HELSINKI — China plans to launch an exoplanet observatory in 2028 with the aim of making a breakthrough detection of a potential second Earth.
Around 5,000 exoplanets have been found since 1995, but no Earth-sized planets in the habitable zones of sun-like stars have been spotted.
Earth 2.0, or ET, proposed by the Shanghai Astronomical Observatory under the Chinese Academy of Sciences (CAS), intends to use six 28-centimeter-aperture wide-field optical telescopes to observe about 2 million stars in the Kepler mission star field and other, larger nearby regions, continuously monitoring for transits of so-called exo-Earths over four years.
The mission is now targeting launch in 2028, according to a new paper authored by the mission principal investigator and others and published in the Chinese Journal of Space Science.
The spacecraft will be launched to Sun-Earth Lagrange point 2—the same gravitationally stable region of space as the James Webb Space Telescope—which will provide a stable orbit, a constant view of deep space, and reduced interference from Earth.
There, ET’s optimized transit telescopes will deliver high photometric precision—the accuracy and consistency with which the telescopes can measure the brightness of stars—that will allow for the detection of small, rocky planets, previously beyond the scope of missions such as NASA’s Kepler exoplanet observatory.
While Earth-like planets have been detected, these have been found in relatively short orbits around quite bright stars or low mass red dwarfs, which emit strong radiation. ET will be able to stare at its target patches of sky for long periods. This extended observation time will allow it to detect planets with longer orbital periods in the habitable zones around sun-like stars, and thus potentially pick up signals of exo-Earths.
Key questions and wandering Earths
The mission will hone in on three key questions: the prevalence of exo-Earths in the galaxy, the formation and evolution of Earth-like planets, and the origin of free-floating planets.
The mission is well-placed to contribute significantly to the search for Exo-Earths, according to Jessie Christiansen, chief scientist at the NASA Exoplanet Science Institute at Caltech/IPAC.
“Given our improved knowledge about the frequency of Earth-like planets, the ET team have been able to design a survey that is much more likely than Kepler was or [the European Space Agency’s] PLATO will be to detect these planets.
“This primarily involves a much larger stellar sample on which the mission will obtain high-precision photometry needed to detect Earths, achieved with both a larger field of view than Kepler and high precision to a fainter magnitude,” Christiansen told SpaceNews.
The question of rogue planets will be investigated using a 35 cm microlensing telescope. That instrument will stare at around 30 million stars in the Galactic bulge to detect microlensing events caused when free-floating, or “rogue” planets. These events occur when planets produce gravitational lensing effects on the light of background stars, picked up by noting characteristic anomalies in the star’s brightness curve. The hope is to find a “wandering Earth,” floating free of stars in the void of deep space.
Tech progress, impact on research
Progress on ET is going well, according to the paper. It describes advanced progress on key technologies for the mission, including the CMOS detector for photometric precision, satellite stability and thermal control. All are near-ready for flight.
Once at Sun-Earth L2 and fully operational, ET can begin filling cutting-edge work in searching for exo-Earths, something that otherwise may not happen for a while.
“Currently the only mission scheduled to fly in the next decade that may detect Earth-like planets is NASA’s Nancy Grace Roman Space Telescope, and that will only detect them with microlensing – a fleeting measurement of a distant signal that disappears rapidly and doesn’t appear again. These planets will be useful for statistically understanding the habitable real estate of the galaxy, but won’t represent the valuable, individual planets that we want to characterize in detail with other telescopes.”
The Roman Space Telescope is expected to launch in 2027.
“Beyond that, NASA’s next flagship mission, the Habitable Worlds Observatory (HWO), is 20-plus years away. If ET is funded and flies in the next decade, it may be able to detect nearby, characterizable habitable planets a decade earlier than otherwise planned.”
Not only does the mission promise candidate exo-Earths, it will allow for follow up observations of such candidates to tease out further characteristics.
ET will work with China’s LAMOST ground-based optical telescope to carry out spectral observations of observation targets, but also other teams and observatories around the world and in space.
This will lead to precise measurement of the mass, density and atmospheric composition of any exo-Earth candidate, contributing to an in-depth study of habitability characteristics, according to the paper.
“Game-changer”
Wang Chi, director of the National Space Science Center (NSSC) under CAS, revealed in April that the ET mission had been selected from a field of astronomical research and space exploration missions. Lunar farside astronomy, extreme space physics, a solar observatory and gravitational wave missions were also approved.
Each of the selected missions looks to push the boundaries of knowledge. And ET could be a game-changer, according to Christiansen.
“We’ve been hunting for Earth 2.0 for a long time, and have been stymied at every junction so far,” says Christiansen. “If ET is able to finally and robustly find a rocky planet in the habitable zone of a Sun-like star, it would be an incredible achievement.
“If it is a planet we can study with other telescopes, such as JWST or, in the future, HWO, it will be a game-changer.”
ObjectiveDescription1. Discover Exo-EarthsTo be the first mission to discover Earth-like exoplanets (Exo-Earths) in the habitable zones of sun-like stars, and measure their occurrence rates.2. Expand Sample of Earth-like PlanetsTo significantly increase the known sample of Earth-like planets, especially those with long orbital periods, for detailed population studies and formation analysis.3. Study Planetary Formation and EvolutionTo explore the formation mechanisms and evolutionary processes of Earth-like planets and other small, rocky exoplanets by conducting statistical population studies.4. Detect Free-Floating PlanetsTo be the first mission to discover and measure the frequency of free-floating (rogue) Earth-like planets, contributing to the understanding of planetary system formation.5. Conduct Microlensing SurveysTo utilize microlensing to detect long-period cold planets and free-floating planets, including detailed characterization of their mass and other properties.6. Combine Transit and Microlensing MethodsTo use a combination of transit and microlensing methods to enhance the likelihood of discovering a wide range of exoplanets, including those difficult to detect by other means.7. Enable Future Exoplanet ResearchTo provide targets and critical data for future direct imaging missions and other exoplanet studies, supporting the next generation of space exploration missions.A table presenting the key objectives of the Earth 2.0/ET mission.