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

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


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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 (?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.

Acceptance Date Jan 18, 2022
Publication Date Feb 26, 2022
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Publisher Oxford University Press
Pages 4551 - 4571
DOI https://doi.org/10.1093/mnras/stac199
Keywords techniques: photometric; radial velocities; planets and satellites: fundamental parameters; interiors; stars: individual: TOI-561 (TIC 377064495, Gaia EDR3 3850421005290172416)
Publisher URL https://academic.oup.com/mnras/article/511/3/4551/6516986

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