Searching for life on distant planets requires lots of data, collected by precise scientific instruments. While much is gained by studying the Earth, the high diversity of lifeforms on our planet is essentially just one data set. By studying planets outside our Solar System – known as exoplanets – scientists can examine the conditions for planet formation and the emergence of life.
The detection of the first planet outside of our Solar System was confirmed in 1992. Since then, scientists have discovered more than 6,000 exoplanets in 4,500 different systems – the vast majority of which have no analogues in our Solar System. Many of these discoveries have been made with Teledyne technology running on major astronomy missions such as the Hubble Space Telescope, Tess, CoRoT, Kepler, and the HARPS and ESPRESSO spectrographs of the European Southern Observatory (ESO). Now that these exoplanets have been documented, astronomers want to know what they look like.
CHEOPS, in the clean room at Airbus Defence and Space Spain, Madrid, in February 2019. Credit: ESA - S. Corvaja
Launched in 2019, CHEOPS (Characterizing Exoplanet Satellite) will target stars already known to have planets; in particular, Earth-to-Neptune-sized planets — one to six Earth radii — also called super-Earths or sub-Neptunes. Utilising a microsatellite bus, CHEOPS is the first small-class mission from ESA, selected in October 2012 and chosen for implementation in Feb. 2014. The mission is a partnership between the ESA Science Program and Switzerland, through the Swiss Space Office (SSO). The University of Bern leads a consortium of 11 contributing ESA Member States represented in the CHEOPS Science Team.
Planetary Transits and First-Step Characterisation
CHEOPS measures exoplanet transits – the dip in light caused when a planet passes in front of its star from the telescope’s point of view. Studying the pattern of the resulting light curve reveals details about the planet’s size. Once the size has been calculated, scientists can combine the known mass, as previously calculated by other observatories, to find the planet’s density and determine a first-step characterisation of the nature of these worlds, including their atmosphere and internal composition: rocky like Earth, gas like Jupiter, or a blend like Neptune.
By understanding their composition and structure, CHEOPS seeks to answer fundamental questions about the diversity of planetary systems in our galaxy, conduct comparative studies, and set new constraints on their structure, formation and evolution. This will not only deepen our understanding of exoplanets, but also that of the Earth and the wider cosmic environment.
The signal of an exoplanet transit can be extremely weak, especially for small planets. Accurate measurements depend on the satellite and its photometer remaining highly stable. CHEOPS features a stiff optical bench mounting that supports a pointing error of less than eight arcseconds of jitter during a 48-hour observation, equivalent to an Olympic archer maintaining a steady aim at the bullseye from a distance of over 3 km (1.8 mi). This means that while the telescope observes a star for hours while the spacecraft moves along its orbit, the image of the star remains within the same group of pixels in the photometer’s detector. Staying within strict boundaries helps ensure that measured changes in light are a result of exoplanet transits, rather than variations caused by the motion of the telescope.
Precise photometric transit measurements demand the lowest possible level of signal noise. Besides the noise associated with the detector itself, stray light reflected off satellite surfaces or outside celestial objects is the main source of noise. Engineers needed to ensure that the only light falling on the CHEOPS detector comes from the star itself. Stray light is minimised through the design of the telescope and by limiting the directions in which the telescope points to avoid solar light reflected by the Earth and/or the Moon from reaching the detector.
Further noise reduction comes from the Sun-synchronous, polar orbit in which CHEOPS is deployed, positioned roughly along Earth's terminator where day turns to night, at an altitude of 800 km (435 mi).
The high precision and pointing flexibility of CHEOPS enable the satellite to return to a target and observe multiple transits. By knowing exactly where and when to look for transits and being able to repeatedly record the same targets, Cheops is the most efficient instrument for studying individual exoplanets.
CHaracterising ExOPlanet Satellite (CHEOPS) Instrument
The bronze-coloured Focal Plane Module (FPM), which houses the CCD detector array and the Front-End Electronics, is seen here mounted on the flat, black optical bench at the rear of the downward-pointing telescope tube (not seen). Credit: University of Bern, ESA
The mission payload of CHEOPS consists of a single instrument: a high-precision photometer attached to a 1.2 m (3.9 ft) Ritchey–Chrétien telescope with a 30 cm (12 in) aperture. The focal plane assembly of the photometer features two back-side illuminated CCD47-20s produced by Teledyne. Each CCD has a resolution of 1024 x 1024, 13 µm pixels, and employs frame transfer and AIMO operation. Detecting light in the VIS/NIR (0.33 - 1.1 µm) range, CHEOPS features a precision of 20 ppm in six hours of integration time for a 9th magnitude star and a precision of 85 ppm in three hours integration time for Neptune-size planets orbiting a 12th magnitude star. This corresponds to a signal to noise of five for a transit of an Earth-sized planet orbiting a solar-sized star of 0.9 solar radii.
Image of HD 70843, the star chosen as the first target for CHEOPS. The star, located around 150 light years away in the constellation of Cancer, is visible at the centre of the image, surrounded by fainter stars in the background. The peculiar shape of the stars in the image displays the deliberate defocusing of the CHEOPS optics, which spreads the light from each star over many pixels. The triangular appearance of the stars is a known effect of the three struts that support the telescope's primary mirror. The image covers about 1000 × 1000 pixels, with each pixel edge representing a tiny angle of about 0.0003 degrees on the sky, equivalent to less than one thousandth of the full Moon's diameter. The inset in the lower right corner shows a region covering about 100 × 100 pixels, centred on the target star. Credit: ESA/Airbus/Mission Consortium
The optical configuration employs additional lenses to de-focus the image of the target star with a PSF (Point Spread Function) covering an area of ~765 cm2 (12.5 in2). Though it may seem counterintuitive, this deliberate “blurring” increases the accuracy of measurements made by CHEOPS since the light from each star is distributed over many pixels, making the instrument far less sensitive to small differences in individual pixel responses to light. It also makes the photometer less susceptible to changes in its orientation.
Achievements
CHEOPS has observed a diverse range of exoplanets, many of which exhibit unusual characteristics, such as large atmospheres and eccentric orbits. These discoveries are helping scientists gain a better understanding of the diversity of exoplanets in our galaxy and the processes that influence their formation and evolution.
- LTT 9779b - Ultra-hot exoplanet that orbits its host star in less than a day. It is covered by reflective clouds of silicate mixed with metals like titanium, making it the shiniest exoplanet ever found.
- WASP-76 b - Exoplanet featuring hints of a phenomenon known as glory – colourful concentric rings of light. This is significant because it is the only known planet outside of Venus to exhibit this phenomenon.
- WASP-103b - An exoplanet orbiting its host star within a day, featuring a deformed oblong shape – more like that of a rugby ball than a sphere. This is the first time that the deformation of an exoplanet has been detected, offering new insights into the internal structure of these star-hugging planets. The high precision of CHEOPS detected a minute signal of tidal deformation, which is responsible for the planet’s shape – an exceptional object with no known equivalent.
- TOI-178 - A unique planetary system consisting of six exoplanets, five of which are locked in a rare rhythmic dance as they orbit their central star. This phenomenon – called orbital resonance – creates repeating patterns, with some planets aligning every few orbits. Planets around stars tend to form in resonance but can be easily disrupted. Accordingly, multi-planet systems that preserve their resonance are rare, with only one per cent remaining in resonance.
- HIP 67522 b - An exoplanet that is exerting its own magnetic influence on its host star due to a very close orbit. The prevailing theory suggests that the planet gathers energy as it orbits and then redirects that energy as waves along the star’s magnetic field lines, much like whipping a rope. When the wave meets the end of the magnetic field line at the star’s surface, it triggers a massive flare, causing the planet to experience six times the radiation it would experience normally. The planet is similar in size to Jupiter but has a density comparable to that of candy floss, making it one of the wispiest exoplanets ever discovered. This is the first time a planet has been seen influencing its host star, overturning the previous assumption that stars behave independently.
- Quaoar - A dwarf planet in our Solar System, with a very unusual ring surrounding it. Too faint to be spotted by any other method, the ring is extraordinary because it is several times further out from its parent body than previously thought possible for a ring system, which may alter theories of ring formation.
Ongoing Science Operations and Support for Other Missions
Well past the 3.5-year science operations lifetime, CHEOPS will continue operations until 2026, as confirmed by ESA’s Science Programme Committee, with a potential extension until 2029, contingent upon ongoing commitments from national contributors and partners.
CHEOPS is providing unique and well-characterised targets to maximise the scientific return of other missions with Teledyne technology. This includes current missions such as the James Webb Space Telescope, and upcoming missions: PLATO (Planetary Transits and Oscillations of stars), Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey), and the ELT (Extremely Large Telescope).
Works Cited
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ESA. (2025, July 2). Clingy planets can trigger own doom, suspect Cheops and TESS. https://www.esa.int/Science_Exploration/Space_Science/Cheops/Clingy_planets_can_trigger_own_doom_suspect_Cheops_and_TESS
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Sentinel Mission. (2025, October 25). CHEOPS (Characterising Exoplanet Satellite) – Definition & Detailed Explanation – Telescopes & Observatories Glossary. https://sentinelmission.org/telescopes-observatories-glossary/cheops-characterising-exoplanet-satellite/
Space.com. (2023, March 12). CHEOPS: A guide to ESA's exoplanet investigating mission. https://www.space.com/36144-cheops-exoplanet-satellite.html
The Planetary Society. (n.d.). CHEOPS, measuring exoplanets. https://www.planetary.org/space-missions/cheops
