CHEOPS Space Telescope

• CHEOPS Space Telescope is an ESA mission that captures ultra-precise transit photometry, providing accurate exoplanet radii and density measurements.
• It employs a thermally stabilized Ritchey–Chrétien telescope and advanced data pipelines to achieve photometric precision within 20 ppm for bright stars.
• The mission underpins detailed studies of exoplanet compositions, enabling refined mass–radius relations, exomoon searches, and atmospheric analyses.

The CHaracterising ExOPlanet Satellite (CHEOPS) is a European Space Agency (ESA) S-class mission developed in partnership with Switzerland and contributions from ten additional member states. CHEOPS was conceived as the first ESA mission devoted exclusively to high-precision, time-resolved photometric monitoring of bright stars known to host exoplanets. Rather than searching for new exoplanets, CHEOPS was engineered to deliver accurate radii and thus bulk densities when combined with radial velocity (RV) measurements for planets in the super-Earth-to-Neptune mass range, with the mission’s principal aims extending to detailed transit characterisation, atmospheric studies, and enabling wide community access to ultra-high precision time-domain photometry.

1. Mission Architecture, Instrumentation, and Ground Segment

CHEOPS operates from a Sun-synchronous, dusk-dawn low-Earth orbit at 700 km altitude, chosen to maximize thermal stability and minimize Earth stray-light contamination (Benz et al., 2020). The spacecraft’s payload is an on-axis Ritchey–Chrétien telescope with a 33.5 cm primary mirror and a sophisticated optical chain (Back End Optics) optimized for PSF stability and photometric uniformity across the visible–near-infrared (400–1100 nm) bandpass (Broeg et al., 2013, Deline et al., 2019). Its single science detector is a back-illuminated, frame-transfer E2V CCD (AIMO CCD47-20), with a 1024×1024 pixel imaging array and precisely controlled thermal environment (operated at 228 K, stability within ±10 mK) (Benz et al., 2020).

The ground segment is architected around two distinct but closely integrated cores: the Mission Operations Centre (MOC, INTA/CEIT, Spain) and the Science Operations Centre (SOC, University of Geneva) (Heitzmann et al., 13 May 2025). The MOC is responsible for orbit determination, spacecraft health, command uplink, and telemetry downlink, employing extensive automation via GMV’s Focus suite and the SCOS-2000 control system. The SOC manages observation requests via a MariaDB-backed web system, implements an automated mission planning solver using genetic algorithms, and processes raw telemetry through pipelines generating science-ready photometric data and archiving products on an open-access platform. The modular, high-automation architecture supports efficient, cost-effective operations with minimal staffing.

2. Calibration, Performance, and In-Flight Systematics

CHEOPS’s high-precision photometric capabilities result from extensive ground-based calibration campaigns and rigorous in-orbit monitoring. The detector is characterized by a bias stability of 0.2 ADU/pixel, a dark current of 0.028 e⁻/s at –40 °C, and a read-out noise of 7–14 e⁻ (depending on clock frequency). Flat-field measurements, using 23 narrow and 4 broad-band filters, yield average per-pixel precision of 6×10⁻⁴, with residual synthetic flat-field errors below 0.07% (Deline et al., 2019).

The on-ground and in-orbit noise floors reach <20 ppm over multi-hour bins for bright stars, and in-flight performance studies demonstrate achievement or surpassing of required photometric precision: 20 ppm (6 h) for V=9 G-type stars and 85 ppm (3 h) for Neptune-sized transits across K-dwarfs (Benz et al., 2020, Fortier et al., 3 Jun 2024). End-to-end simulations confirm these values, yielding SNRs of 10–25 in typical transit depth regimes.

Instrumental systematics—such as “flux ramps” following repointing due to thermal settling—are modelled and removed using a combination of principal component analysis (PCA) and direct correlation techniques. For example, a linear de-correlation with temperature,

$$F_\mathrm{corrected} = F_\mathrm{measured} \times (1 + \beta \, \Delta T)$$

with $\Delta T$ representing the deviation from a reference thermal sensor, is routinely employed, with $\beta = 140-400$ ppm/°C depending on aperture size (Fortier et al., 3 Jun 2024). The performance, including growing hot pixel prevalence and cosmic ray damage, is tracked through a dedicated monitoring and characterisation programme, with weekly bad-pixel map updates and PSF stability assessments.

Data pipelines (e.g., DRP, PyCHEOPS, PIPE) implement advanced detrending techniques, including explicit roll-angle correction (via

$$f(t) = c + \sum_j \alpha_j \sin(j\phi(t)) + \beta_j \cos(j\phi(t))$$

) and PSF shape metrics, to reach or approach the photon noise floor in the majority of visits (Maxted et al., 2021, Fortier et al., 3 Jun 2024).

3. Science Drivers: Refining the Mass–Radius Relation and Exoplanet Composition

The core science objective of CHEOPS is to enable precise measurements of planetary radii for targets with known mass (from RV or transit survey data), facilitating robust bulk density estimates

$$\rho = \frac{M_\text{p}}{\frac{4}{3}\pi R_\text{p}^3}$$

and, thus, constraining interior structure and composition (Broeg et al., 2013). CHEOPS targets stars hosting

• Planets with known RV orbits but unknown radii (non-transiting or previously undetected shallow transits),
• Planets with ground-based transit detections requiring radius refinement.

This precision is critical in the super-Earth–to–Neptune regime (1–4 R$_\oplus$), where theoretical models predict a diversity of possible compositions (from predominantly rocky to volatile- and envelope-rich), and where mass–radius demarcations (e.g., the “radius valley”) inform core accretion, migration, and atmospheric loss models.

Over its planned 3.5-year nominal mission (extended into 2026), CHEOPS was tasked to observe approximately 500 targets, with project requirements emphasizing a radius precision of 10% for Neptune-sized exoplanets—sufficient to break the degeneracy in inferred super-Earth/neptunian interior composition (Broeg et al., 2013, Benz et al., 2020). Early science validation demonstrates that derived transit depths and timings from CHEOPS reach uncertainties of a few tenths of a percent, rivaling larger platforms such as Kepler and Spitzer for bright hosts (Maxted et al., 2021).

4. Advanced Time-Domain Photometry: Phase Curves, Secondary Eclipses, and Community Science

CHEOPS’s flexible pointing and high-cadence photometry enable a broad suite of secondary science, including phase curve mapping, secondary eclipse (occultation) photometry, and the detection of dynamical effects (e.g., gravity darkening, tidal deformation), as well as time-critical or unusual transit events.

Phase-curve and secondary-eclipse studies for ultra-hot Jupiters such as WASP-189b (Lendl et al., 2020), HD 209458b (Brandeker et al., 2022), and HD 189733b (Krenn et al., 2023) exploit CHEOPS’s optical bandpass to probe reflected light and thermal emission. For HD 209458b, a geometric albedo of $A_g = 0.096 \pm 0.016$ was measured, consistent with a cloud-free atmosphere where Rayleigh scattering by H$_2$ and Na absorption set the visible reflectivity. For HD 189733b, $A_g = 0.076 \pm 0.016$, supporting a low-albedo, sodium-absorber dominated atmosphere.

As a complementary follow-up instrument to TESS, CHEOPS enables disambiguation between reflected and thermal components in secondary eclipses, and, via multi-bandpass comparisons, the identification of atmospheric haze via Rayleigh effects—where the difference between CHEOPS and TESS transit depths can be parameterized as

$$\Delta\delta = \frac{\delta_\mathrm{CHEOPS} - \delta_\mathrm{TESS}}{\delta_\mathrm{TESS}}$$

and is sensitive to haze particle sizes at the 0.04–0.05 μm scale (Gaidos et al., 2017). Such characterizations directly inform JWST spectroscopic follow-up by flagging systems where infrared transmission features remain unobscured by optical hazes.

Community-driven science is explicitly supported, with up to 20% open time allocated annually for competitive proposals covering topics ranging from stellar variability and asteroseismology to Solar System stellar occultations (e.g., of the TNO Quaoar (Morgado et al., 2022)).

5. Solar System and Ancillary Science

CHEOPS’s design, while exoplanet-focused, supports ancillary observations. The detection and analysis of resident space object (RSO) trails via Hough transform pipelines has yielded a statistically significant survey of orbital debris and satellite populations in low-Earth orbit, with identifiable signatures (e.g., the rise of Starlink satellites and per-year increases in occurrence rates) (Billot et al., 27 Nov 2024).

In Solar System studies, CHEOPS achieved sub-milliarcsecond astrometry and upper limits of 85 nbar on the methane atmosphere of the transneptunian Quaoar via high-cadence, high-precision occultation photometry, demonstrating that 3-second temporal sampling suffices to resolve km-scale structures in TNO environments and to constrain surface/subsurface properties (Morgado et al., 2022).

6. Exomoon and Exocomet Sensitivity

Simulations and analyses project CHEOPS’s potential in exomoon discovery. Using an optimized decision algorithm based on the Photocentric Transit Timing Variation (PTV) signal, the detection threshold is estimated around Earth-sized moons orbiting Neptune-sized or larger planets; an 80–85% detection rate can be obtained in 5–6 observed transits, with a controllable false-alarm rate via bootstrapping statistics (Simon et al., 2015). These techniques can be extended by mirroring and folding light curves, or by photodynamical modeling, and suggest CHEOPS can contribute to the first robust exomoon detections with appropriate target selection and sustained monitoring.

CHEOPS has also acquired the first photometric evidence for exocomet transits, as illustrated by the detection of a 340 ppm event in HD 172555 after careful delta-Scuti pulsation removal and empirical exocomet transit modeling. The inferred properties—nucleus radius $\approx$2.5 km, dust production rate $\approx$10<sup\>5</sup> kg/s—are analogous to those known from the Solar System and $\beta$ Pic, confirming dynamical similarities in young planetary systems and opening photometric access to small-body coma and tail characteristics (Kiefer et al., 2023).

7. Lessons Learned, Ageing Effects, and Legacy for Future Missions

CHEOPS’s first 3.5 years of operation have demonstrated that state-of-the-art, photon-limited precision photometry is attainable in a cost-constrained, small-class mission architecture—provided that ground segment automation, in-flight calibration, and robust data pipelines are instituted from the outset. Ageing effects, primarily hot pixel accumulation and slowly increasing CTI, have only marginally degraded performance, with weekly dark map updates and PSF-fitting extractions maintaining high-quality light curves (Fortier et al., 3 Jun 2024).

Key legacy recommendations include:

• A continuously updated monitoring and characterisation programme, with systematic “bad pixel” tracking and PSF modeling;
• Sufficient flexibility in data reduction pipelines to adapt to unforeseen degradation modes (e.g., custom aperture PSF photometry and roll-angle decorrelation);
• Mission planning automation capable of optimizing science output under complex operational constraints;
• Open access to high-quality photometric and calibration datasets to maximize community engagement and secondary science output.

CHEOPS’s successful strategies—both technical (thermal stabilization to mK precision, use of a defocused PSF, photon-limited performance even for faint targets) and organizational (consortium-lead operations, modular ground segment, 90%+ science duty cycle)—provide a template for future small-class exoplanet characterization missions.

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CHEOPS has established itself as a central resource for exoplanet characterization via transit photometry, providing the statistical sample and precision required to advance planet formation and evolution models, inform atmospheric studies, and enable community-driven investigations across a broad astrophysical landscape (Broeg et al., 2013, Benz et al., 2020, Heitzmann et al., 13 May 2025, Fortier et al., 3 Jun 2024).