TOI-5108 b and TOI 5786 b: Two transiting sub-Saturns detected and characterized with TESS, MaHPS and SOPHIE
Creators: name: Thomas, Luis
name: Hébrard, Guillaume
name: Kellermann, Hanna
name: Korth, Judith
name: Heidari, Neda
name: Forveille, Thierry
name: Sousa, Sérgio G.
name: Schöller, Laura
name: Riffeser, Arno
name: Gössl, Claus
name: Bell, Juan Serrano
name: Kiefer, Flavien
name: Hara, Nathan
name: Grupp, Frank
name: Ehrhardt, Juliana
name: Murgas, Felipe
name: Collins, Karen A.
name: Bieryla, Allyson
name: Parviainen, Hannu
name: Belinski, Alexandr A.
name: Esparza-Borges, Emma
name: Ciardi, David R.
name: Clark, Catherine A.
name: Fukui, Akihiko
name: Gilbert, Emily A.
name: Hopp, Ulrich
name: Ikuta, Kai
name: Jenkins, Jon M.
name: Latham, David W.
name: Narita, Norio
name: Nielsen, Louise D.
name: Quinn, Samuel N.
name: Palle, Enric
name: Pippert, Jan-Niklas
name: Polanski, Alex S.
name: Ries, Christoph
name: Schmidt, Michael
name: Schwarz, Richard P.
name: Seager, Sara
name: Strakhov, Ivan A.
name: Striegel, Stephanie
name: van Eyken, Julian C.
name: Watanabe, Noriharu
name: Watkins, Cristilyn N.
name: Winn, Joshua N.
name: Ziegler, Carl
name: Zöller, Raphael
Publisher: name: arXiv
Year: 2025
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the discovery and characterization of two sub-Saturns from the Transiting Exoplanet Survey Satellite (\textit{TESS}) using high-resolution spectroscopic observations from the MaHPS spectrograph at the Wendelstein Observatory and the SOPHIE spectrograph at the Haute-Provence Observatory. Combining photometry from TESS, KeplerCam, LCOGT, and MuSCAT2 with the radial velocity measurements from MaHPS and SOPHIE we measure precise radii and masses for both planets. TOI-5108 b is a sub-Saturn with a radius of $6.6 \pm 0.1$ $R_\oplus$ and a mass of $32 \pm 5$ $M_\oplus$. TOI-5786 b is similar to Saturn with a radius of $8.54 \pm 0.13$ $R_\oplus$ and a mass of $73 \pm 9$ $M_\oplus$. The host star for TOI-5108 b is a moderately bright (Vmag 9.75) G-type star. TOI-5786 is a slightly dimmer (Vmag 10.2) F-type star. Both planets are close to their host stars with periods of 6.75 days and 12.78 days respectively. This puts TOI-5108 b just inside the bounds of the Neptune desert while TOI-5786 b is right above the upper edge. We estimate hydrogen-helium envelope mass fractions of $38 \%$ for TOI-5108 b and $74 \% $ for TOI-5786 b. However, using a model for the interior structure that includes tidal effects the envelope fraction of TOI-5108 b could be much lower ($\sim 20\,\%$) depending on the obliquity. We estimate mass-loss rates between 1.0 * $10^9$ g/s and 9.8 * $10^9$ g/s for TOI-5108 b and between 3.6 * $10^8$ g/s and 3.5 * $10^9$ g/s for TOI-5786 b. Given their masses, this means that both planets are stable against photoevaporation. We also detect a transit signal for a second planet candidate in the TESS data of TOI-5786 with a period of 6.998 days and a radius of $3.83 \pm 0.16$ $R_\oplus$. Using our RV data and photodynamical modeling, we are able to provide a 3-$σ$ upper limit of 26.5 $M_\oplus$ for the mass of the potential inner companion to TOI-5786 b.
description: 25 pages, 13 figures. Accepted for publication in A&amp;A