Two years ago we were able to learn about several Chinese space telescope projects aimed at studying exoplanets. One of these proposals was the Earth 2.0 (ET) Observatory, also called Earth 2.0 (地球2.0) or 系外地球 (I’m fine“exo-Earth” in Mandarin), is designed to search for exoplanets using transit methods and micro-gravitational lensing. Recently, ET/Earth 2.0 was confirmed, and if all goes well, it will launch in 2028. Tierra 2.0 is designed as a 3.2-ton observatory that combines the features of NASA’s Kepler telescope and the European PLATO mission. While it won’t be as ambitious and complex as the European Space Agency’s satellite, Earth 2.0 hopes to discover between ten and twenty exoplanets with similar dimensions to Earth — 0.8 to 1.25 Earth radii — and located in the habitable zone like Earth’s Sun, i.e. exo-Earths, or “Earth 2.0s.”
Earth 2.0 will observe the sky from the L2 Lagrange point in the Earth-Sun system (ESL-2) using multiple telescopes to cover a larger surface than the celestial sphere, a system used by planet hunters such as TESS or PLATO. It will have six 28-cm telescopes with a field of view of 550 square degrees each to search for exoplanets using the transit method and a 35-cm telescope with a field of view of 4 degrees to detect exoplanets using the microlensing method. The six main telescopes will observe about two million stars of the spectral type FGKM during the four years of the primary mission. Each telescope includes a -40 °C cooled camera with four GSENSE1081 CMOS sensors with a resolution of 8900 × 8210 pixels. By the end of this mission, Earth 2.0 hopes to discover more exoplanets than PLATO: up to twenty, compared to two or three for the European observatory (the exact number will obviously depend on the actual frequency of exoplanets around the Sun). – The type of stars, a number we do not know exactly). The Chinese mission plans to discover a greater number of Earth-like worlds despite being a less sophisticated observatory, because it will use a “trick”: observing the same star field as Kepler.
While this means that Earth 2.0 will “rediscover” about 2,700 exoplanets that Kepler discovered in the same star field, the dataset collected by NASA’s observatory over four years can be used to improve the signal-to-noise ratio (Kepler discovered a total of 2,778 exoplanets, but some during the extended mission when they were no longer pointing to the original field). As a result, Earth 2.0 is expected to discover about 8,100 giant Jupiter- or Saturn-type planets, about 22,000 exoneptons — the most common type of planet — 11,000 super-Earths, and about 3,200 Earth-sized planets. Due to biases in the transit method, most of these worlds will be very close to their stars. For its part, the 35-centimeter telescope will observe approximately thirty million stars in the central region of the Milky Way for half a year each year, and it is believed that it will be able to detect about a thousand exoplanets using the gravitational microlensing method – a planet temporarily increases the brightness of a background star thanks to general relativity, and its mass can be estimated at around 300. Most of them will be wandering worlds wandering among the stars, which will allow us to better understand the inhabitants of this type of object.
Earth 2.0 will have a payload mass of 1.2 tons — remember, the total mass of the satellite is 3.2 tons — and the mission is expected to send up to 730 gigabits of data back to Earth each day (each transit telescope generates an image every 10 seconds and the microgravity lens telescope every 10 minutes). Its average power consumption is about 1.5 kilowatts. It will launch in 2028 using the CZ-3B Long March or CZ-7A, which will put it into a halo orbit with a radius of 13,500 kilometers around the ESL-2 point. Unlike the larger academy led by the Chinese Academy of Sciences (CAS), which is developing the project in collaboration with the Shanghai Astronomical Observatory (OAS) and several institutes and universities across the country. The project manager is Ji Jian (葛健) of the Organization of American States.
If the missions proposed by CASC under the instructions of the central government are similar to NASA’s flagship missions, the space missions led by the Academy of Sciences are more similar to Discovery or New Frontiers missions in the sense that they are proposals that have emerged from universities and research institutes across the country and which must then undergo a selection process. In the case of Earth 2.0/ET, the mission was selected at the expense of CHES (Nearby Habitable Exoplanet SurveyCHES). CHES was a space telescope with a 1.2-meter-diameter monolithic mirror, and was intended to search for potentially habitable planets relatively close by, up to 32 light-years away. Unlike other proposals, CHES planned to search for exoplanets using astronomical measurements.
Earth 2.0/ET does not have the ability to analyze the exoplanets it discovers, and will only be able to detect them. But these discoveries will be essential for future space- and ground-based telescopes that will have to characterize these worlds for signs that they could be habitable, such as the ambitious HWO (Habitable Worlds ObservatoryNASA’s Miin (觅音) and Tianlin (天邻) missions in China. Miin consists of five independent space telescopes that will collect their images using optical interferometry to achieve very high resolution. With the units at ESL-2, Miin could be deployed before 2030, after which telescopes could be added to increase resolution and sensitivity. Tianlin, for its part, is a proposal for a large space telescope with a 6-meter telescope that will be launched by CZ-9 and will observe the sky in ultraviolet, visible, and near-infrared light. While it is very likely that the exoplanets discovered by Earth 2.0 are too distant to be accurately characterized by these future missions, there is no doubt that finding planets that are potentially similar to our own is an exciting challenge.
References:
- https://cjss.ac.cn/cn/article/doi/10.11728/cjss2024.03.yg05
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