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China is devising a strategy to find Earth 2.0

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China is planning its first space mission to survey the skies for exoplanets similar to Kepler-186f, an Earth-size planet orbiting a distant star (artist’s impression).

China is now looking at distant solar systems after sending robots to the Moon, landing them on Mars, and building its own space station. Scientists will release detailed plans for the country’s first mission to find exoplanets later this month.

The mission’s goal is to find the first Earth-like planet orbiting in the habitable zone of a star similar to the Sun outside of the Solar System in other parts of the Milky Way. Astronomers believe that such a planet, dubbed Earth 2.0, would have the right conditions for liquid water to exist, as well as the possibility of life.

More than 5,000 exoplanets have already been discovered in the Milky Way, primarily using NASA’s Kepler telescope, which was operational for 9 years before running out of fuel in 2018. Some of the planets were rocky Earth-like bodies orbiting small red dwarf stars, but none of them met the definition of an Earth 2.0.

According to Jessie Christiansen, an astrophysicist at the NASA Exoplanet Science Institute at California Institute of Technology in Pasadena, finding the signal of small, Earth-like planets with current technology and telescopes is extremely difficult when their host stars are one million times heavier and one billion times brighter.

Earth 2.0, a Chinese mission, hopes to change that. It is being funded by the Chinese Academy of Sciences and is in the early stages of development. If the designs are approved by an expert panel in June, the mission team will receive funding to begin building the satellite. The spacecraft will be launched on a Long March rocket before the end of 2026, according to the team.

Seven eyes

The Earth 2.0 satellite will carry seven telescopes to observe the sky for four years. Six telescopes will collaborate to survey the Cygnus–Lyra constellations, the same patch of sky that the Kepler telescope surveyed. “The Kepler field is low-hanging fruit because we have very good data from there,” says Jian Ge, the astronomer in charge of the Earth 2.0 mission at the Chinese Academy of Sciences’ Shanghai Astronomical Observatory.

The telescopes will look for exoplanets by detecting small changes in the brightness of a star that indicate a planet has passed in front of it. Using multiple small telescopes together provides scientists with a broader field of view than a single large telescope like Kepler.

Earth 2.0’s six telescopes will look at approximately 1.2 million stars across a 500-square-degree patch of sky, which is approximately 5 times wider than Kepler’s view. At the same time, Earth 2.0 will be able to observe fainter and farther away stars than NASA’s Transiting Exoplanet Survey Satellite (TESS), which surveys bright stars close to Earth.

“Our satellite can be 10–15 times more powerful than NASA’s Kepler telescope in its sky-surveying capacity,” says Ge.

The seventh instrument on the satellite will be a gravitational microlensing telescope for surveying rogue planets (free-roaming celestial objects that do not orbit any star) and exoplanets (like Neptune) that are far from their star. It detects changes in starlight when the gravity of a planet or star warps the light of a background star in front of which it is passing.

The telescope will be aimed at the center of the Milky Way, which contains a massive number of stars. According to Ge, if the launch is successful, this will be the first gravitational microlensing telescope to operate from space.

“Our satellite can essentially conduct a census that identifies exoplanets of different sizes, masses and ages. The mission will provide a good collection of exoplanet samples for future research,” he says.

Doubling the data

Kepler was launched by NASA in 2009 with the goal of determining how common Earth-like planets are in the Galaxy. Astronomers must measure the time it takes an exoplanet to orbit its sun in order to confirm that it is Earth-like. Such planets should have a similar orbital period to Earth and transit their suns about once a year.

According to Chelsea Huang, an astrophysicist at the University of Southern Queensland in Toowoomba, scientists need at least three transits to calculate a precise orbital period, which takes about three years of data, and sometimes more if there are gaps in the data.

However, four years into the Kepler mission, parts of the instrument failed, preventing the telescope from staring at a single patch of sky for an extended period of time. According to Huang, who has worked as a data-simulation consultant with the Earth 2.0 team, Kepler was on the verge of discovering some truly Earth-like planets.

Astronomers may have another four years of data with Earth 2.0, which, when combined with Kepler’s observations, may help to confirm which exoplanets are truly Earth-like.

“I am very excited about the prospect of returning to the Kepler field,” Christiansen says, adding that he hopes to study Earth 2.0 data if it becomes available.

Ge is hoping to discover a dozen Earth 2.0 planets. He says he intends to publish the data within a year or two of its collection. “There will be a lot of data,” he says, “so we’ll need all the hands we can get.” The team already has about 300 scientists and engineers, the majority of whom are Chinese, but Ge hopes that more astronomers from around the world will join. “Earth 2.0 provides an opportunity for improved international cooperation.”

The European Space Agency is also planning an exoplanet mission called Planetary Transits and Oscillations of Stars (PLATO) for 2026. PLATO’s design includes 26 telescopes, giving it a much larger field of view than Earth 2.0. However, the satellite will shift its focus every two minutes. However, the satellite’s gaze will be shifted every two years to observe different regions of the sky.

REFERENCE/S:
China is devising a strategy to find Earth 2.0.|nature.com

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