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

G Lacedelli*, T G Wilson, L Malavolta, M J Hooton, A Collier Cameron, Y Alibert, A Mortier, A Bonfanti, R D Haywood, S Hoyer, G Piotto, A Bekkelien, A M Vanderburg, W Benz, X Dumusque, A Deline, M López-Morales, L Borsato, K Rice, L FossatiD W Latham, A Brandeker, E Poretti, S G Sousa, A Sozzetti, S Salmon, C J Burke, V Van Grootel, M M Fausnaugh, V Adibekyan, C X Huang, H P Osborn, A J Mustill, E Pallé, V Bourrier, V Nascimbeni, R Alonso, G Anglada, T Bárczy, D Barrado y Navascues, S C C Barros, W Baumjohann, M Beck, T Beck, N Billot, X Bonfils, C Broeg, L A Buchhave, J Cabrera, S Charnoz, R Cosentino, Sz Csizmadia, M B Davies, M Deleuil, L Delrez, O Demangeon, B-O Demory, D Ehrenreich, A Erikson, E Esparza-Borges, H-G Florén, A Fortier, M Fridlund, D Futyan, D Gandolfi, A Ghedina, M Gillon, M Güdel, P Gutermann, A Harutyunyan, K Heng, K G Isaak, J M Jenkins, L Kiss, J Laskar, A Lecavelier des Etangs, M Lendl, C Lovis, D Magrin, L Marafatto, A F Martinez Fiorenzano, P F L Maxted, M Mayor, G Micela, E Molinari, F Murgas, N Narita, G Olofsson, R Ottensamer, I Pagano, A Pasetti, M Pedani, F A Pepe, G Peter, D F Phillips, D Pollacco, D Queloz, R Ragazzoni, N Rando, F Ratti, H Rauer, I Ribas, N C Santos, D Sasselov, G Scandariato, S Seager, D Ségransan, L M Serrano, A E Simon, A M S Smith, M Steinberger, M Steller, Gy Szabó, N Thomas, J D Twicken, S Udry, N Walton, J N Winn

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Citations (Scopus)
6 Downloads (Pure)


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 RM =2.0 M), c (P = 10.78 d, R =2.91 RM = 5.4 M), d (P = 25.7 d, R = 2.82 RM =13.2 M) and e (P = 77 d, R =2.55 RM = 12.6 R). Moreover, we identify an additional, long-period signal (>450 d) in theRVs, which could be due to either an external planetary companion or to stellar magnetic activity. The precise masses and radii obtained for the four planetsallowed us to conduct interior structure and atmospheric escape modelling. TOI-561b is confirmed to be the lowest density (ρb = 3.8 ± 0.5 gcm−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 band c having experienced significant gas loss, and planets d and e showing anatmospheric 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.

Original languageEnglish
Pages (from-to)4551-4571
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date28 Jan 2022
Publication statusPublished - 1 Apr 2022


  • Planets and satellites: interiors
  • Stars: individual: TOI-561 (TIC 377064495
  • Gaia EDR3 3850421005290172416)
  • Techniques: photometric
  • Techniques: radial velocities
  • Planets and satellites: fundamental parameters


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