AO-3: Approved Programmes

Announced on 27 May 2022

The third and final Announcement of Opportunity (AO-3) for the CHEOPS Guest Observers Programme in the nominal mission came out on 15 February 2022, and closed 4 weeks later on 15 March 2022. A total of 38 proposals were submitted to the AO, with an oversubscription factor of ~ 2.4 on the time available at the opening of the Call. Given the demand on CHEOPS observing time it was agreed with the mission PI that observing time that was not allocated from Year 2 could be taken forward to Year 3. As a result a total of 25% of the science time in the period 1 July 2022 to 24 September 2023 is available to the GO Programme. 

It was decided to allocate approximately 85% of the GO Programme time to proposals submitted to AO-3, with the remainder to be made available to the Community through the Discretionary Programme. Approximately 30% more orbits than can be physically scheduled in the AO-3 component of the GO Programme have been allocated to enable efficient scheduling of the very large number of time-critical observations in CHEOPS programmes. 

The CHEOPS Time Allocation Committee (TAC) met on 11-13 May, 2022. Based on the recommendations made by the TAC, the Director of Science has awarded observing time on CHEOPS to the proposals listed in the table below.  Successful proposals, known as programmes, have been awarded observing time on CHEOPS totalling 1546.8 orbits. Targets that are part of these programmes have been added to the Reserved Target List and cannot be included in observing programmes of others until the AO-3 observations have been completed. 

All programmes have been assigned a priority from 1 (high) to 3 (low), in some cases at the level of individual targets. This priority is taken into account by the automated planning tool used in the weekly 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 will be executed, and that more observing time has been awarded than can be physically scheduled to minimise the time that the CHEOPS satellite is idle.

Principal Investigators of proposals that have been awarded time will be 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 in the document at this link. Feedback will be provided to the PIs of proposals that were not awarded observing time. 

* Programmes #21 and #35 have a target in common. Given the similarity of the proposed observations, a single observation will be made and the data shared.

Abstracts

ID0001 — Masses, radii, and limb darkening on the lower main sequence (PI: Guy Worthey)
For stars on the lower main sequence whose interiors are largely convective, theory misses the observed mass-radius relation by several percent. A strong empirical mass-radius relation, along with star spotting characterization, can help narrow alternative convection prescriptions and the treatment of the superadiabatic layer. Eclipsing binaries are the laboratories needed to measure radius. Among low-mass eclipsing binary stars where convection is expected to dominate both components, only three systems are bright enough for accurate CHEOPS observations and only one is observable: CU Cnc = GJ 2069A, nearly equal-mass M5 Ve stars. We propose to obtain a light curve much more accurate than any previous attempt to refine the mass and radius solution, and also to investigate limb darkening laws and starspots in this flare star.

ID0002 — Investigating the multicolor phase curves of underexplored giant exoplanets (PI: Szilard Kalman)
By analysing the full phase curves of exoplanets (including the beaming and ellipsoidal effects, the reflection of the planetary atmosphere and the secondary eclipses), we can put constrains on the planetary atmospheres and interiors. We have selected four targets (WASP-7b, WASP-13b, WASP-156b and XO-6b) where we have found evidence of the secondary eclipses by analysing their TESS light curves. We therefore propose the observation of these four targets using CHEOPS. As the two satellites have different bandpasses, observation of the phase curves in two colours can help untangle the light reflected by the planetary atmosphere and the emitted light by the planet. We request observations with CHEOPS covering the 30-30% of the orbital phase centred at the orbit and the eclipse for these planets.

ID0003 — Two sub-Neptunes in short orbit around two bright young G and K stars (PI: Alexis Heitzmann)
To understand the formation and evolution of planetary systems, there is a crucial need to map exoplanets parameters as a function of time. Young planetary systems are the most important to focus on, as they provide live snapshots of the processes at play and are currently dramatically under sampled. For planets discovered by survey mission such as the Transiting Exoplanet Survey Satellite (TESS), follow-ups required to characterise planetary orbits and atmospheres are only possible with a precise knowledge of the planets transit times. Thanks to CHEOPS, we will be able to refine the transit time ephemerides of two sub Neptunes less than 500 Myr old and in short-orbit, TOI-5082 b and TOI-5169 b. Five years from now and for both planets, the transit mid-time uncertainty will shrink from ~2 hours when solely relying on TESS data to less than 10 minutes with CHEOPS observations.

ID0004 — Determining the internal structure of the young hierarchical triple HD 144548 (PI: Hans Deeg)
HD 144548 is a young (8-11 Myr) triply eclipsing triple star in the Upper Scorpius OB association that displays complex eclipses from simultaneous occultations of all components (syzygys). This system is unique for the availability of both precise Kepler/K2 eclipse light curves from and radial velocities of all three components. In Alonso et al. (2015) we derived the components’ physical and orbital parameters with high precision, indicating significantly inflated stellar sizes, albeit the size of the system’s largest component is in tension with established models of young stars. With CHEOPS we intend to reobserve several eclipses towards an actualized determination of the systems parameters. Particularly, this should lead to a much improved determination of the system’s osculating orbital parameters, due to a coverage that would span 9 years versus the 79 days from K2. Such improved osculating parameters should permit a derivation of the internal mass-distribution of the system’s inner binary through determination of the apsidal constant k_2. Reducing uncertainties on the orbital parameters found by Alonso et al. will also significantly improve the precision of the system’s physical parameters, and permit a calibration of pre-main sequence star parameters and of the evolution of their stellar interiors. CHEOPS is uniquely suited to perform the observations of the long complex eclipses, with a required (and expected) precision that should rival the original Kepler/K2 data.

ID0008 — Bridging CHEOPS and Twinkle: forming long TTV baselines for the multi-planet system K2-266 (PI: Ing-Guey Jiang)
The measuring of transit timing variations (TTVs) allows one to explore the dynamical interactions of planets within a system, shedding light on the properties of these bodies. Here propose to observe the transits of two planets in the K2-266 system, K2-266d and K2-266e, in order to more robustly constrain the previously reported TTVs. These data will allow us to obtain much more precise mass and orbital parameters of this unique system. TESS has observed the system already, adding to the time baseline relative to the original K2 observations, but the TESS data are not accurate enough to distinguish between competing models. However, as discussed in the scientific justification of this proposal, the CHEOPS data proposed here is able to distinguish between different models with small differences in the system parameters. The ability to precisely distinguish between competing models means the CHEOPS data will provide a high precision measurement and bring a large impact into the study of this astronomically important planet system. Furthermore, as members of the Twinkle Space Mission’s exoplanet TTV Working Group, we aim to observe this system with Twinkle in the future. While the data proposed for here is valuable in its own right, combining data from both these small low-Earth orbit satellites over a long baseline will yield better results than using either one alone.

ID0009 — CHEOPS @ School - building bridges between education and space science (PI: Kate Isaak)
The topic of exoplanets speaks directly to the very simple and profound question of whether we are alone in the Universe and is easy to relate to. As such, it provides an excellent means through which to engage young people in some of the thrills of science and, critically, a vehicle with which to teach analytical skills. We propose to use CHEOPS observations as the basis for a new ESA Education classroom project to run in 2022/2023. We request a total of 20 CHEOPS orbits for the 2-part project, in which teams of 15 - 19 year olds will write a short CHEOPS proposal to observe a transiting exoplanet and analyse the resulting light curves, with support from ESA and local early-career mentors. We plan to involve the CHEOPS/exoplanets community in the project as mentors and project reviewers, also in publicising of the activity.

ID0010 — Two baby sub-Neptunes, one visit: a snapshot of atmosphere evolution (PI: Nataliea Lowson)
Sub-Neptunes are the most abundant population of planets around Sun-like stars, yet we have no analogue within our own Solar System. Therefore, atmosphere analysis of adolescent sub-Neptunes is the best way to investigate the mechanisms that shape this dominant population. We will use CHEOPS to confirm and characterise two sub-Neptunes (HIP113103.01 and HIP113103.02) around a bright ~500 Myr old star, HIP113103. With an irradiation 50 times greater than Earth, we hypothesise HIP113103.01 is undergoing primordial atmospheric evaporation while HIP113103.02 maintains its primordial atmosphere. Such a situation makes this system a perfect case study to determine to what extent are these atmospheres a product of evaporation or outgassing with the next generation of space and ground-based telescopes. We propose 10 orbits to observe both planets in a single visit, for CHEOPS is the only facility capable of detecting the 600 ppm transits of HIP113103.01 with a single transit. These observations will also refine the transit depth of both planet candidates by 40%, providing the necessary precision required for future in-depth atmosphere characterisations.

ID0011 — Transit Timing Variations of XO-3b: gaining insights into the orbital evolution of a hot Jupiter (PI: Fan Yang)
XO-3b is a hot Jupiter (Winn et al. 2008) with reported transit timing variations (TTVs), which provides evidence for a decaying orbital period (Yang & Wei 2022). The physical origin of the TTVs are not clearly known. The high sampling rate and precision of CHEOPS data would result in accurate transit timing, which would distinguish between different physical scenarios for the orbital evolution of XO-3b. The timing difference between a constant period model and a period decay model would be 8 minutes for CHEOPS AO3, which would resolve the uncertainties that arise from the scatter in early measurements from ground-based telescopes. Measuring the transit times with such high precision will provide the first constraints on the interior structure of XO-3b if tidal dissipation is the dominant mechanism for orbital evolution, and along with the results for WASP-161b, will reveal the physical processes that resulted in the formation of these hot Jupiters. We therefore propose observing three primary transit events of XO-3b, with each visit lasting 7 orbits, using CHEOPS.

ID0013 — Exploiting the color difference between CHEOPS and TESS for exoplanet candidate validation (PI: Zoltan Garai)
The most reliable technique to validate a planet candidate is measuring its true mass applying the radial velocity (RV) method, together with the transit photometry method. However, RV observations are difficult in many cases, e.g., in cases of fast rotating stars, hot stars, or stars with few absorption lines. An alternative method is the multicolor transit photometry. Following this method, we propose to utilize the color difference between TESS and CHEOPS for validation of 2 TESS exoplanet candidates, namely TIC 335630746 and TIC 91987762, where the RV method could be difficult. In the case of TIC 91987762 we also aim at determining the true orbital period of the transiting object.

ID0014 — Atmospheric characterisation of the hot-Jupiter WASP-79 b with CHEOPS (PI: Quentin Changeat)
Hot Jupiters are prime targets for atmospheric characterisation due to their large atmospheric signatures. However, despite their intense scrutiny in the last decade, their nature and properties remain largely unknown. Theoretical studies have suggested that they might come in two distinct classes depending on their temperatures and the presence of visible absorbers. Nevertheless, as of today, only a few studies have been able to assess the validity of those predictions and the presence of visible absorbers in the hottest planets remains largely evasive. Here, we propose to observe the hot Jupiter WASP-79 b with CHEOPS to investigate its atmosphere. WASP-79 b, which sits in at the transition between the two regimes, is one of the best targets for atmospheric characterisation. The data obtained with CHEOPS during four eclipses will allow us to characterise the presence of visible absorbers and determine if thermal inversions occur in the atmosphere of WASP-79 b. The presence of those absorbers has ambiguously been suggested in a number of studies for this planet, both at the day-side and the terminator, but CHEOPS is in a unique position to confirm or refute those claims. We will also combine the obtained dataset with spectroscopic observations by the Hubble Space Telescope, which will provide a new perspective on the atmosphere of this fascinating planet.

ID0016 — CHEOPS Phase Curves to Characterize the Clouds Composition of Hot Jupiters (PI: Lucas Teinturier)
Observing and modeling efforts have recently been made to begin the atmospheric characterization of It is thought that clouds are ubiquitous in the atmosphere of Hot Jupiter. However, distinguishing between the different kinds of condensates is hard, as degeneracies emerge when analyzing photometric and spectral data. High precision optical phase curves can help disentangle these degeneracies as the peak offset of the optical phase curve is sensitive to the cloud's composition. We aim to observe two phase curves of the Hot Jupiter HIP 65A b and three phase curves of the Hot Jupiter WASP-77A b. HIP 65A b has an equilibrium temperature of 1410 K, an orbital period of 0.98 days and a poorly constrained radius between 2.64 and 1.54 RJ. WASP-77A b has an equilibrium temperature of 1715 K, an orbital period of 1.35 days and a radius of 1.230 RJ. These observations will allow us to refine the constraints on the planetary radius (in particular for HIP 65A b), to constrain the geometric albedo and potentially to discriminate between different forming clouds. They will also be useful for the preparation and interpretation of future observations by JWST and ARIEL as well as characterizations by high spectral resolution observations by SPIRou or CRIRES.

ID0017 — Refining radii for planets in the radius valley (PI: Hannah Osborne)
The Kepler mission revealed that small planets could be split into two subgroups – super-Earths and sub-Neptunes – divided by a radius valley, where very few planets are found. The precise location of the valley informs our understanding of planetary evolution i.e. how planets lose their atmospheric layers and cross from above the valley to below. As well as this, the emptiness of the valley can be used to indicate whether planets typically form with a homogeneous core composition or not i.e. a mixed core composition model predicts a less-empty radius valley. A small number of planets have been observed to apparently sit within the radius valley, however uncertainties in planetary radii mean it is challenging to confirm their location precisely. We propose to target 3 planets thought to be inside the radius valley. The greater precision of the CHEOPS transits will improve radius measurements for the targeted planets, allowing us to confirm whether they are really inside the radius valley. If all three planets are confirmed to be located within the radius valley, then this will reinforce the theory of a mixed core composition formation scenario. The precise location of these planets can also be used to refit the location of the radius valley, which in turn provides an indication of which evolution pathway is most likely.

ID0018 — Constraining the morning and evening limbs of the hot jupiters WASP-79b and WASP-101b (PI: Jayshil Patel)
Observations of an exoplanet during a transit event has provided vast information regarding its bulk and atmospheric properties with photometric and spectroscopic observations. The usual assumption made while analysing thus observed lightcurves is that the thin terminator region around the disk of an exoplanet, the part seen in these kinds of observations, is uniform throughout the planetary disk. However, the most recent theoretical developments using 3D general circulation models of the planetary atmospheres have shown that, especially in the case of planets with high equilibrium temperatures, the approximation of uniform terminator is not good enough. These planets have a temperature gradient across the terminator that can produce chemical and aerosol inhomogeneities in this area. This non-uniform terminator will, in turn, produce asymmetric lightcurves, in both photometry and spectroscopy. This program aims to detect the direct evidence of this effect in exoplanet atmospheres using ultra-high precision photometry from CHEOPS. The effect of these asymmetries on the transit lightcurve can be as large as 1000 ppm for the most suitable targets, which CHEOPS should detect comfortably. To this end, based upon previous observations made with HST, WASP-79b and WASP-101b would be the most suitable targets to explore this effect with CHEOPS. Constraints from CHEOPS observations on aerosol properties would be very crucial in spectroscopic analysis of them with future observations.

ID0019 — Cloudiness of three warm Sub-Neptunes (PI: Amelie Gressier)
Substantial efforts have been made to analyze and interpret exoplanetary atmospheres this past decade. However, while the development of forward and retrieval codes, lead to atomic and molecular absorption discoveries in both emission and transmission spectroscopy, clouds remain difficult to characterize and detect. We want to assess the cloudiness of three warm Sub-Neptunes using CHEOPS bandpass in the optical. While there is no doubt that these planets have a light, hydrogen dominated, atmosphere, the presence of clouds or hazes in the upper atmosphere is more difficult to assess, and this can be investigated with an accurate optical measurement for which CHEOPS is in an excellent position to provide. The goals of our study are, first, to refine the planetary transit depth in the optical, and, secondly, by comparing the value found by CHEOPS in transmission, to atmospheric clear and cloudy forward models, confirm or refute the presence of clouds in the atmosphere of Sub-Neptunes. Besides, we want to compare CHEOPS value to previous atmospheric measurements of the targets, particularly with the Hubble Space Telescope in the near-infrared, and, constrain the cloudiness of this type of planets to better define observational strategies for the James Webb Space Telescope and ARIEL. CHEOPS observations offer the unique opportunity to study consistently the cloudiness of an intriguing population of planets.

ID0021 — Exploring the Diversity of Small Planet Compositions (PI: Diana Dragomir)
The sizes and masses of planets provide a powerful constraint on their composition. But until recently, the exoplanet community had access to large samples of just radii for small planets. TESS is enabling us to finally also measure the masses of an ensemble of small planets. Complementarily, we propose to use CHEOPS to measure precise radii for seven small TESS planets in order to constrain their interior composition, and explore the period-radius valley. The proposed observations will also serve to constrain the orbital eccentricity of these planets, provide an anchor point at visible wavelengths for infrared transmission spectroscopy observations. For one planet, it may also significantly refine the ephemeris, since it is not yet known whether TESS will re-observe it.

ID0022 — Confirming the Eclipse Variability of the Ultrahot Neptune, LTT 9779b (PI: James Jenkins)
The study of the chemistry, dynamics, and evolution of exoplanetary atmospheres is providing a treasure-trove of discoveries that are allowing us to understand the nature of planets in the galaxy to an extent not previously possible. Although most efforts have been focused on hot Jupiters, (given they represent the best target population due to their relatively large atmospheres), we are still only scratching the surface. More recently, hot Neptune-like exoplanets have been found orbiting stars bright enough that we can perform detailed atmospheric analyses. One standout member of this population is LTT 9779b, the first Ultrahot Neptune, a rare world that orbits its star in only 19 hours, placing it firmly in the Neptune Desert. Using CHEOPS we observed 10 eclipses of the planet behind the star, yet only six of the light curves show evidence for any occultation, and they show surprisingly deep eclipses of >100 ppm. Therefore, it is highly possible that we are witnessing the evolution of the reflective layer of the planet's atmosphere, being modulated by its rotational spin. In order to confirm planetary eclipse variability for the first time for such a world, we request an additional 10 eclipse observations, such that we can search for a modulating signal, building up the statistics to allow for a robust detection. We shall then use the latest global circulation models to precisely constrain the scattering properties of the layer, along with its temporal evolution.

ID0023 — The TTVs of a compact planetary system around an early-type star with CHEOPS (PI: Tyler Fairnington)
The planetary formation and evolution process around early type stars are poorly mapped. We propose to use CHEOPS to observe a benchmark multi-planetary system (TOI-5126) around a young F star. The system hosts a Neptune sized planet with a 5.46 day orbital period (TOI-5126.01), and a sub-Neptune on a 17.9 day orbit (TOI-5126.02). Further, there are potential Transit Timing Variations (TTVs) exhibited in the TESS data at the 5-minute level. We will use CHEOPS to validate the candidate TTV signals of TOI-5126.01, as well as put mass constraints on the planet. These observations are essential for the confirmation of the planetary nature of the TOI-5126 system, enabling future atmospheric characterization and density measurements.

ID0024 — Photometric Confirmation of The Brightest-Host Transiting Habitable Zone Terrestrial Exoplanet (PI: Nicholas Scarsdale)
We propose to use CHEOPS photometry to confirm the existence of the Transiting Exoplanet Survey Satellite (TESS) planet candidate TOI4353.01 (TIC176797879). This object is nominally a terrestrial-size exoplanet with two transits separated by 718 days in the TESS photometry. However, one transit occurs near the peak of a large spike in background flux. Because of the ease of matching a single valid transit to transit-like noise, it is difficult to establish with high confidence that this first transit event is real, though we take several measures to do so. By searching for another transit with CHEOPS, we can test the existence of a transiting candidate around this star. Based on the transit duration and a priori likelihood of transit, the true period is very unlikely to be 718 days, and instead is more likely to be 25-60 days. If the signal is confirmed to be real by the proposed observations, it would pave the way for future followup. Such a signal would represent a terrestrial planet candidate with an overwhelming probability of being in the optimistic habitable zone (OHZ) and potential for future mass measurement and even transmission spectroscopy thanks to its bright (for an M star) host. Because of the great rarity of terrestrial OHZ exoplanets and the difficulty of discovering more with TESS due to its short sectors, followup of candidates like this one is of critical importance.

ID0026 — Ephemeris Refinement of Key Targets for the ESA-Ariel Mission (PI: Billy Edwards)
The ESA-Ariel mission seeks to observe the atmospheres of around 1000 exoplanets. To maximise the science yield of the mission, the sample of planets studied must be as diverse as possible, probing all areas of the exoplanet population. To allow for this, the catalogue of planets from which the Ariel sample is chosen must be significantly larger. For each of these potential targets, a likely requirement is that number of key parameters are well characterised. One piece of knowledge that is undeniably obligatory is the predicted transit time of the planet in question. Keeping track of such a large number of worlds necessitates a well-structured follow-up programme and so the Ariel consortium has launched ExoClock, which allows professional and amateur astronomers from around the world to support this effort. Nevertheless, there are some planets which simply cannot be studied from the ground, and for which TESS data is also of insufficient quality, so another solution is required. Here we propose to use the excellent photometric performance of CHEOPS to study 9 key worlds for the Ariel mission. By observing these targets, CHEOPS will be ensuring they are available for future atmospheric characterisation and thereby providing key support for another ESA mission.

ID0027 — Readying a warm sub-Neptune orbiting around the bright, Sun-like star TOI-4320, for future atmospheric characterisation (PI: Nora Eisner)
Sub-Neptune sized planets are amongst the most common types of exoplanets in our Galaxy, however, their formation and composition are still widely debated. Through transit spectroscopy we can probe their atmospheric composition and thus help constrain theories of how these systems form and evolve. We propose to refine the ephemerides and radius of a sub-Neptune sized planet orbiting around a bright, Sun-like star, that was identified in the TESS data by citizen scientists taking part in the Planet Hunters TESS project. The refined orbital period that we can determine with just one additional transit observation with CHEOPS will ensure that the mid-transit time of this target will be sufficiently constrained for future atmospheric characterisation with JWST or Ariel. Furthermore, the refined planet radius, combined with our ongoing radial velocity campaign to determine the mass of this planet, will allow us to constrain the bulk composition and thus infer the internal structure of the planet. Even though sub-Neptune sized planets are common, this one stands out due to its long orbital period (~25 days) compared to the majority of TESS planets, and due to the brightness of the host star (Vmag = 9.17). To date, less than 3% of all known sub-Neptune size planets transit stars with a Vmag brighter than10, highlighting the unique opportunity presented by this system.

ID0028 — Hunting for a Transit of a HARPS-N Known Exoplanet Target around a TESS Blend (PI: Thomas Wilson)
Since the birth of the exoplanet field, the complementarity of the transit and radial velocity (RV) techniques has been well-utilised with a key example being the transit follow-up of the RV planet; HD 209458 b. Importantly, the extended observing campaigns of RV instruments can yield detections of longer-period planets that, if transiting, can give a unique glimpse into a currently observationally rare space, see Nu2 Lupi. The HARPS-N Consortium has been conducting a decade-long survey of bright stars that yielded the discovery of the four planet system HD 219134 that was found to be transiting by Spitzer. There are several more long-period candidates that need to be followed-up with transit photometry to try to obtain a planetary radius and density. Due to TESS’s observing strategy it is unlikely to find longer-period bodies and because of the large pixel scale transits may be hidden if there is a close, bright stellar contaminant that causes blending. CHEOPS is unique and necessary to search for transits of small RV targets with contaminants that would be too shallow for ground-based instruments. We find one target with a >8 sigma RV signal on a period of 24d whose host star is part of a binary system with a separation of 18". We propose for a modest amount of time to hunt for a transit of a low-mass planet around a TESS blended target that could open up a new avenue for characterisation of Earth-like, long-period planets that could open up a new utility for CHEOPS.

ID0029 — Constraining Refractory Species and Characterising the Stellar Environment of the Inflated hot-Jupiter WASP-17 b (PI: Arianna Saba)
In the known population of exoplanets, puffy hot-Jupiters are extremely rare and yet crucial in order to understand planetary formation and evolution. Their large radii coupled with the high S/N ratio achievable thanks to their extended atmospheres, makes them the perfect target for current observational facilities. In this restricted pool of planets, WASP-17b is an ideal case study. With an equilibrium temperature of ~1800 K, this inflated hot-Jupiter is expected to display signatures of metal oxides and hydrides in absorption. However, recent studies showed that the host star, a F6 dwarf, appears to be considerably more active than expected. The ability of stellar spots and faculae to mimic absorption features in a planetary atmosphere or hide them altogether, is one of the most serious obstacles in exoplanet transmission studies. The high-precision photometer aboard CHEOPS is the optimal instrument to monitor the transit of WASP-17b for two main reasons. Being centred at 0.6 µm, the CHEOPS photometer targets specifically the wavelengths where the stellar spectrum peaks and where optical absorbers display strong absorption features. Hence, CHEOPS data is essential to solve the ongoing disputes on this planetary system regarding the activity of the host star and the presence of refractory elements in the atmosphere of the companion planet. At least 11 transits with a very high coverage (>65%) are expected to occur during AO-3, making WASP-17 b an excellent target for CHEOPS.

ID0030 — Unveiling the origin of an in-phase additional dimming in the TESS light curve of a known exoplanet (PI: Jorge Lillo-Box)
Along the past years, accumulations of gas and dust have been hinted for the first time at the Lagrangian points of some young forming planets. Also, the first planet-like co-orbital candidates have been published. These discoveries represent the rise of a new window to exoplanet exploration: the search for co-orbital worlds. These bodies contain key dynamical and chemical information about the history of planetary systems. They are also predicted to form in a relatively frequent number of multi-planetary systems (13-30% according to theoretical studies) and to survive inward migration with the planet at the cost of an increase in their libration amplitude around the Lagrangian points. In the path towards the confirmation of the first exotrojan planet, we detected a new candidate through observations from the TESS mission and subsequent ground-based follow-up with the SPECULOOS telescopes. The analysis from both datasets shows independently a 2-σ dimming at the Lagrangian point L5 of a confirmed extrasolar hot-Jupiter. The candidate transits also show some variations in the timing possibly due to the expected libration. Here we propose to use CHEOPS to definitively confirm this dimming with sufficient statistical confidence and to characterize this potential first exotrojan planet.

ID0032 — Pinning down orbital period, transit ephemeris, and radius of the infant planet V1298 Tau e (PI: Mario Damasso)
V1298 Tau is a very young (20±10 Myr) star that hosts four planets discovered by Kepler/K2. It is a unique laboratory to test planet formation and evolution theories right after system's birth. The outermost planet V1298 Tau e was observed in transit only once by K2, and a second transit was detected later on by TESS, thus orbital period and ephemeris are presently not yet constrained. This greatly affects the analysis of radial velocities and the mass determination of the planet. We are intensively studying and monitoring V1298 Tau. Since 2019 we are intensively following it up with HARPS-N and CARMENES to determine planet masses and bulk densities, with focus on planet e. So far, our published results provided preliminary estimates which need to be revised once the transit ephemeris of planet e will be accurately known. K2 and TESS observations also revealed that the transit depth is hampered by stellar activity, thus observing transits with CHEOPS will allow for a more accurate measurement, needed to pin down the bulk density of the planet. CHEOPS will provide crucial results to characterise physical and architectural properties of the whole planetary system. CHEOPS observations will complement our large dataset of radial velocities, allowing for fully exploitation. The results will be crucial to carry on a thorough study of the formation and dynamical evolution of the system at the earliest phases, and to predict the future evolution of planets' atmospheric structure.

ID0034 — Rescuing Longer Period TESS Planet Candidates for Future Atmospheric Characterisation (PI: Billy Edwards)
Keeping track of the colossal number of new exoplanet detections is becoming ever more difficult. Yet, if we wish to conduct a thorough characterisation of these worlds, by studying its atmosphere for instance, we must be sure of when the planet is going to transit. Due to the short observing sectors of TESS (< 30 days), our knowledge of the ephemerides of these worlds is quickly decaying. In fact, by the time most planet confirmation papers are published the uncertainty on the transit time is often into the tens of minutes and, when followed-up, large shifts are often seen in the transit times. The situation is even worse for longer period planets, which are also more rarely found by the TESS mission. Hence, we propose to conduct timely observations of seven of the best longer period TESS planet candidates for atmospheric characterisation. These data will ensure that our knowledge of their ephemeris remains up-to-date and so will facilitate the possibility for atmospheric characterisation in the near-future. CHEOPS is the only facility capable of these observations and the dataset proposed will be of great value to the exoplanet community.

ID0035 — Dynamical masses and bulk compositions of newly-found TESS and ASTEP-confirmed sub-Neptunes (PI: Bruno Merin)
The Sub-Neptunes recently detected by the TESS, Kepler and K2 surveys, continue to challenge our understanding of planetary systems, in particular in relationship to the radius valley. Their physical nature also constitutes a mystery with their bulk compositions still allowing them to be either rocky, water worlds or gas-giants (e.g. Rogers and Seager 2010), possibly in connection to photoevaporation of their initial atmospheres as they migrate towards the stars (e.g. Owens and Wu, 2013) or due to core-powered mass-loss (e.g. Rogers et al. 2021). We request high-quality CHEOPS observations to dynamically determine the planet masses and radii via transit timing variation (TTV) measurements, to identify prime targets for atmospheric follow-up characterisation with JWST, to put constraints on their formation histories and to search for further non-transiting and still undetected long-period planets via the TTVs. The five planets are the unique best targets for this science case in CHEOPS AO-03 to our knowledge (observable both by CHEOPS and ASTEP, not observed by CHEOPS yet and not in the CHEOPS Reserved Target List).

ID0036 — Stellar Occultation by Minor Bodies in our Solar System with CHEOPS (PI: Altair Ramos Gomes Junior)
Stellar Occultation is one of the leading techniques in accessing important physical characteristics of solar system bodies. With it, we were able to discover rings around the Centaur Chariklo and the TNO Haumea, detect sub-km TNOs and drastic variations of Pluto's atmospheric pressure. The knowledge of such characteristics has been important in understanding the origin and evolution of the solar system. Since the technique translates photometric resolution into astrometric resolution, a better photometric condition will provide improved constraints in the determination of the body’s physical characteristics. As a natural step forward, observing stellar occultation by spacecraft will provide better photometric circumstances avoiding the noise caused by atmospheric fluctuations. With CHEOPS, five interesting stellar occultations have considerable chances of being observed, involving the Neptunian satellite Triton, where we can further study its atmosphere, the TNO Quaoar and Centaur Chiron, where we can improve the knowledge about the presence of rings, and the TNO 2002MS4, where we can explore its environment. Note: We do not use the CHEOPS Exposure Time Calculator for this science case because we need the highest time resolution possible in imagette mode.

ID0037 — K2-149: unveiling planet interaction of the 2nd most highly packed system with four new sub-Neptunes (PI: Octavi Fors)
We announced the discovery of four new sub-Neptunes (c, d, e, and f) orbiting K2-149, an M1 G=13.779 mag star. K2-149 c, d, e and f orbital periods and radii were found to be 16.37, 21.38, 37.23 and 57.47 days, and 1.61, 1.82, 1.53 and 2.08R⊕, respectively. K2-149 b, also a sub-Neptune, was previously confirmed as the first planet in the system. The four discovered planets were observed by K2 30min cadence data in early 2016. Given the long orbital periods and the relatively long transit durations, this results in large MCMC uncertainties, especially for T0. Moreover, these T0 uncertainties once forward-propagated to current day lead to as high as ~2 days for planet d. These large T0 uncertainties could be linked to the possible interaction between K2-149 d - K2-149 e as seen in our mean-motion resonances analysis, probably detectable via transit-timing variations (TTVs). K2-149 is the second most packed multi-planetary system of transiting sub-Neptunes after Kepler-444. Other systems such as K2-138, Kepler-11, Kepler-20 show orbital gaps between the innermost and outermost planets. CHEOPS is the only facility that will efficiently allow us to refine the transit ephemeris. Thus, we propose CHEOPS observations of K2-149 to investigate in detail the architecture of this multiplanetary system and, in particular, the possible interactions between K2-149 planets.

ID0038 — Precise Mass Measurement for the Compact Object in LS 5039: The Heaviest Neutron Star or the Lightest Stellar-mass Black Hole? (PI: Jianfeng Wu)
LS 5039 is a gamma-ray binary containing a compact object within the mass gap (2-5 solar masses). It could potentially be the heaviest neutron star or the lightest stellar-mass black hole. However, to precisely constrain its mass, high-precision optical photometry at the level of 0.1 mmag (100 ppm) is required. Therefore, we propose a CHEOPS monitoring campaign to obtain the needed light curve.

This page was last updated on 27 May 2022.