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The CHaracterising ExOPlanet Satellite (CHEOPS) is a partnership between the European Space Agency and Switzerland with important contributions by 10 additional ESA member States. It is the first S-class mission in the ESA Science Programme. CHEOPS has been flying on a Sun-synchronous low Earth orbit since December 2019, collecting millions of short-exposure images in the visible domain to study exoplanet properties. A small yet increasing fraction of CHEOPS images show linear trails caused by resident space objects crossing the instrument field of view. To characterize the population of satellites and orbital debris observed by CHEOPS, all and every science images acquired over the past 3 years have been scanned with a Hough transform algorithm to identify the characteristic linear features that these objects cause on the images. Thousands of trails have been detected. This statistically significant sample shows interesting trends and features such as an increased occurrence rate over the past years as well as the fingerprint of the Starlink constellation. The cross-matching of individual trails with catalogued objects is underway as we aim to measure their distance at the time of observation and deduce the apparent magnitude of the detected objects. As space agencies and private companies are developing new space-based surveillance and tracking activities to catalogue and characterize the distribution of small debris, the CHEOPS experience is timely and relevant. With the first CHEOPS mission extension currently running until the end of 2026, and a possible second extension until the end of 2029, the longer time coverage will make our dataset even more valuable to the community, especially for characterizing objects with recurrent crossings.

HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright (\\(G=9.0\\) mag) solar analogue. TESS photometry revealed the star to have two candidate planets with radii of \\(3.9 \\pm 0.044\\) \\(R_\\oplus\\) (HIP 9618 b) and \\(3.343 \\pm 0.039\\) \\(R_\\oplus\\) (HIP 9618 c). While the 20.77291 day period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-day gap in the time series, leaving many possibilities for the period. To solve this issue, CHEOPS performed targeted photometry of period aliases to attempt to recover the true period of planet c, and successfully determined the true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE and CAFE revealed a mass of \\(10.0 \\pm 3.1 M_\\oplus\\) for HIP 9618 b, which, according to our interior structure models, corresponds to a \\(6.8\\pm1.4\\%\\) gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of \\(< 18M_\\oplus\\). Follow-up and archival RV measurements also reveal a clear long-term trend which, when combined with imaging and astrometric information, reveal a low-mass companion (\\(0.08^{+0.12}_{-0.05} M_\\odot\\)) orbiting at \\(26^{+19}_{-11}\\) au. This detection makes HIP 9618 one of only five bright (\\(K<8\\) mag) transiting multi-planet systems known to host a planet with \\(P>50\\) d, opening the door for the atmospheric characterisation of warm (\\(T_{\\rm eq}<750\\) K) sub-Neptunes.

We report the discovery of two warm sub-Neptunes transiting the bright (G = 9.5 mag) K-dwarf HD 15906 (TOI 461, TIC 4646810). This star was observed by the Transiting Exoplanet Survey Satellite (TESS) in sectors 4 and 31, revealing two small transiting planets. The inner planet, HD 15906 b, was detected with an unambiguous period but the outer planet, HD 15906 c, showed only two transits separated by \\(\\sim\\) 734 days, leading to 36 possible values of its period. We performed follow-up observations with the CHaracterising ExOPlanet Satellite (CHEOPS) to confirm the true period of HD 15906 c and improve the radius precision of the two planets. From TESS, CHEOPS and additional ground-based photometry, we find that HD 15906 b has a radius of 2.24 \\(\\pm\\) 0.08 R\\(_\\oplus\\) and a period of 10.924709 \\(\\pm\\) 0.000032 days, whilst HD 15906 c has a radius of 2.93\\(^{+0.07}_{-0.06}\\) R\\(_\\oplus\\) and a period of 21.583298\\(^{+0.000052}_{-0.000055}\\) days. Assuming zero bond albedo and full day-night heat redistribution, the inner and outer planet have equilibrium temperatures of 668 \\(\\pm\\) 13 K and 532 \\(\\pm\\) 10 K, respectively. The HD 15906 system has become one of only six multiplanet systems with two warm (\\(\\lesssim\\) 700 K) sub-Neptune sized planets transiting a bright star (G \\(\\leq\\) 10 mag). It is an excellent target for detailed characterisation studies to constrain the composition of sub-Neptune planets and test theories of planet formation and evolution.

Context: TOI-2076 is a transiting three-planet system of sub-Neptunes orbiting a bright (G = 8.9 mag), young (\\(340\\pm80\\) Myr) K-type star. Although a validated planetary system, the orbits of the two outer planets were unconstrained as only two non-consecutive transits were seen in TESS photometry. This left 11 and 7 possible period aliases for each. Aims: To reveal the true orbits of these two long-period planets, precise photometry targeted on the highest-probability period aliases is required. Long-term monitoring of transits in multi-planet systems can also help constrain planetary masses through TTV measurements. Methods: We used the MonoTools package to determine which aliases to follow, and then performed space-based and ground-based photometric follow-up of TOI-2076 c and d with CHEOPS, SAINT-EX, and LCO telescopes. Results: CHEOPS observations revealed a clear detection for TOI-2076 c at \\(P=21.01538^{+0.00084}_{-0.00074}\\) d, and allowed us to rule out three of the most likely period aliases for TOI-2076 d. Ground-based photometry further enabled us to rule out remaining aliases and confirm the \\(P=35.12537\\pm0.00067\\) d alias. These observations also improved the radius precision of all three sub-Neptunes to \\(2.518\\pm0.036\\), \\(3.497\\pm0.043\\), and \\(3.232\\pm0.063\\) \\(R_\\oplus\\). Our observations also revealed a clear anti-correlated TTV signal between planets b and c likely caused by their proximity to the 2:1 resonance, while planets c and d appear close to a 5:3 period commensurability, although model degeneracy meant we were unable to retrieve robust TTV masses. Their inflated radii, likely due to extended H-He atmospheres, combined with low insolation makes all three planets excellent candidates for future comparative transmission spectroscopy with JWST.

We report the discovery and characterisation of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in TESS photometry. To characterise the system, we performed and retrieved CHEOPS, TESS, and ground-based photometry, HARPS high-resolution spectroscopy, and Gemini speckle imaging. We characterise the host star and determine \\(T_{\\rm eff, \\star}=4734\\pm67\\) K, \\(R_{\\star}=0.726\\pm0.007\\) \\(R_{\\odot}\\), and \\(M_{\\star}=0.748\\pm0.032\\) \\(M_{\\odot}\\). We present a novel detrending method based on PSF shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of \\(P_{\\rm b}=6.44387\\pm0.00003\\) d, a radius of \\(R_{\\rm b}=2.59\\pm0.04\\) \\(R_{\\oplus}\\), and a mass of \\(M_{\\rm b}=13.5_{-1.8}^{+1.7}\\) \\(M_{\\oplus}\\), whilst TOI-1064 c has an orbital period of \\(P_{\\rm c}=12.22657^{+0.00005}_{-0.00004}\\) d, a radius of \\(R_{\\rm c}=2.65\\pm0.04\\) \\(R_{\\oplus}\\), and a 3\\(\\sigma\\) upper mass limit of 8.5 \\({\\rm M_{\\oplus}}\\). From the high-precision photometry we obtain radius uncertainties of \\(\\sim\\)1.6%, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterised sub-Neptunes, with a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further RVs are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass-radius space, and it allows us to identify a trend in bulk density-stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.

A crucial chemical link between stars and their orbiting exoplanets is thought to exist. If universal, this connection could affect the formation and evolution of all planets. Therefore, this potential vital link needs testing by characterising exoplanets around chemically-diverse stars. We present the discovery of two planets orbiting the metal-poor, kinematic thick-disk K-dwarf TOI-2345. TOI-2345 b is a super-Earth with a period of 1.05 days and TOI-2345 c is a sub-Neptune with a period of 21 days. In addition to the target being observed in 4 TESS sectors, we obtained 5 CHEOPS visits and 26 radial velocities from HARPS. By conducting a joint analysis of all the data, we find TOI-2345 b to have a radius of \\(1.504\\substack{+0.047\\\-0.044}\\) R\\(_\\oplus\\) and a mass of \\(3.49\\pm0.85\\) M\\(_\\oplus\\); and TOI-2345 c to have a radius of \\(2.451\\substack{+0.045\\\-0.046}\\) R\\(_\\oplus\\) and a mass of \\(7.27\\substack{+2.27\\\-2.45}\\) M\\(_\\oplus\\). To explore chemical links between these planets and their host star, we model their interior structures newly accounting for devolatised stellar abundances. TOI-2345 adds to the limited sample of well characterised planetary systems around thick disk stars. This system challenges theories of formation and populations of planets around thick disk stars with its Ultra-Short Period super-Earth and the wide period distribution of these two planets spanning the radius valley.

The CHaracterising ExOPlanet Satellite (CHEOPS) was selected in 2012, as the first small mission in the ESA Science Programme and successfully launched in December 2019. CHEOPS is a partnership between ESA and Switzerland with important contributions by ten additional ESA Member States. CHEOPS is the first mission dedicated to search for transits of exoplanets using ultrahigh precision photometry on bright stars already known to host planets. As a follow-up mission, CHEOPS is mainly dedicated to improving, whenever possible, existing radii measurements or provide first accurate measurements for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys and to following phase curves. CHEOPS will provide prime targets for future spectroscopic atmospheric characterisation. Requirements on the photometric precision and stability have been derived for stars with magnitudes ranging from 6 to 12 in the V band. In particular, CHEOPS shall be able to detect Earth-size planets transiting G5 dwarf stars in the magnitude range between 6 and 9 by achieving a photometric precision of 20 ppm in 6 hours of integration. For K stars in the magnitude range between 9 and 12, CHEOPS shall be able to detect transiting Neptune-size planets achieving a photometric precision of 85 ppm in 3 hours of integration. This is achieved by using a single, frame-transfer, back-illuminated CCD detector at the focal plane assembly of a 33.5 cm diameter telescope. The 280 kg spacecraft has a pointing accuracy of about 1 arcsec rms and orbits on a sun-synchronous dusk-dawn orbit at 700 km altitude. The nominal mission lifetime is 3.5 years. During this period, 20% of the observing time is available to the community through a yearly call and a discretionary time programme managed by ESA.

We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K-M binary system. Analysis of the spectra and rotation period reveal it to be a young star with an age of \\(200^{+400}_{-200}\\)Myr. TOI-815b has a 11.2-day period and a radius of 2.94\\(\\pm\\)0.05\\(\\it{R_{\\rm\\mathrm{\\oplus}}}\\) with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62\\(\\pm\\)0.10\\(\\it{R_{\\rm\\mathrm{\\oplus}}}\\), based on observations of three non-consecutive transits with TESS, while targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6\\(\\pm\\)1.5 \\(\\it{M_{\\rm \\mathrm{\\oplus}}}\\) (\\(\\rho_\\mathrm{P}\\)=1.64\\(^{+0.33}_{-0.31}\\)gcm\\(^{-3}\\)) and 23.5\\(\\pm\\)2.4\\(\\it{M_{\\rm\\mathrm{\\oplus}}}\\) (\\(\\rho_\\mathrm{P}\\)=7.2\\(^{+1.1}_{-1.0}\\)gcm\\(^{-3}\\)) respectively. Thus, the planets have very different masses, unlike the usual similarity of masses in compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those suffering strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere constituting a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3\\(\\sigma\\) level.

Superconducting Tunnel Junction (STJ) detectors offer wavelength-resolved detection of single optical photons with high time resolution. To meet the needs of future astronomical instruments, large detector arrays must be developed. We describe recent activities aimed to secure such performance, and highlight results achieved with prototype arrays when employed at the William Herschel Telescope.

We present observations of 11 quasars, selected in the range z = 2.2-4.1, obtained with ESA's Superconducting Tunnel Junction (STJ) camera on the WHT. Using a single template quasar spectrum, we show that we can determine the redshifts of these objects to about 1%. A follow-up spectroscopic observation of one QSO for which our best-fit redshift (z = 2.976) differs significantly from the tentative literature value (z ~ 2.30) confirms that the latter was incorrect.