Refined parameters of the HD 22946 planetary system and the true orbital period of planet d

Abstract

Context. Multi-planet systems are important sources of information regarding the evolution of planets. However, the long-period planets in these systems often escape detection. These objects in particular may retain more of their primordial characteristics compared to close-in counterparts because of their increased distance from the host star. HD 22946 is a bright (G = 8.13 mag) late F-type star around which three transiting planets were identified via Transiting Exoplanet Survey Satellite (TESS) photometry, but the true orbital period of the outermost planet d was unknown until now. Aims. We aim to use the Characterising Exoplanet Satellite (CHEOPS) space telescope to uncover the true orbital period of HD 22946d and to refine the orbital and planetary properties of the system, especially the radii of the planets. Methods. We used the available TESS photometry of HD 22946 and observed several transits of the planets b, c, and d using CHEOPS. We identified two transits of planet d in the TESS photometry, calculated the most probable period aliases based on these data, and then scheduled CHEOPS observations. The photometric data were supplemented with ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) radial velocity data. Finally, a combined model was fitted to the entire dataset in order to obtain final planetary and system parameters. Results. Based on the combined TESS and CHEOPS observations, we successfully determined the true orbital period of the planet d to be 47.42489 ± 0.00011 days, and derived precise radii of the planets in the system, namely 1.362 ± 0.040 R⊕, 2.328 ± 0.039 R⊕, and 2.607 ± 0.060 R⊕ for planets b, c, and d, respectively. Due to the low number of radial velocities, we were only able to determine 3σ upper limits for these respective planet masses, which are 13.71 M⊕, 9.72 M⊕, and 26.57 M⊕. We estimated that another 48 ESPRESSO radial velocities are needed to measure the predicted masses of all planets in HD 22946. We also derived stellar parameters for the host star. Conclusions. Planet c around HD 22946 appears to be a promising target for future atmospheric characterisation via transmission spectroscopy. We can also conclude that planet d, as a warm sub-Neptune, is very interesting because there are only a few similar confirmed exoplanets to date. Such objects are worth investigating in the near future, for example in terms of their composition and internal structure.

TESS and CHEOPS discover two warm sub-Neptunes transiting the bright K-dwarf HD 15906

“…This work has demonstrated how CHEOPS can be used to follow-up TESS duotransits to expand the sample of long-period planets. Figure 17 (Garai et al 2023). Through our CHEOPS duotransit programme, we have contributed to the discovery of 6 planets with periods longer than 20 days, radii smaller than 5 R ⊕ and host stars brighter than G = 12 mag.…”

“…We select most of our targets from the TESS Objects of Interest (TOI) Catalog (Guerrero et al 2021) and from our specialised duotransit pipeline (Tuson & Queloz 2022). Through our CHEOPS programme, we have recovered the periods of two duotransits in the TOI 2076 system , one duotransit in the HIP 9618 system (Osborn et al 2023), one duotransit in the TOI 5678 system (Ulmer-Moll et al 2023) and one duotransit in the HD 22946 system (Garai et al 2023).…”

The “Drake Equation” of Exomoons—A Cascade of Formation, Stability and Detection

“…Somewhat surprisingly, the number of promising targets in our search area has increased very significantly in the past few years, and 6 out of 9 systems were discovered in 2021 or later. This rapidly increasing rate of discoveries can be devoted to the long observation time basis for planet search (e.g., repeated TESS visits) and the evolution of the observation strategies (e.g., follow-up long-period TESS planet candidates to determine the accurate period, (e.g., [122,[131][132][133]), which help the precise characterization of longperiod transiting planets. The chances of obtaining more and more appropriate candidates within the exomoon search region are increased due to the longer time coverage of the observations and also the better resolution of the full sky with appropriately long and precise observations (as indicated by the arrows in Figure 5.…”