HD 23472: a multi-planetary system with three super-Earths and two potential super-Mercuries,

Abstract

Context. Comparing the properties of planets orbiting the same host star, and thus formed from the same accretion disc, helps in constraining theories of exoplanet formation and evolution. As a result, the scientific interest in multi-planetary systems is growing with the increasing number of detections of planetary companions. Aims. We report the characterisation of a multi-planetary system composed of five exoplanets orbiting the K-dwarf HD 23472 (TOI-174). Methods. In addition to the two super-Earths that were previously confirmed, we confirm and characterise three Earth-size planets in the system using ESPRESSO radial velocity observations. The planets of this compact system have periods of P d " 3.98 , P e " 7.90 , P f " 12.16 , P b " 17.67, and P c " 29.80 days and radii of R d " 0.75 , R e " 0.82,, R f " 1.13 , R b " 2.01, and, R c " 1.85 R C . Because of its small size, its proximity to planet d's transit, and close resonance with planet d, planet e was only recently found. Results. The planetary masses were estimated to be M d " 0.54 ˘0.22, M e " 0.76 ˘0.30, M f " 0.64 `0.46 ´0.39 , M b " 8.42 `0.83 ´0.84 , and M c " 3.37 `0.92 ´0.87M C . These planets are among the lightest planets, with masses measured using the radial velocity method, demonstrating the very high precision of the ESPRESSO spectrograph. We estimated the composition of the system's five planets and found that their gas and water mass fractions increase with stellar distance, suggesting that the system was shaped by irradiation. The high density of the two inner planets (ρ d " 7.5 `3.9 ´3.1 and ρ e " 7.5 `3.9 ´3.0 g.cm ´3) indicates that they are likely to be super-Mercuries. This is supported by the modelling of the internal structures of the planets, which also suggests that the three outermost planets have significant water or gas content. Conclusions. If the existence of two super-Mercuries in the system is confirmed, this system will be the only one known to feature two super-Mercuries, making it an excellent testing bed for theories of super-Mercuries formation. Furthermore, the system is close to a Laplace resonance, and further monitoring could shed light on how it was formed. Its uniqueness and location in the continuous viewing zone of the James Webb space telescope will make it a cornerstone of future in-depth characterisations.

A Comparison of the Composition of Planets in Single-planet and Multiplanet Systems Orbiting M dwarfs

“…In other words, we might expect the innermost planets in multiplanet systems to be more iron-rich than their outer siblings. For instance, Barros et al (2022) characterized the HD 23472 system, which contains 5 small planets orbiting a K-dwarf, and they found a clear decrease in the core mass fractions of the planets with increasing period. Similarly, they found that the water and gas mass fraction increases as a function of the period such that the innermost planets are dry and dense and the outermost ones have much larger fractions of water and gas.…”

“…These ultra-dense planets could be super Mercuries (see, e.g.,Santerne et al 2018;Adibekyan et al 2021;Schulze et al 2021;Rodríguez Martínez et al 2023, andBarros et al 2022), but they have relatively large mass uncertainties so more observations are needed to robustly determine their composition. 9 K2-18 c has been recently validated byRadica & Artigau (2022), however, we do not include it in our analysis as it does not have a radius measurement.…”

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

“…Of more than 5300 confirmed exoplanets 18 , there are 66 sub-Neptune sized planets (1.75 < R P / R ⊕ < 3.5) transiting bright stars (G ≤ 10 mag). Only 18 of these have an insolation flux less than HD 15906 b and only 5 have an insolation flux lower than HD 15906 c -GJ 143 b (Dragomir et al 2019), 𝜈 2 Lupi d (Delrez et al 2021), HD 23472 c (Barros et al 2022), HD 73583 c (Barragán et al 2022) and Kepler-37 d (Marcy et al 2014). HD 15906 c is therefore one of the most lowly irradiated sub-Neptune planets transiting such a bright star.…”

“…HD 15906 c is therefore one of the most lowly irradiated sub-Neptune planets transiting such a bright star. Furthermore, there are only 5 other multiplanet systems with two warm (T eq ≤ 700 K) sub-Neptune sized planets (1.75 < R P / R ⊕ < 3.5) transiting a bright (G ≤ 10 mag) star -HD 108236 (Bonfanti et al 2021), 𝜈 2 Lupi (Delrez et al 2021), HD 191939 (Orell-Miquel et al 2023, HD 23472 (Barros et al 2022) and TOI 2076 . The HD 15906 system is therefore an interesting target for future follow-up studies, discussed further in Section 7.4.…”

DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections

“…Without the context of hosting an S-type planet, DMPP-3AB is not a particularly close binary, but it is the most compact binary system to harbour an S-type planet observed thus far. It is also one of the few systems containing a radial velocity (RV) detected S-type super-Earth around an FGK star (B20; Unger et al 2021;Barros et al 2022). Su et al (2021) compare the known S-type RV discoveries, and show that DMPP-3A b is an extreme outlier in their sample.…”