TOI-5108 b and TOI 5786 b: Two transiting sub-Saturns detected and characterized with TESS, MaHPS, and SOPHIE

February 2025 • 2025A&A...694A.143T

Authors • Thomas, Luis • Hébrard, Guillaume • Kellermann, Hanna • Korth, Judith • Heidari, Neda • Forveille, Thierry • Sousa, Sérgio G. • Schöller, Laura • Riffeser, Arno • Gössl, Claus • Serrano Bell, Juan • Kiefer, Flavien • Hara, Nathan • Grupp, Frank • Ehrhardt, Juliana • Murgas, Felipe • Collins, Karen A. • Bieryla, Allyson • Parviainen, Hannu • Belinski, Alexandr A. • Esparza-Borges, Emma • Ciardi, David R. • Clark, Catherine A. • Fukui, Akihiko • Gilbert, Emily A. • Hopp, Ulrich • Ikuta, Kai • Jenkins, Jon M. • Latham, David W. • Narita, Norio • Nielsen, Louise D. • Quinn, Samuel N. • Palle, Enric • Pippert, Jan-Niklas • Polanski, Alex S. • Ries, Christoph • Schmidt, Michael • Schwarz, Richard P. • Seager, Sara • Strakhov, Ivan A. • Striegel, Stephanie • van Eyken, Julian C. • Watanabe, Noriharu • Watkins, Cristilyn N. • Winn, Joshua N. • Ziegler, Carl • Zöller, Raphael

Abstract • We report the discovery and characterization of two sub-Saturns from the Transiting Exoplanet Survey Satellite (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, along with the radial velocity measurements from MaHPS and SOPHIE, we measured precise radii and masses for both planets. TOI-5108 b is a sub-Saturn, with a radius of 6.6 ± 0.1 R_⊕ and a mass of 32 ± 5 M_⊕. TOI-5786 b is similar to Saturn, with a radius of 8.54 ± 0.13 R_⊕ and a mass of 73 ± 9 M_⊕. 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 within the bounds of the Neptune desert, while TOI-5786 b is right above the upper edge. We estimated hydrogen-helium (H/He) envelope mass fractions of 38% for TOI-5108 b and 74% for TOI-5786 b. However, when using a model for the interior structure that includes tidal effects, the envelope fraction of TOI-5108 b could be much lower (~20%), depending on the obliquity. We estimated mass-loss rates between 1.0 x 10⁹ g/s and 9.8 x 10⁹ g/s for TOI-5108 b and between 3.6 x 10⁸ g/s and 3.5 x 10⁹ g/s for TOI-5786 b. Given their masses, both planets could be stable against photoevaporation. Furthermore, at these mass-loss rates, there is likely no detectable signal in the metastable helium triplet with the James Webb Space Telescope (JWST). We also detected 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 ± 0.16 R_⊕. Using our RV data and photodynamical modeling, we were able to provide a 3-σ upper limit of 26.5 M_⊕ for the mass of the potential inner companion to TOI-5786 b.

Links

• PREPRINT http://arxiv.org/abs/2501.03803
• POSTSCRIPT https://www.aanda.org/10.1051/0004-6361/202451676/postscript
• ELECTR https://doi.org/10.1051%2F0004-6361%2F202451676
• PDF https://www.aanda.org/10.1051/0004-6361/202451676/pdf
• DATA http://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/694/A143