Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N and TESS

Abstract

We present a precise characterization of the TOI-561 planetary system obtained by combining previously published data with TESS and CHEOPS photometry, and a new set of 62 HARPS-N radial velocities (RVs). Our joint analysis confirms the presence of four transiting planets, namely TOI-561 b (𝑃 = 0.45 d, 𝑅 = 1.42 R ⊕ , 𝑀 = 2.0 M ⊕ ), c (𝑃 = 10.78 d, 𝑅 = 2.91 R ⊕ , 𝑀 = 5.4 M ⊕ ), d (𝑃 = 25.7 d, 𝑅 = 2.82 R ⊕ , 𝑀 = 13.2 M ⊕ ) and e (𝑃 = 77 d, 𝑅 = 2.55 R ⊕ , 𝑀 = 12.6 R ⊕ ). Moreover, we identify an additional, long-period signal (> 450 d) in the RVs, which could be due to either an external planetary companion or to stellar magnetic activity. The precise masses and radii obtained for the four planets allowed us to conduct interior structure and atmospheric escape modelling. TOI-561 b is confirmed to be the lowest density (𝜌 b = 3.8 ± 0.5 g cm −3 ) ultra-short period (USP) planet known to date, and the low metallicity of the host star makes it consistent with the general bulk density-stellar metallicity trend. According to our interior structure modelling, planet b has basically no gas envelope, and it could host a certain amount of water. In contrast, TOI-561 c, d, and e likely retained an H/He envelope, in addition to a possibly large water layer. The inferred planetary compositions suggest different atmospheric evolutionary paths, with planets b and c having experienced significant gas loss, and planets d and e showing an atmospheric content consistent with the original one. The uniqueness of the USP planet, the presence of the long-period planet TOI-561 e, and the complex architecture make this system an appealing target for follow-up studies.

LHS 1903 provides evidence for gas-depleted formation of planets around M-dwarfs

“…Our model is The LHS 1903 planetary system presented in detail in ref. [42], and more information can be found in the Methods and in previous studies [41,[43][44][45][46][47]. Our analysis shows that the H 2 /He gas mass fraction in the three innermost planets increases with orbital distance (see Extended Data Fig.…”

“…• The eccentricities, e, for all planets were constrained with half-Gaussian zeromean priors following refs. [41,173] with the width determined by the eccentricity distribution of compact multi-planet systems [174], such as LHS 1903, justified by dynamical arguments.…”

“…A previous study [40] found that kinematic thick disk stars, such as LHS 1903, likely do not host terrestrial planets. With our discovery that LHS 1903 b is below the radius valley, this system joins TOI-561 [41] as a rocky planet thick disk host and thus is an exception to this finding. Interestingly, we find that LHS 1903 e lies above the observed radius valley making it the only small (R p <4 R ⊕ ), long-period planet relative to other M-dwarf bodies with a refined radius and mass (∆ R p <5% & ∆ M p <33%) and a radius that is larger than literature model predictions [2][3][4], see Figure 3.…”

Generalized Peas-in-a-Pod: Extending Intra-System Mass Uniformity to Non-TTV Systems via the Gini Index

“…Most commonly, such long-period planets are often gas giants that form beyond the snowline of their system, and therefore represent an entirely different regime of both size and composition when compared to their close-in companions. For systems that instead contain ultra-short period (USP) members, such as TOI-561 (Weiss et al 2021, Lacedelli et al 2022, it has been shown that the USP planets are often markedly smaller than other members of the same system, and may have experienced a much more dynamically volatile formation history. (Winn et al 2018).…”