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TOI-1296b and TOI-1298b observed with TESS and SOPHIE: two hot Saturn-mass exoplanets with different densities around metal-rich stars

C. Moutou, J.-M. Almenara, G. Hébrard, N. C. Santos, K. G. Stassun, S. Deheuvels, S. C. C. Barros, P. Benni, A. Bieryla, I. Boisse, X. Bonfils, P. T. Boyd, K. A. Collins, D. Baker, P. Cortés-Zuleta, S. Dalal, F. Debras, M. Deleuil, X. Delfosse, O. Demangeon, Z. Essack, T. Forveille, E. Girardin, P. Guerra, N. Heidari, K. Hesse, S. Hoyer, J. M. Jenkins, F. Kiefer, P. C. König, D. Laloum, D. W. Latham, T. A. Lopez, E. Martioli, H. P. Osborn, G. R. Ricker, S. Seager, R. K. Vanderspek, M. Vezie, J. Villaseñor, J. N. Winn, B. Wohler, C. Ziegler

Abstract
We present the discovery of two new transiting extrasolar planet candidates identified as TOI-1296.01 and TOI-1298.01 by the Transiting Exoplanet Survey Satellite (TESS). The planetary nature of these candidates has been secured with the SOPHIE high-precision spectrograph through the measurement of the companion’s mass with the radial velocity method. Both planets are similar to Saturn in mass and have similar orbital periods of a few days. They, however, show discrepant radii and therefore different densities. The radius discrepancy might be explained by the different levels of irradiation by the host stars. The subgiant star TOI-1296 hosts a low-density planet with 1.2 RJup while the less luminous, lower-size star TOI-1298 hosts a much denser planet with a 0.84 RJup radius, resulting in bulk densities of 0.198 and 0.743 g cm−3, respectively.In addition, both stars are strongly enriched in heavy elements, having metallicities of +0.44 and +0.49 dex, respectively. The planet masses and orbital periods are 0.298 ± 0.039 MJup and 3.9443715 ± 5.8 ± 10−6 days for TOI-1296b, and 0.356 ± 0.032 MJup and 4.537164 ± 1.2 ± 10−5 days for TOI-1298b. The mass measurements have a relative precision of better than 13%.

Keywords
methods: observational / techniques: photometric / techniques: radial velocities / planets and satellites: gaseous planets / planetary systems

Notes
Based on observations collected with the SOPHIE spectrograph on the 1.93 m telescope at the Observatoire de Haute-Provence (CNRS), France.

Astronomy & Astrophysics
Volume 653, Article Number A147, Number of pages 12
2021 September

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia