As of this article’s writing, NASA has indicated that 5,030 extrasolar planets have been confirmed in 3,772 systems, with another 8,974 candidates awaiting confirmation. With next-generation instruments like the James Webb Space Telescope (JWST) coming online, the number and diversity of confirmed exoplanets are expected to grow exponentially. In particular, astronomers anticipate that the number of known terrestrial planets and Super-Earths will drastically increase.

In the coming years, the opportunities for exoplanet studies will increase considerably as thousands more are discovered using various methods. In a recent study, a team led by the Chinese Academy of Sciences (CAS) described a new space-telescope concept known as the Closeby Habitable Exoplanet Survey (CHES). This proposed observatory will search for Earth-like planets in the habitable zones (HZs) of Sun-like stars within approximately 33 light-years (10 parsecs) using a method known as micro-arcsecond relative astrometry.

The branch of astronomy known as astrometry consists of taking precise measurements of the positions and proper motions of celestial bodies by comparing them to background reference stars. Examples of this method include the ESA’s Gaia Observatory, which has been measuring the motion of 1 billion stars in the Milky Way (as well as 500,000 distant quasars) since 2013. This data will be used to create the most precise three-dimensional map of our Galaxy ever made.


Artist’s impression of the ESA’s Gaia Observatory. Credit: ESA

In this case, researchers from the Chinese Academy of Sciences (CAS) and multiple Chinese observatories and Universities propose a space telescope that could take high-precision astrometry measurements of Sun-like stars to detect exoplanets orbiting them. The proposed CHES mission will operate at the Sun-Earth L2 Lagrange point – where NASA’s James Webb Space Telescope (JWST) currently resides – and observe target stars for five years. These targets will include 100 stars within 33 light-years of the Solar System that fall into the F, G, and K types.

Whereas F-type stars (yellow-white dwarfs) are hotter, brighter, and more massive than our Sun, G-type stars (yellow dwarf) are consistent with our Sun – a main-sequence G2V star. Meanwhile, K-type stars (orange dwarf) are slightly dimmer, cooler, and less massive than our Sun. For each star it observes, CHES will measure the small and dynamical perturbances induced by orbiting exoplanets, which will provide accurate estimates of their masses and orbital periods.

As a space-based observatory, CHES will not be subject to interference due to Earth’s precession and atmosphere and will be able to make astrometry measurements accurate enough to fall into the micro-arcsecond domain. Dr. Jianghui Ji is a professor at the CAS Key Laboratory of Planetary Sciences in Nanjing, the University of Science and Technology, and the lead author on the study. As he told Universe Today via email:

“For an Earth-mass planet at 1 AU around a solar-type star at 10 pc, the astrometry wobble of the star caused by the Earth Twin is 0.3 micro-arcsecond. Thus the micro-arcsecond level measurement is required. The relative astrometry for CHES can accurately measure micro-arcsecond level angular separation between one target star and 6-8 reference stars. Based on the measurements of these tiny changes, we can detect whether there are terrestrial planets around them.”

Specifically, CHES will make the first direct measurements of the true masses and inclinations of Earth analogs and super-Earths that orbit within their stars’ HZ and are considered “potentially habitable.” The primary payload for this mission, said Dr. Ji, is a high-quality mirror with a diameter of 1.2 meters (ft) and a field of view (FOV) of 0.44° x 0.44°. This mirror is part of a coaxial three-mirror anastigmat (TMA) system, where three curved mirrors are used to minimize optical aberrations.

CHES also relies on Mosaic Charge-Coupled Devices (CCDs) and the laser metrology technique to conduct astrometric measurements in the 500nm~900nm range –
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