◎ Science and Technology Daily reporter Jin Feng

Whether there is other life in the universe, and whether there is a second home suitable for human habitation, has attracted scientists to explore deeper into the universe for thousands of years.

Recently, the reporter learned from the Purple Mountain Observatory of the Chinese Academy of Sciences that Chinese scientists have proposed a "Nearby Habitable Planet Survey" (hereinafter referred to as "CHES"), planning to launch a 1.2-meter-diameter high-precision astrometric space telescope, which will operate routinely at the Sun-Earth Lagrange L2 point for at least 5 years, and intends to detect 100 solar-type stars about 32 light-years away from the Earth, hoping to discover the first habitable zone outside the solar system, "Earth 2.0". This will be the first international space exploration mission dedicated to searching for habitable Earth-like planets around nearby solar-type stars.

"As far as the evolution of the universe is concerned, we cannot predict what will happen to the Earth's environment in 50-100 years. We are trying to explore whether there are other habitable planets in the universe, especially around the neighboring stars of the solar system, and then explore whether there is life or advanced civilization on these planets." Ji Jianghui, a researcher at the Purple Mountain Observatory of the Chinese Academy of Sciences and project leader of the "Neighboring Habitable Planet Survey", told Science and Technology Daily that scientists hope to bring a pair of wise eyes to the search for "Earth 2.0" by implementing this plan.

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We will search for nearby planets with a mass comparable to that of Earth and in the habitable zone.

Since the discovery of the first Jupiter-like planet outside the solar system in 1995, humans have discovered and confirmed more than 5,000 exoplanets. These planets vary in size and shape, including hot Jupiters, sub-Neptunes, rocky planets, super-Earths and other types.

Gliese 581c, discovered in 2007, is considered the first Earth-like planet discovered by humans in the habitable zone. In 2016, astronomers discovered another Earth-like planet in the habitable zone around Proxima Centauri, the closest star to Earth. Its minimum mass is 1.3 Earth masses and its orbital period is only 11.2 days.

"The habitable zone in the solar system is between Mars and Venus, and the Earth is right in the middle of it." Ji Jianghui introduced that astronomers call the range of planetary orbits suitable for the existence of life in a planetary system the "habitable zone."

In the "habitable zone", the average surface temperature of the planet can maintain the stable existence of liquid water, so it may have conditions for the existence of life similar to that of the Earth. At the same time, the stellar radiation and activity here will not be too strong to ionize the water molecules and carbon dioxide molecules in the planet's atmosphere, or even strip the planet's atmosphere.

"There are about 50 terrestrial planets discovered in the habitable zone outside the solar system, but most of them have a mass several to ten times that of the Earth, equivalent to a 'super-Earth'. Most of them are far away from the Earth, up to thousands of light-years. Moreover, many of the discovered planets are located around red dwarfs, whose surface temperatures are below 3500 Kelvin, and the space environment is harsh with strong flares. Therefore, we are more concerned about whether there is an 'Earth 2.0' in the habitable zone around stars similar to the Sun, which are about 32 light-years away from the Earth." Ji Jianghui said that the "Earth 2.0" or "twin Earth" that the CHES plan is looking for is a planet with a mass comparable to that of the Earth, an orbit in the habitable zone, and liquid water in the atmosphere or on the surface to sustain life.

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"Even if humans do not have the ability to reach these Earth-like planets now, it does not affect our thinking about where humans will go in the universe and whether there are other planets like the Earth," said Ji Jianghui.

The transit method, which can outline the contours of exoplanets, is difficult to "weigh" planets

In the vast starry sky, whether the Earth is the only lonely planet with life has attracted scientists from all over the world to spare no effort in deep space exploration. They deploy "planet hunters" in space and on Earth to capture the traces of planets.

In space, the United States' Kepler space telescope and TESS satellite have taken over the detection of exoplanets, and have discovered more than 3,400 so far.

On the ground, the Spanish-German CARMENES project uses a 3.5-meter telescope in southern Spain, combining near-infrared observations with an optical step-grating spectrometer to search for Earth-like planets around red dwarfs. The new spectropolarimeter "SPIRou" developed by the Canada-France-Hawaii Telescope and the "Habitable Zone Planet Finder" of the McDonald Observatory in the United States use the radial velocity method to search for habitable planets around red dwarfs in the near-infrared band.

Ji Jianghui introduced that the current methods for detecting exoplanets include transit method, radial velocity measurement method, astrometry method, direct imaging method, microlensing method, etc. Most of the exoplanets were discovered through radial velocity measurement method and transit method.

"About 73% of the stars in the universe are red dwarfs. Since red dwarfs have low effective temperatures, their habitable zones are close to their host stars, and their mass and volume are small, planets around red dwarfs are easily detected by the transit method and the radial velocity method." Ji Jianghui explained that "transit" means that when a planet passes in front of a star, it blocks the light emitted by the star, so the transit events of exoplanets can be tracked through the periodic dimming of the star's brightness. This is like the transit of Venus. When Venus moves slowly across the surface of the sun, it blocks a very small part of the sun, and the sun appears to become darker. Transit observations can infer the size and orbital period of exoplanets based on the periodic changes in the luminosity of stars. However, Ji Jianghui does not deny that although the transit method is effective, its detection efficiency is limited.

"First, if you want to see the transit phenomenon, there are special requirements for the planet's orbit. The planet needs to pass through the direction of the star toward the earth, almost in the direction of the observer's line of sight; but the orbits of the planets are randomly distributed, so the probability of a transit detected by some space telescopes is only 5/1000. Secondly, the weakening and dimming of the star's luminosity may be caused by sunspots or stellar activity, so the transit method still needs to be confirmed by other ground-based detection methods. So even though TESS has observed nearly 4,000 planet candidates, only more than 200 planets can be confirmed. Third, the transit method can only measure the radius of the planet, and cannot directly give the mass of the planet, which is a key parameter for describing Earth 2.0," said Ji Jianghui.

Innovative detection method, CHES detection accuracy will reach micro-arcsecond level

Unlike satellites such as TESS that use transit detection methods, the CHES program will use high-precision astrometry at the micro-arcsecond level in space to accurately measure the micro-arcsecond level of interstellar distances between a target star and 6-8 reference stars. This subtle change reflects the very small swing of the target star caused by the gravitational perturbations of its orbiting planet.

"In layman's terms, if there is a planet around a star, the planet will cause the star to produce a small periodic swing. After excluding the star's own motion, the smaller the swing amplitude of the star, the smaller the mass of the planet around it, and vice versa. By observing tiny changes in the position of a star, we can find out whether there are habitable planets around the star and calculate their true mass and orbital parameters," said Ji Jianghui.

Detecting "Earth 2.0" in the vast universe requires a pair of sharp eyes. Ji Jianghui said that the scientific payload of CHES is a high-quality, low-distortion, and highly stable optical telescope with an aperture of 1.2 meters and a focal length of 36 meters, which can achieve near-diffraction-limited imaging of the entire field of view.

According to the plan, the CHES space telescope will be sent into the Halo orbit of the second Lagrange point of the Sun-Earth system and maintain stable operation in this orbit for at least 5 years. During this period, it will conduct scientific detection of 100 solar-type stars, with each star being observed no less than 50 times. It is expected to discover about 50 Earth-like planets.

The detection accuracy of CHES will reach an unprecedented micro-arcsecond level. Ji Jianghui gave an analogy, "This is equivalent to looking at the moon from the earth and distinguishing the edge of a one-yuan coin placed on the moon." He said that after years of hard work, the CHES team has made breakthroughs in key technologies such as micro-pixel inter-satellite distance measurement, which can meet the detection accuracy requirements for habitable planets. If CHES is approved, it will be the first space exploration mission in the world to use high-precision astrometry to specifically search for habitable planets.

Ji Jianghui said that in addition to detecting habitable planets, this mission will also make corresponding contributions to cutting-edge scientific research on dark matter, black holes, etc.

If CHES can successfully "capture" the orbits of these Earth-like planets, how can we determine whether they will become habitable "chosen ones"? Ji Jianghui explained, "In addition to judging the mass of the planets and whether they are habitable, we also need to see whether they have water and oxygen, and see how these planets differ from the planets that have been discovered." In addition, there are many other conditions for the stable existence of life, such as long enough star and planet lifespans for life to occur, suitable star luminosity, stable near-circular planetary orbits and rotation inclinations, suitable planetary atmospheres and planetary magnetic fields, etc.

How can humans reach the Earth-like planets in the habitable zone 32 light-years away from the Earth? Ji Jianghui imagined, "First of all, this requires the blessing of interstellar navigation capabilities. For example, the flight speed may need to reach sub-light speed. In addition, we also need to understand the distribution of these planets. This requires a comprehensive "interstellar census" of solar-like stars 32 light-years away from the Earth to expand space for the future development of mankind."

Source: Science and Technology Daily

Editor: Zhang Shuang

Review: Julie

Final judge: Wang Yu