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CHEOPS

CHEOPS (CHaracterising Satellite) is a mission led by the (ESA) dedicated to measuring the sizes of known transiting around bright, nearby stars using ultra-high precision photometry. Launched on 18 December 2019 from Europe's in , , aboard a / , the 280 kg operates in a sun-synchronous at an altitude of approximately 700 km. As ESA's first small (S-class) mission under the 2015–2025 programme, selected in October 2012, CHEOPS features a single instrument: an on-axis Ritchey-Chrétien with a 30 cm clear , which directs light to a frame-transfer back-illuminated detector for broadband visible-light observations over a 19 × 19 arcminute . The mission's core objective is to determine the radii of exoplanets ranging from to sizes, enabling the calculation of their bulk densities when combined with measurements from ground-based telescopes, thus providing insights into their internal structures, compositions, and formation histories. CHEOPS targets brighter than 12 that host previously detected exoplanets, achieving photometric precision of 150 parts per million for a 7th-magnitude star over six hours, and has observed hundreds of such systems during its nominal 3.5-year phase. In March 2023, the mission was extended until the end of 2026, allowing continued contributions to science, including searches for exomoons and detailed studies of multi-planet systems. Notable achievements include precise characterizations of over 50 (as of ), such as measurements of the WASP-18b's radius and the unlocking of a rare resonant six-planet system around in 2023, as well as 2025 findings of an potentially triggering explosive flares on its host star HIP 67522 b and "clingy" planets susceptible to . The mission is a collaboration involving ESA, as the primary partner, and contributions from , , , , , , , , , and the , with scientific operations supported by the CHEOPS Science Ground Segment.

Mission Overview

Objectives and Scope

The CHaracterising Satellite (CHEOPS) mission is dedicated to measuring the radii of known transiting exoplanets in the super-Earth to size range, spanning 1 to 6 radii, that orbit bright stars with visual magnitudes less than 12. By providing precise photometric observations of these transits, CHEOPS enables the determination of planetary densities when combined with ground-based mass measurements, offering insights into the bulk compositions of these worlds. The mission's scope is narrowly focused on characterizing already discovered transiting exoplanets, with no capability for detecting new ones, emphasizing high-precision follow-up observations rather than wide-field surveys. CHEOPS targets stars bright enough to facilitate complementary ground-based , prioritizing those hosting planets with orbital periods under 50 days. The instrument covers a broad range from 330 to 1100 , capturing visible and near-infrared to achieve photometric stability. A key performance goal is to reach a of 10 parts per million () for a 6-hour observation of a 9th-magnitude , enabling radius measurements accurate to about 10% for planets in the 1–6 radii range. Launched in December 2019, CHEOPS operates with a nominal duration of 3.5 years, which has been extended through the end of 2026 to maximize scientific return. This extension allows for continued observations of priority targets, including those that probe the mass-radius relationship for low-mass . The 's characterization priorities center on elucidating the formation mechanisms, evolutionary pathways, and potential atmospheric properties of these exoplanets, particularly how they differ from solar system bodies and inform models of planetary diversity. Approximately 20% of the observing time is allocated for guest investigator programs, broadening community access to the .

Scientific Significance

The CHaracterising Satellite (CHEOPS) represents the first (ESA) mission dedicated exclusively to the characterization of s, marking a pivotal step in ESA's 2015-2025 program. As such, it addresses critical gaps in exoplanet demographics by focusing on planets with radii between and (approximately 1-6 radii), bridging the observational divide between terrestrial worlds and gas giants. This emphasis is particularly significant for probing the "radius gap" observed around 1.5-2 radii, a transitional zone where few planets are detected, potentially revealing insights into atmospheric retention and planetary evolution. CHEOPS's unique value lies in its high-precision photometric follow-up of known exoplanets identified by missions like Kepler and TESS, as well as ground-based surveys. By targeting bright host stars, it achieves radius measurements with approximately 10% precision, enabling the construction of detailed mass-radius diagrams when combined with data. These diagrams facilitate inferences about planetary compositions, distinguishing rocky cores from those enveloped in hydrogen-helium atmospheres and testing models of core accretion and envelope collapse. Beyond demographics, CHEOPS contributes to broader understandings of planet formation, , and potential by constraining the es and structures of low-mass planets. For instance, it helps discern migration pathways through analysis of orbital and atmospheric mass loss influenced by stellar irradiation. This work synergizes with future observatories like the (JWST), providing precisely characterized targets for atmospheric to explore habitability indicators.

Development and History

Proposal and Approval

The CHaracterising ExOPlanet Satellite (CHEOPS) mission originated from conceptual work initiated in 2009 by a consortium led by the , under the direction of principal investigator Willy Benz, in response to a Swiss National Science Foundation call for feasibility studies on characterization missions. This early effort built on growing interest in precise photometry for known , leading to a formal proposal submitted to the (ESA) in March 2012 as part of the call for small (S-class) missions within the 2015–2025 programme. CHEOPS was selected on 19 October 2012 from 26 competing proposals, marking it as the inaugural S-class mission in ESA's programme, designed for rapid development and lower costs compared to larger missions. Following selection, a competitive Phase A/B1 definition study commenced in 2013, issued via an at the end of March, to refine the mission design and assess feasibility within stringent constraints. The mission was formally adopted by ESA's Science Programme Committee in February 2014, transitioning to full implementation after successful completion of the definition phase. As an ESA-Switzerland partnership led by the Swiss Space Office, CHEOPS involved contributions from ten additional ESA Member States—Austria, , , , , , , , , and the —totaling eleven countries in the . Funding was structured to align with S-class guidelines, with ESA capping its contribution at €50 million (including launch), while Switzerland provided primary non-ESA support through the Swiss Space Office, supplemented by investments from partner nations to cover the overall mission cost estimated at around €100 million. This distributed model emphasized international collaboration to distribute technical and financial responsibilities. The proposal process highlighted key challenges inherent to S-class missions, including an ESA cost cap of €50 million, a total mission cost limit of approximately €150 million, and a compressed development timeline of approximately four years from adoption to launch. To meet these, the design prioritized cost-effectiveness through the use of off-the-shelf components, commercial hardware where possible, and a modular that minimized custom , ensuring high scientific return within fiscal and schedule bounds.

Construction and Testing

The construction of the CHEOPS spacecraft was led by in as the prime contractor for the satellite platform, while the science instrument was developed by a consortium headed by the , with contributions from the and other European partners. The implementation phase (Phases B/C/D) began in July 2014 following project approval, encompassing detailed design, procurement, assembly, integration, and testing, and extended through 2019 until launch readiness. Key milestones included the delivery of the fully assembled science instrument to Airbus in Madrid in May 2018, enabling the start of spacecraft-level integration later that year. The platform and payload were then integrated into the complete satellite at Airbus facilities, with initial functional checks verifying subsystem interfaces and overall architecture. In July 2018, the integrated spacecraft was shipped to Airbus's Engineering Validation Test facility in Toulouse, France, for thermal-vacuum testing, where it was subjected to simulated space conditions, including cooling to -165°C to assess thermal stability and performance under vacuum. Following this, it underwent mechanical vibration and shock tests at RUAG Space in Zurich, Switzerland, to simulate launch loads and confirm structural integrity. The satellite was then transported to the European Space Research and Technology Centre (ESTEC) in , , for comprehensive system-level testing in 2018, including end-to-end simulations that confirmed the photometric stability required for measurements, achieving noise levels below 150 ppm for bright stars. These environmental and system verifications ensured the met mission requirements for precise photometry in the harsh space environment. To control costs within the S-class mission budget, CHEOPS incorporated (COTS) components where feasible, balancing reliability with affordability. Additionally, the instrument's baffle was specifically designed and extended to suppress from and , enhancing observation accuracy through minimized background noise.

Launch and Commissioning

The CHEOPS satellite was launched on 18 December 2019 as a secondary payload on the Soyuz-2.1b/Fregat VS23 mission, operated by Arianespace from Europe's Spaceport in Kourou, French Guiana. The mission also carried the Italian CSG-1 Earth observation satellite as the primary payload. Following separation from the Fregat upper stage approximately 90 minutes after liftoff, CHEOPS was inserted into its nominal Sun-synchronous dusk-dawn orbit at an altitude of 700 km and an inclination of 98.3°. The Launch and Early Orbit Phase (LEOP) proceeded nominally, with initial spacecraft activation and health checks confirming stable communications and basic functionality; LEOP was successfully completed on 23 December 2019. In-orbit commissioning began in January 2020 and extended through March, encompassing detailed checkout of the subsystems, performance verification, and activities. Key milestones included the deployment and opening of the baffle and cover on 29 January 2020, followed by first light acquisition on 7 February 2020, which captured an intentionally defocused image of the star HD 70843 to assess the point spread function. During this phase, the satellite demonstrated pointing stability exceeding requirements, achieving better than 1 arcsecond root-mean-square error through fine-pointing s using the field and digital reconstruction processor. Minor thermal variations in the tube, causing ramps of 100–1000 over 2.5–12 hours, were identified and mitigated via detrending algorithms and optimized pointing strategies to ensure photometric precision. By late March 2020, commissioning confirmed that CHEOPS met or surpassed all performance specifications, including a measured flux 25–30% lower than pre-launch predictions due to aperture and refinements, which were accounted for in subsequent . Routine science operations commenced in April 2020, with the satellite allocated over 90% of its time to guest observer programs targeting transits.

Spacecraft Design

Overall Architecture

The CHEOPS adopts a compact, hexagonal-prismatic configuration optimized for low-cost characterization, with overall dimensions of approximately 1.5 m in height and a base footprint fitting within a 1.6 m circle. Its launch is 280 kg, including a of around 60 kg, while the dry is approximately 250 kg. The structure utilizes aluminum panels for lightweight rigidity and thermal stability. The platform is derived from the AS-250, a flight-proven bus that enables three-axis stabilization with high pointing accuracy suitable for precise photometry. Power is provided by body-mounted solar panels using GaAs triple-junction cells, delivering an average of about 150 W to support spacecraft and instrument operations. Subsystems include an S-band communication system with a transceiver for downlinks exceeding 1 Gbit per day, and an onboard computer based on a LEON-3 with 128 MB for command processing and handling. This architecture emphasizes cost efficiency and simplicity, leveraging components to minimize development risks while achieving the mission's photometric goals. The instrument benefits from via radiators exposed to deep , maintaining stable temperatures without active cryogenic systems.

Attitude and Orbit Control System

The Attitude and Orbit Control System (AOCS) of CHEOPS is a 3-axis stabilized platform designed to maintain precise for high-precision photometry while operating in a nadir-locked configuration, ensuring the remains Earth-oriented during orbits. The system relies exclusively on magnetic actuation for fine , with no chemical thrusters dedicated to attitude adjustments; instead, a separate 30 kg propulsion module handles occasional orbit-raising maneuvers using propellant to counteract atmospheric drag. This design prioritizes simplicity and reliability for the mission's platform, inherited from the DS AS250 line with custom adaptations for CHEOPS' requirements. Key components include a of four reaction wheels arranged in hot redundancy to provide primary for slewing and , enabling rapid reorientation between targets. Attitude determination is achieved using two star trackers mounted directly on the Assembly for high-fidelity measurements, supplemented by coarse sensors for redundancy. Magnetotorquers serve as actuators for wheel desaturation and compensation of environmental disturbances, such as magnetic s from Earth's field or residual momentum buildup. The system incorporates redundant sensor chains to ensure fault-tolerant operation, with onboard software processing star centroids from the science instrument to correct low-frequency pointing errors in . CHEOPS operates in a Sun-synchronous dawn-dusk at approximately 700 km altitude and 98° inclination, selected to minimize effects and thermal variations while providing repeated viewing opportunities. maintenance is performed semi-autonomously through ground-commanded firings, with over 3.5 years of operations consuming just 6.5 m/s of delta-v while preserving substantial margins for extension to a 5-year lifetime. No continuous compensation is required, as the 's stability supports the without frequent adjustments. The AOCS delivers pointing stability better than 1 arcsec over 48-hour visits, exceeding the requirement of 4 arcsec and enabling photometric down to 15 ppm for bright stars over 6 hours. This performance supports 1- to 5-day observation campaigns with over 95% efficiency in target illumination time, accounting for brief interruptions from occultations or slews. In-flight data confirm sub-arcsecond jitter in both PITL (Pointing In The Loop, using instrument feedback) and NOPITL modes, with ensuring continued operations despite minor anomalies.

Power and Thermal Management

The power subsystem of the CHEOPS is designed to generate, store, and distribute electrical reliably throughout its nominal 3.5-year lifetime, drawing on established flight heritage for and . Primary is provided by body-mounted arrays integrated into the sun shield, consisting of triple-junction GaAs cells with approximately 28% and a total active area of about 1.85 m² across three fixed panels angled at 67° for optimal exposure. These arrays generate an orbit-average of 138 , sufficient to support continuous operations at 60 while providing margins for other subsystems, including brief peaks up to 70 . is handled by a with 36 Ah capacity at 28 V, capable of sustaining full operations during up to 1-hour eclipse periods in the low-Earth . distribution occurs via an unregulated primary bus (22–34 V) with DC-DC converters providing regulated voltages, including redundant units for critical components like the back-end electronics, ensuring overall average consumption remains around 55–60 with end-of-life margins exceeding 20%. Thermal management emphasizes passive techniques to minimize power draw and maintain , supplemented by active elements for control. The system employs (MLI) blankets and oriented toward deep to passively cool key components, with the (CCD) detector maintained at 233 (-40°C) via a dedicated focal plane . Survival and operational heaters, including PID-controlled units on the tube and focal plane assembly, provide active stabilization, consuming approximately 20–25 W for the (targeting -10°C) and 3.9–4.2 W for the front-end electronics, with total in-orbit heating averaging 81.2 W. The nadir-pointing orbital configuration and sun shield further reduce thermal variations, achieving CCD temperature better than 0.05 /hour and overall instrument within 0.1 /hour to support photometric . Ground testing validated the integrated power and thermal performance, including a thermal vacuum/balance campaign in July 2018 that confirmed operation within design limits under simulated conditions, with models correlating well to early in-orbit from December 2019 launch and commissioning. The AOCS draws approximately 10–15% of total power for trackers and wheels, integrated seamlessly to handle transitions without subsystem interruptions.

Instrument System

Telescope Design

The CHEOPS telescope employs an on-axis Ritchey-Chrétien optical design, featuring a primary mirror with a physical diameter of 320 mm and an effective of 300 mm. The effective is 1600 mm, yielding an f-ratio of f/5, which supports high-precision photometry by delivering a compact image scale suitable for observations. The field of view spans 0.32 degrees (approximately 19 arcminutes square), enabling targeted monitoring of individual bright stars while minimizing contamination from nearby sources. The optical system consists of two aspheric mirrors fabricated from , a low-thermal-expansion glass-ceramic, both coated with protected silver to optimize reflectivity across the operational bandpass of 330–1100 nm. An uncoated fused silica entrance window protects the optics from contamination, and the design incorporates additional refractive elements to achieve a defocused , enhancing photometric stability against pointing errors. This configuration ensures broad spectral coverage without dedicated narrowband filters, prioritizing throughput in the visible and near-infrared regimes. Stray light suppression is achieved through an integrated baffle system, including internal vanes within the telescope tube and a central field stop to block off-axis light, complemented by an external aluminum baffle that extends beyond the secondary mirror. An mask further mitigates reflections from the and satellite structures, reducing background noise to levels below 1 photon per pixel per second under nominal orbital conditions. This design draws heritage from the CoRoT mission but is scaled for CHEOPS's compact and on-axis . Performance metrics confirm the telescope's suitability for ultra-precise photometry, with a defocused point spread function enclosing 90% of energy within a radius of approximately 12 pixels (corresponding to about 12 arcseconds at the plate scale of 1 arcsecond per pixel). Optical throughput exceeds 60% across the primary bandpass, accounting for mirror coatings, window transmission, and minor losses from the defocusing optics. The system operates in a diffraction-limited regime at wavelengths around 780 nm, ensuring sharp imaging for stars down to visual magnitude 12 while maintaining stability over long exposures.

Photometer and Detector

The photometer in CHEOPS employs a custom Focal Plane Module (FPM) centered on a high-sensitivity charge-coupled device (CCD) detector optimized for stable, low-noise photometry of bright stars. The core component is an e2v CCD47-20 back-illuminated frame-transfer CCD featuring a 1024 × 1024 pixel imaging array with a 13 µm pixel pitch, enabling a plate scale of approximately 1 arcsec per pixel across a 0.32° field of view. This configuration, combined with advanced inverted mode operation (AIMO), minimizes dark current while supporting rapid charge transfer from the image area to a masked storage area, facilitating continuous exposure without mechanical shutters. The detector's read noise is 7 e⁻ RMS when operated at a readout frequency of 100 kHz, contributing to the instrument's target photometric precision of 20 ppm for a 9th-magnitude star over 6 hours. Thermal management is achieved through passive cooling via a dedicated radiator oriented toward cold space, maintaining the CCD at a nominal temperature of 233 with no active cryocooler required; this setup ensures exceptional stability of 1 /min to mitigate noise from thermal variations. During commissioning, the operating temperature was adjusted to 228 for optimal performance, with overall stability better than 10 . The readout system incorporates 2×2 binning to enhance for lower-flux targets, supporting frame rates up to 1 kHz in binned modes while preserving the full well capacity of approximately 120,000 e⁻ per pixel. Quantum efficiency peaks at around 80% near 600 nm, providing high sensitivity across the visible bandpass (400–1100 nm) where exoplanet transits are observed. Observation data modes emphasize efficient photometry, with full-frame readout reserved for and sub-array windowing (typically 200 × 200 pixels centered on the target) employed for science observations of bright stars (V < 12 mag) to minimize data volume and readout time. Onboard processing includes , enabling the transmission of up to 1.2 Gbit/day of compressed science data via the S-band link (at ~1143 kbps during ground contacts) while preserving photometric accuracy. The Sensor Electronics Module () handles analog-to-digital conversion at 14 bits and initial , ensuring robust performance against cosmic rays and jitter through correlated double sampling.

Calibration Features

The in-flight calibration of the CHEOPS relies on the Monitoring and Characterisation (M&C) programme, a sky-based approach that compensates for the absence of an onboard shutter, , or other dedicated mechanisms, ensuring no moving parts are involved. This programme conducts periodic observations to monitor instrument stability, (PSF) evolution, and gain variations, using starlight illumination from dedicated sky fields to assess performance without introducing additional hardware. Key components of the M&C programme include weekly observations for bad pixel detection and dark current assessment, quarterly checks for random telegraph signal (RTS) pixels, and annual PSF monitoring across a grid of 25 positions on the CCD to track shape and position stability. These efforts enable the detection of flux sensitivity changes at the parts-per-million level, such as an initial degradation rate of -25 ppm per day that stabilized to -10 ppm per day, far exceeding the mission's requirements for photometric precision. Data from these observations are integrated into the Data Reduction Pipeline (DRP) for corrections, including hot pixel masking, dark current subtraction, and PSF fitting via the PSF Imagette Photometric Extraction (PIPE) tool, to mitigate systematics and maintain data quality. Flat-fielding, essential for correcting pixel-to-pixel variations, is primarily derived from pre-launch measurements with color-dependent maps achieving 0.5-5% precision, but in-flight M&C indirectly supports this by monitoring related effects like charge transfer inefficiency (CTI) and broadening due to aging. is minimized through careful observation planning, such as avoiding the within 25° and filtering Earth limb contamination, ensuring reliable calibration data. The detector's , which necessitates these ongoing checks due to temperature-dependent variations of approximately 1 per , is addressed through this framework as detailed in the and detector description.

Operations

Orbital Configuration

The CHEOPS spacecraft is in a Sun-synchronous dawn-dusk orbit at an altitude of approximately 688 km (perigee 681 km, apogee 695 km) with an inclination of 98.15° as of November 2025. This configuration positions the satellite such that it crosses the equator at approximately 06:00 local time on the ascending node, ensuring consistent lighting conditions over its lifetime. The orbital period is about 99 minutes, enabling roughly 14 orbits per day. This orbital setup offers key advantages for the mission's photometric observations. The dawn-dusk alignment maintains a stable thermal environment by keeping perpetually behind the , minimizing eclipses and temperature fluctuations that could affect instrument performance. It also provides access to 50–60% of the sky at any time, with the low-Earth orbit facilitating avoidance of the Earth's limb to reduce and enable high-precision measurements of transits. The Attitude and Orbit Control System supports fine pointing within these constraints but does not perform major orbit adjustments. Orbit maintenance relies on due to the absence of for or major corrections beyond initial injection adjustments. Atmospheric drag at this altitude is minimal, allowing natural decay to be monitored from the ground without intervention, which contributes to a predicted mission lifetime exceeding 7 years. As of November 2025, the remains stable, with ESA announcing an enhanced sky observability fraction through optimized operational procedures that expand accessible targets.

Observation Strategy

The CHEOPS mission employs a Guest Observer (GO) program to facilitate community-driven target selection, where proposals are submitted annually and evaluated by a Time Allocation Committee based on scientific merit. Priorities are given to bright host stars with visual magnitudes between 6 and 12 that are already known to host transiting exoplanets, particularly super-Earths and Neptunes, to enable high-precision follow-up observations. Crowded stellar fields are systematically avoided to mitigate contamination from nearby sources, which could compromise the defocused point spread function used for photometry. Observation visits typically last 1 to 5 continuous days per target, achieving up to an 80% to maximize on-sky time while accommodating orbital constraints. The mission schedules 400 to 600 targets annually, balancing guaranteed time observations with GO allocations. A Discretionary Programme reserves up to 25% of GO time for time-sensitive transients, such as newly discovered exoplanets, with rolling calls like Announcement of Opportunity 6, which opened in March 2025. CHEOPS operates primarily in standard photometry mode for monitoring but supports specialized modes for curves and secondary eclipses/occultations to exoplanet atmospheres and albedos. Efficiency is enhanced through strategies that minimize sky , including precise pointing and the use of a defocused design. avoidance angles greater than 20° are enforced to reduce contamination, and all observation plans undergo pre-execution to ensure feasibility and quality.

Ground Segment and Data Handling

The CHEOPS ground segment comprises the Mission Operations Centre (MOC) at the Instituto Nacional de Técnica Aeroespacial (INTA) in , , and the Science Operations Centre (SOC) at the , , which together manage satellite commanding, data reception, processing, and scientific analysis. The primary ground station is Cebreros in , part of ESA's network, providing 4-6 daily S-band contacts of 7-10 minutes each for telemetry and science data downlink, with in serving as the backup station to ensure redundancy, particularly during critical phases. Science data, totaling 10-100 GB per month after onboard compression, flows from the MOC to the SOC via secure FTP transfer, where an automated pipeline processes raw telemetry into calibrated light curves in FITS format, including preprocessing, quick-look assessments, and full data reduction steps. Quick-look products, such as pass reports on pointing accuracy and source detection, are generated within 24 hours and made available to principal investigators during the proprietary period. The processed data are archived at the SOC's Mission Archive and mirrored at the Italian Space Agency's Space Science Data Center (SSDC) for redundancy, with public access via the CHEOPS Archive Browser after a one-year proprietary phase; long-term data migrate to the ESA archive after 10 years. Automation is central to operations, with the Mission Planning System (MPS) at the using a to generate weekly activity plans optimizing observations around orbital constraints and ground contacts, minimizing manual intervention. User tools include the Proposal Handling Tool phase 2 (PHT2), a web interface for submitting guest observer proposals, and the CHEOPS Visibility Tool for assessing target observability. In 2025, enhancements expanded the observable sky fraction from 71.3% to 77.6% by relaxing the Sun Exclusion Angle to 115 degrees as of October 2025, enabling more targets in key fields like those of Kepler and . The Discretionary Programme was introduced as a continuous channel for rapid-response observations meeting merit criteria, complementing the sixth Announcement of Opportunity () cycle, which ran from to May 2025 for observations through September 2026.

Scientific Results

Early Observations and Discoveries

CHEOPS commenced its scientific operations in April 2020, following the successful completion of its in-orbit commissioning phase. The mission's inaugural observations targeted transiting exoplanets around bright stars, demonstrating the satellite's capability for high-precision photometry. One of the first targets was the hot Jupiter KELT-11b, orbiting a subgiant star 320 light-years away, where CHEOPS captured an 8-hour transit and refined the planet's diameter to 181,600 ± 4,300 km—five times more precise than prior ground-based measurements. These early transits also included confirmations for systems like WASP-76b and MASCARA-3b (also known as KELT-24b), validating their orbital parameters and providing initial photometric data to support atmospheric studies. A landmark early discovery came in 2021 with CHEOPS's observations of the TOI-178 system, revealing a compact architecture of six transiting around a nearby K-dwarf star. Five of these form an intricate chain of mean-motion resonances, with orbital periods ranging from 1.09 to 20.7 days, challenging models of planetary formation and migration. CHEOPS refined the radii of the inner five to between 1.7 and 2.6 radii, with periods locked in ratios close to 2:4:6:9:12, while also updating their orbital eccentricities and improving mass constraints through combined data. This resonant chain, spanning to sizes, highlighted CHEOPS's strength in characterizing multi-planet systems and provided new insights into their dynamical stability. In 2023, CHEOPS contributed to unlocking another rare resonant system around , a nearby Sun-like star, revealing six planets in a chain of 3:2 mean-motion resonances spanning orbital periods from 9 to 55 days. CHEOPS provided precise photometry to confirm transits of the inner three planets, refining their radii to 1.94–2.85 radii and enabling density estimates via follow-up. This pristine sextuplet, the longest known resonant chain, offers critical insights into the preservation of orbital resonances over billions of years and the inward migration processes during planet formation. Between 2022 and 2023, CHEOPS expanded its early science outputs through detailed phase curve analyses of s, mapping their atmospheric heat circulation and dayside-nightside contrasts. For instance, observations of WASP-43b yielded a phase curve with a dayside of approximately 1,750 K and evidence of efficient heat redistribution, consistent with a monotonic temperature-pressure profile and low coverage. Similarly, the phase curve of WASP-189b, an ultra- Jupiter, revealed a hot dayside reaching over 3,000 K and a significant phase offset, indicating strong eastward winds transporting heat from the substellar point. These studies exemplified CHEOPS's role in probing atmospheric dynamics without the need for spectroscopy, using visible-light photometry to infer circulation patterns and cloud properties. CHEOPS also captured the secondary eclipse (occultation) of the super-Earth 55 Cnc e in 2022–2023, detecting a depth of 12 ± 3 ppm across 41 visits, indicative of thermal emission from its dayside. This measurement implies a dayside of about 2,000 K, with a 2σ upper limit on the of 0.31, suggesting a dark, possibly rocky or metallic surface with minimal reflected light. The detection ruled out a highly reflective atmosphere and supported models of a volatile-poor, high-temperature dayside, advancing understanding of the innermost planets in multi-planet systems. Overall, these early observations validated CHEOPS's photometric precision, achieving 150 for 6-hour transits on 7th-magnitude , as designed, and enabling refined measurements for over 50 by combining precise radii with existing masses. This improved bulk compositions for a diverse sample, from hot Jupiters to super-Earths, bridging gaps in exoplanet demographics and confirming the mission's ability to deliver sub-percent radius uncertainties for bright targets.

Recent Findings and Impacts

In 2024, CHEOPS contributed to the potential first detection of a 'glory' effect on the ultra-hot Jupiter exoplanet WASP-76b, located 637 light-years away, through 23 observations spanning three years that revealed an unexpected brightness increase at the planet's eastern terminator. This rainbow-like optical phenomenon, caused by light scattering on uniform spherical cloud particles, suggests stable atmospheric conditions on the dayside reaching 2,400°C, challenging models of extreme weather on hot Jupiters and providing insights into cloud formation processes. The finding, corroborated by data from TESS, Hubble, and Spitzer, enhances understanding of exoplanet atmospheres and could inform searches for biosignatures like ocean glint on habitable worlds. A major 2025 discovery involved HIP 67522 b, a low-density Jupiter-sized orbiting a young Sun-like star, where CHEOPS and TESS observations detected 15 powerful flares—each 100 times more energetic than typical—predominantly during the planet's , indicating the planet's triggers stellar activity. This reverses traditional views of energy flow from star to planet, as the flares erode the 's atmosphere at a rate that could reduce it to size within 100 million years, highlighting destructive star-planet interactions in young systems. Led by researchers at ASTRON, the study underscores CHEOPS's precision in monitoring photometric variability for such . CHEOPS has also advanced the search for hot water worlds, or hycean planets, through 2025 observations of candidates TOI-238 b and TOI-1685 b, refining their radii to 1.8 and 2.1 radii, respectively, and modeling internal structures with thick H/He envelopes atop water-rich cores. These sub-Neptunes, with equilibrium temperatures around 800 K, show lifetimes exceeding the age of the for certain compositions, making them prime targets for JWST to detect signatures. The programme demonstrates CHEOPS's role in bridging surveys like TESS with atmospheric , identifying only a handful of well-characterized planets in the 1.5–2.5 radii range critical for studies. In November 2025, CHEOPS, in collaboration with TESS and HARPS-N, confirmed a transiting (HD 85426 b, radius approximately 2.5 radii) and detected two outer non-transiting companions around the bright solar-analog star HD 85426. CHEOPS's high-precision photometry refined the inner planet's transit depth and orbital period (around 11 days), enabling mass estimates via radial velocities and density calculations that suggest a volatile-rich composition. This system enhances the sample of multi-planet architectures around Sun-like stars, aiding studies of planetary formation in environments similar to our own. These findings have broadened CHEOPS's impact beyond initial exoplanet radius measurements, enabling detailed atmospheric and dynamical analyses that inform formation theories and prepare for missions like . By 2025, after 3.5 years of nominal operations, the satellite's photometric precision—stable at sub-millimagnitude levels—has exceeded expectations, supporting over 500 target observations and fostering international collaborations via its Guest Observers Programme. Extended operations to 2026, with potential to 2029, position CHEOPS to refine masses and densities for dozens more systems, enhancing the catalog of exoplanet properties for planetology.

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