AO-6: Approved Programmes

Announced on 30 June 2025

The 6th Announcement of Opportunity (AO-6) for the CHEOPS Guest Observers (GO) Programme opened on 18 March 2025 and closed on 8 May 2025. It is the third call of the first extended mission and covers observing time in the period from 1 October 2025 until 30 September 2026. The available GO share of the science observing time was increased from 20% (in the Nominal Mission) to 30% (in the first Extended Mission).

A total of 33 proposals were received in reply to AO-6, requesting 3953 orbits (with each orbit circa 99 minutes in duration). The requests represented 260% of the available science observing time foreseen in the AO-6 observing cycle (circa 1505.5 orbits).

The CHEOPS Time Allocation Committee (TAC) met on 3 - 4 June 2025. Based on the TAC's recommendations, the Director of Science has awarded CHEOPS observing time to the proposals listed in the table below. In the end, 16 proposals were awarded observing time totaling 1562 orbits. Of these, 12 proposals were on exoplanet science, three on stellar science and one in Solar System science. The TAC-recommended allocation of observing time represents up to 104% of the available GO science observing time foreseen in the AO-6 observing cycle.

Succesful proposals will be implemented as GO programmes. Targets that are part of these GO programmes can generally not be included in other observing programmes unless in specific cases. All programmes have been assigned a priority from Priority 1, or P1 (high), to Priority 3, or P3 (low). This priority is taken into account by the automated planning tool used in the weekly/biweekly planning, and is a strong indicator of the likelihood that observations will be scheduled. Observers are reminded that the award of observing time provides no guarantee that the observations can be executed, and that generally more observing time is awarded than can be physically scheduled to minimise idle time.

Principal Investigators (PIs) of proposals that have been awarded time have been contacted by email, and are required to complete and submit observation requests at their earliest convenience. Guidelines on how to prepare observation requests can be found here. The TAC feedback has been provided to all PIs of proposals.

A fraction of up to 25% of the GO Programme time will remain available to the community to apply for time via the Discretionary Programme (DP), which is foreseen to continue running throughout the mission lifetime. This is in line with a possible over-allocation of up to 133% to facilitate the efficient scheduling of time critical observations.

* Excluding two targets in two higher-ranked proposals.
** The total awarded observing time is 1562 orbits, as stated above, before applying the exclusion indicated with * in the table.

Abstracts

ID0003 — Probing rapid quasi-periodic pulsations in flares of active Solar-like stars (PI: Dario Fritzewski)
The mechanisms contributing to the heating of stellar coronae remain poorly understood. One method probe stellar coronae is through quasi-periodic pulsations (QPPs) observed during flares. These pulsations occur within the flaring region and modulate the flaring light curve. Despite their frequent observations on the Sun the origin of QPPs is still elusive. To differentiate between several proposed models, observations of QPPs on stars with magnetic properties different to these of the Sun are required. One key discriminating factor is the pulsation period. Solar QPPs are observed with pulsation periods ranging from a few seconds to one minute. We propose CHEOPS observations of four active solar-mass stars to search for QPP with such short periods. If we detect fast QPPs with periods of a few seconds in flares on our targets, we can narrow down the potential models of QPPs. CHEOPS is currently the only space mission fulfilling the cadence requirements of a few seconds, allowing to probe pulsations with periods shorter than ten seconds. By bridging Solar and stellar physics, we aim to open pathways to coronal seismology on cool stars, thereby, probing coronal properties that govern the coronal heating.

ID0005 — Plucking large magnetic loops with a planet: test of a new mechanism for star-planet interaction (PI: Ekaterina Ilin)
We propose CHEOPS observations of the young planetary system HIP 67522 to investigate the physical mechanism driving planet-induced stellar flaring. Planet-induced flaring has recently been confirmed in HIP 67522 using TESS and CHEOPS observations on a 3-year baseline (Nature, in press). Our goal is to test whether the flare rate remains elevated near the innermost planet’s transit two years later, and whether the energy distribution of these flares differs from that of intrinsic stellar activity. The flare rate in HIP 67522 is elevated by a factor of six around the transit of the innermost planet compared to the intrinsic flare rate. These additional flares tentatively appear to be triggered preferentially at high energies, suggesting interaction with large stellar magnetic loops — a new mechanism not accounted for in current models. We propose to observe HIP 67522 for a total of 270 orbits in May–June 2026 to coincide with the inner planet's transit phases, complementing scheduled TESS coverage in March–April. Together, these data will double the sample of both planet-induced and intrinsic flares, extending the persistence of interaction to 5 years. Importantly, the data will allow us to test if the flare energy distributions of intrinsic and planet-induced flares differ from each other, as expected if the planet selectively perturbs large coronal structures. This result could not be achieved with any other instrument in the near future.

ID0006 — An evolving gas dwarf or a freshly formed water world? (PI: Saugata Barat)
Two competing theories for the origin of mini-Neptunes have emerged: the gas dwarf scenario and the water world scenario. At mature ages it is difficult to distinguish between gas dwarfs and water worlds from their size. However, recent modelling studies have shown at extremely young ages (<50 Myr) gas dwarfs are expected to be inflated, whereas water worlds do not undergo significant evolution. Therefore, precise radius measurements for transiting planets younger than 50 Myr can help differentiate between these two proposed scenarios. TOI-7038.01 is the youngest (16 Myr) known mini-Neptune sized candiadate with 6 transits in TESS. The precision on the optical radii from these observations is 6.7% which makes this candidate consistent with both scenarios. We propose to observe and validate the first potential young water-world candidate, TOI-7038.01 using 2 transits with CHEOPS. We expect an improvement in the optical radius by a factor of 2.5 compared to TESS and is sufficient to break the gas-dwarf-water world degeneracy. Furthermore, the proposed observations would help us to maintain its ephemeris and pave the way for follow-up JWST observations.

ID0007 — Dust Content of A Disintegrating Planet During Spectroscopic Diagnosis (PI: Everett Schlawin)
Disintegrating exoplanets offer a valuable opportunity to study the composition of rocky planets outside the Solar System. They exhibit transit depths of ~1% and transit durations much longer than the planet crossing time, which indicates that the escaping dust extends as long tails trailing and/or leading the planet. Up until now, these disintegrating bodies (found by Kepler) were too faint for spectroscopic characterization. The new discovery of the TESS disintegrating planet candidate BD +05 4868 A b, however, opens the window for detailed high dispersion and high SNR spectroscopy. While the BD +05 4868 system exhibits deep and regularly spaced transits of dusty material, the amount and extent of dust varies wildly from one orbit to the next. We propose to collect CHEOPS photometry to aid in spectroscopic campaigns to provide simultaneous data on the dust content. This data will be critical to ground-based campaigns, which cannot cover the full transit duration and baseline. Furthermore, the CHEOPS photometry will reveal the location of the dust relative to the gas to better understand the disintegration and trajectory of the debris.

ID0008 — Probing the micro-flaring activity of bright M dwarfs ahead of PLATO (PI: Julien Poyatos)
Stellar flares are key signatures of magnetic activity, with profound implications for stellar physics and planetary habitability. While large flares on M dwarfs have been extensively studied, the micro-flare regime (10²⁵–10²⁸ erg) remains largely unexplored due to sensitivity limitations of past missions. This program leverages CHEOPS’ high cadence and exquisite photometric precision to detect and characterise micro-flares down to ~10²⁶ erg, well below the detection thresholds of TESS and Kepler. We aim to extend Flare Frequency Distributions (FFDs) to lower energies to assess whether flare generation mechanisms change across energy regimes, examine the influence of a radiation-convection transition layer on flare rates, and evaluate whether quasi-periodic signals may arise from clustered low-energy flares. Observations target both partially and fully convective M dwarfs from the PLATO M Dwarf Sample (P4), serving as essential groundwork for PLATO’s upcoming stellar science studies. We request 500 CHEOPS orbits to be executed as fillers, distributed across 5 bright (Gmag < 9), flare-active M dwarfs, selected to maximise year-round observability. Using imagettes, tailored detrending, wavelet denoising, and flare-fitting techniques, this program will robustly detect and characterise micro-flaring activity, extending our understanding of M dwarf magnetism and laying the foundation for interpreting high-cadence PLATO data.

ID0009 — Hide-and-seek with HIP41378 d: ready or not, here we come! (PI: Alexandre Santerne)
The bright star HIP41378 hosts a fascinating system with five exoplanets transiting at long orbital period (between 2 weeks and 1.5 years). This target is thus a rare laboratory to probe the atmosphere (with JWST and ARIEL) of planets that are not intensively irradiated or strongly affected by tides. However, to characterise these exo-atmospheres, one needs accurate ephemeris. With an orbital period of 278 d (9 months), HIP41378 d is such a long-period transiting planet. Two of its transits have been observed by K2 in 2015 and 2018, and a Rossiter-McLaughlin effect has been partly detected in 2019. CHEOPS attempted to detect its transit in 2023 but missed it due to unexpectedly large transit timing variations (TTVs). With this proposal, we aim at observing HIP41378 near the next transit times of planet d taking into account for large TTVs. Detecting this transit with CHEOPS will allow us to 1- better understand the architecture of the system, 2- improve the planets ephemeris on time for the ARIEL mission, 3- constrain the TTVs of planet d and other planets in the system. Given the long period of this exoplanet, this is the last opportunity to observe this event before 2029 or 2032.

ID0011 — Case Study: Measuring M-dwarf Radius Inflation from a 22-hr Eclipse of the 72-day EB TOI-2065 (PI: Daniel Stevens)
Despite low-mass, M-dwarf stars’ ubiquity and large, recent efforts to find small exoplanets around them, low-mass stellar evolutionary models still predict radii and effective temperatures that are 5-15% smaller and hotter than the measured values: these discrepancies are likely caused by a combination of incomplete/inaccurate model physics that govern the predicted bulk properties and/or binary-star interactions that produce deviations in the measured bulk properties from those of solitary stars. Although double-lined eclipsing binaries (DLEBs) are the gold standard systems for measuring accurate stellar masses, radii, and temperatures; only 1 DLEB (2 M-dwarfs) thus far has both stellar parameters measured to percent-level precision and accuracy and a wide enough orbit such that binary interactions should be negligible. We propose to add a third M-dwarf to this sample by measuring the mass, radius, and temperature of the M-dwarf secondary in the long-period (72 d) single-lined EB (SLEB) TOI-2065 to 2% precision and accuracy from a CHEOPS observation of its 22 hr primary eclipse. TOI-2065 B will become the longest-period well-characterized M-dwarf, and our CHEOPS analysis will lay the groundwork for future work on even-longer-period SLEB M-dwarfs in our sample.

ID0013 — CHEOPS transits of an additional planet candidate in TOI 451 (PI: Madyson Barber)
Young transiting planets provide valuable windows into the early stages of planet evolution. Multi-planet systems are especially valuable, as they allow for direct comparisons between planets from the same formation conditions. However, due to differences in precision between the Kepler and TESS missions, the decreased sensitivity to young small transiting planets in TESS data have raised questions surrounding the true distribution of young planets from TESS in comparison to the mature planets from Kepler. As we are better able to handle young stellar activity, we are becoming increasingly sensitive to additional smaller planets in known young systems. Here, we propose to confirm an additional small planet candidate in a known 120 Myr transiting system.

ID0015 — CHEOPS observations of TOI-2537b: a temperate, eccentric, long-period (94.1 d) Warm Jupiter with Transit Timing Variations (PI: Neda Heidari)
TOI-2537b is a transiting, long-period (94.1 d), eccentric (e = 0.36) warm Jupiter with a mass of 1.31 ± 0.09 MJ, a radius of 1.00 ± 0.06 RJ, and an equilibrium temperature of 307 ± 15 K. It exhibits significant transit timing variations (TTVs) with an amplitude of approximately 22 minutes, likely induced by gravitational interactions with a massive, non-transiting outer companion, TOI-2537c (P = 1920 ± 185 d, M sin i = 7.2 ± 0.5 MJ, e = 0.29 ± 0.06). However, the current dynamical constraints are limited by the availability of only three transits observed by TESS. We propose a CHEOPS observation to capture a full transit of TOI-2537b, which would yield a precise mid-transit time, refine the TTV model, improve the system’s dynamical characterization, and potentially constrain the true mass of TOI-2537c. A single CHEOPS visit is expected to reduce the uncertainty in the planet-to-star radius ratio by ~98% and significantly refine the orbital period. This period refinement is crucial for follow-up studies, including measurements of spin–orbit alignment via the Rossiter–McLaughlin effect and future atmospheric characterization. As no additional TESS observations are planned, and given that the planet’s long (5-hour) and infrequent transits are difficult to observe from the ground, CHEOPS presents a unique opportunity to investigate gravitational interactions in a system hosting two massive planets, long-period, eccentric, transiting giant.

ID0017 — Observation of a second transit of a long-period (40.4 d) planet to refine its orbital period and radius (PI: Neda Heidari)
Warm Jupiters—giant exoplanets with orbital periods between 10 and 200 days—provide valuable insights into the formation and migration of planetary systems. However, they remain poorly discovered and well-characterized, particularly at longer periods (>40 days). TOI-5893b is a transiting warm Jupiter with an orbital period of 40.312 ± 0.032 days. It was initially identified by TESS as a single-transit candidate. We initiated a radial velocity (RV) follow-up campaign and confirmed its planetary nature. A joint analysis of the RV data and TESS photometry yields a planetary radius of 0.91 ± 0.05 RJ, and a mass of 1.07 ± 0.12 MJ. Although a second TESS visit helped refine the period estimate, the predicted transit was missed due to a data gap. We propose to observe the next transit of TOI-5893b with CHEOPS to further constrain its orbital period and significantly improve the precision of its radius measurement. This period refinement is essential for enabling future studies, including measurements of spin-orbit alignment via the Rossiter–McLaughlin effect. The CHEOPS observation is expected to reduce the radius ratio uncertainty from ~2.3% to ~0.03%, an improvement by a factor of ~80, placing TOI-5893b among the most precisely characterized long-period warm Jupiters.

ID0019 — Stellar Occultations by Outer Solar System Objects with CHEOPS (PI: Altair Ramos Gomes Junior)
Stellar occultations occur when a Solar System body passes in front of a star from the perspective of an observer. These events offer valuable opportunities to investigate and infer the physical properties of the occulting object, such as its size, shape, rings, atmosphere, or surroundings. Observing an occultation requires precise geometric alignment between the observer, the occulting body, and the background star. When such events are observed simultaneously by CHEOPS and ground-based telescopes, the combined datasets significantly enhance the spatial coverage and accuracy of the observations, leading to a more detailed and reliable characterization of the occulting object. Thus, we propose the observation of 11 stellar occultations by Trans-Neptunian Objects (TNOs), Centaurs and the Jovian satellite Io.

ID0023 — Confirming long-period giant exoplanets around M-dwarfs with CHEOPS (PI: Caleb Canas)
The known giant exoplanets orbiting M-dwarfs (GEMS) have so far been predominantly found to transit early M dwarfs on short-period orbits (P<10 days). These planets, with their large masses, are difficult to explain around low-mass stars with current theories of core accretion. This problem is further exacerbated for mid to late M-dwarfs, given the lower stellar masses. We propose to confirm long-period GEMS around mid M-dwarfs to increase the sample size. Using CHEOPS, we propose to confirm the transits and refine the system parameters for two long-period (P>10 days) GEMS. This program will increase the sample in sparsely populated regions of the parameter space for GEMS (long periods and low mass stars). Additionally, the large transit depths of GEMS (>3%) make them excellent candidates for measurements of stellar obliquity and atmospheric characterization to probe other tracers of formation pathways in the future.

ID0024 — Measuring masses and radii of young, resonant multi-planetary systems (PI: Hritam Chakraborty)
Young planets are excellent laboratories to test planet formation and evolution pathways. However, the accurate characterisation of parameters like the masses and radii is limited by the enhanced activity of the host star. However, the timing of individual transits is far less affected by the stellar activity. Thus, transit timing variations arising due to dynamical interactions provide a unique opportunity to measure precise masses of young exoplanets. In this proposal, we aim to survey young, resonant multi-planetary systems with ages < 100 Myr to constrain their precise masses by measuring transit timing variations. Additionally, we aim to quantify the variations in transit depth associated with stellar activity and use this information to derive accurate radii of planets in the sample. Our sample consists of HIP 67522 b and c, V1298 Tau c and d, HD109833 b and c, and TIC 434398831 b and c.

ID0025 — Chasing the lost transit of a unique giant planet in an open cluster (PI: Alexander Venner)
Open clusters are important laboratories for stellar and planetary physics. A small number of exoplanets in open clusters are currently known, but this sample is dominated by small planets, and no transiting Jupiter-sized planets are currently known in any open cluster. We have discovered a long-period giant planet in archival K2 observations of a subgiant star in a nearby open cluster. Originally detected from a single transit, we have collected follow-up RV observations that have resolved the orbital period and confirmed that this is a Jovian planet. We request time on CHEOPS to track down the lost transit of this unique planet. With a constrained period, mass, radius, age, and formation environment, this planet will be a key benchmark among the population of long-period transiting giant planets.

ID0029 — Probing Transit Timing Variations in HD 332231 b with CHEOPS (PI: Gracjan Maciejewski)
We propose CHEOPS observations of five consecutive transits of the warm giant planet HD 332231 b in July-September 2026 to refine the transit timing variation (TTV) signal previously detected in TESS. HD 332231 b exhibits coherent TTVs with a ~6.7-year periodicity and an amplitude of ~45 minutes, likely driven by an undetected outer planetary companion. CHEOPS’s high-precision photometry is essential to maintaining phase coverage, as the system is not scheduled for TESS observations until late 2026. The proposed transits occur near a key phase of the TTV cycle and may exhibit short-timescale "chopping" signatures - subtle deviations induced by close orbital interactions. Detecting such signals can help resolve dynamical degeneracies in mass and orbit solutions. These observations will significantly constrain the architecture of the HD 332231 system and improve our understanding of planetary perturbations in systems with long-period companions.

ID0030 — Beating stellar variability with simultaneous CHEOPS+JWST observations of the young sub-Neptune V1298 Tau c (PI: Hinna Shivkumar)
Young planets serve as a bridge between a planet’s formation and maturity. By unveiling their young atmospheres, we can study their origin, evolution and ultimate fate. In the NIR, JWST is revolutionising our view into these young atmospheres. However, stellar variability introduces major degeneracies that NIR data alone cannot resolve. We propose simultaneous CHEOPS and JWST/NIRSpec observations of the young (~23 Myr) sub-Neptune V1298 Tau c to anchor its optical transit depth, thereby constraining stellar contamination, correcting flux offsets, and enabling accurate constraints on the planet’s atmospheric properties. These simultaneous observations will additionally calibrate flux offsets introduced by stellar variability for the non-simultaneous NIRISS observations of V1298 Tau c, ensuring robust cloud characterization.

This page was last updated on 30 June 2025.