China could soon begin its first space-based hunt for exoplanets, if a proposal from the Shanghai Astronomical Observatory (SAO) gets the go-ahead this summer.
The Earth 2.0 Telescope would spend four years orbiting sun-Earth Lagrange point 2, about 930,000 miles (1.5 million kilometers) from Earth. There, it would fixed its seven telescopes on a portion of the sky toward the galactic center and watch for signs of dimming as planets transit, or pass in front of, a star as they orbit.
The main targets are roughly Earth-size exoplanets with similar orbits around sun-like stars. This requires high sensitivity to spot the signals of small-planet transits, as well as long-term monitoring to glimpse planets that take an Earthly year to go around their star.
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The Earth 2.0 Telescope would not be able to confirm an Earth twin on its own; rather, it would measure planets' sizes and orbital periods to identify candidates for follow-up observations for potential habitability, Ge Jian, a professor at the SAO, told Space.com via email.
"These planet candidates can be followed up with ground-based telescopes to obtain radial velocity measurements to determine their masses and densities," Ge said. "Some of these planet candidates around bright stars can be further followed up with ground-based or space-based spectroscopy to obtain transmission spectra of planets to study their atmospheres' compositions."
The mission would follow up on observations of an area of space that NASA's Kepler space telescope studied for nine years, but the Earth 2.0 Telescope would have a much greater field of view, meaning it would be able to observe a larger area and more stars, Ge said.
Kepler's field of view was 115 square degrees; it observed half a million stars and discovered 2,392 exoplanets, with a similar number of candidate planets awaiting confirmation. Although the telescope detected some terrestrial planets, none around sunlike stars were potential Earth twins.
The Earth 2.0 Telescope, in comparison, would cover 500 square degrees and monitor 1.2 million dwarf stars for four years with six of its seven 11.8-inch-aperture (30 centimeters) telescopes. For reference, the apparent area of the moon in the sky is about 0.5 square degrees, while the whole sky is about 41,000 square degrees. The telescope would also be able to view dimmer and more distant stars, adding to its survey capabilities.
"As the transit method is a statistical game, the more suitable solar-type stars you search, the higher chance you would detect an Earth 2.0," Ge said. "If the Earth 2.0's occurrence rate is 10%, then we need to search roughly 2,000 relatively bright, quiet solar-type stars to detect an Earth 2.0's transit.
"Our survey simulations show that we expect to detect about 30,000 new planets, including around 5,000 terrestrial-like planets, by our ET [Earth 2.0 Telescope] mission," Ge said, adding that the six-telescope design and better signal-to-noise ratio detectors would also boost its capabilities.
The seventh telescope, meanwhile, would have the sensitivity to detect cold or free-floating rocky planets — also known as rogue planets — as small as Mars, by looking for the effects of a planet’s gravity bending starlight as it passes by. It would also be able to spot cold planets orbiting stars at distances comparable to those at which Mars and Neptune orbit the sun.
"The ability of the Earth 2.0 telescope to continue the survey started by Kepler, and extend this to planets on longer and cooler orbits, is incredibly exciting," said Elizabeth Tasker, an associate professor at the Japan Aerospace Exploration Agency. "Our current exoplanet discoveries have not yet been able to probe these regions thoroughly, leaving us unable to properly mine the data to seek trends and patterns that would tell us how planets form, including rocky worlds in similar orbits to the Earth."
Tasker and her students have been using machine learning to try to identify patterns. Better data would help reveal trends that could provide valuable insight into planetary formation.
"This mission will provide a lot of data for the international planet-hunting communities to study, and also planet candidates for them to conduct follow-up studies to measure their properties such as masses, densities and atmospheric compositions," Ge said.
The Earth 2.0 telescope will be able to find Earth-size worlds on similar orbits to our planet's. "This is the first step in finding a planet that might be habitable," Tasker said.
But a planet's radius and orbit alone do not tell us about its surface conditions. "Such a planet might well host a Venus or Mars environment on its surface, or perhaps something even more alien still," Tasker said. "To discover if a planet is Earth-like and might be habitable or even inhabited, we will have to wait until we can probe the atmosphere or maybe even properties of the surface."
The Earth 2.0 proposal is part of a Chinese Academy of Sciences space-science satellite program. Other mission proposals are competing for funding in areas such as astronomy and space science, solar and space physics, planetary science, and Earth observation.
Decisions on funding are expected in June. If the Earth 2.0 Telescope mission is selected, the team will begin readying the satellite for a 2026 launch. Another exoplanet proposal, which seeks to find exoplanets by measuring how a star wobbles around the center of mass of the system due to the influence of planets' gravity, is also in the running.
China only recently began developing its own space-science missions, while other areas of space activities — such as human spaceflight, lunar exploration, and remote sensing and communications — have been thriving for years. China's first round of space-science missions included the Wukong dark matter probe, the Hard X-ray Modulation Telescope and the Mozi quantum science satellite, all of which launched between 2015 and 2017.
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