Abstract DGP2026-70

One for all, all for one: The TESS-HARPS-PFS-CHEOPS synergy reveals three mini-Neptunes transiting the late F-type star TOI-261.

S. Grziwa (1); D. Gandolfi (2) , H. P. Osborn (3, 4) , K. Gozdziewski (5) + KESPRINT and CHEOPS Consortium

(1) Rheinisches Institut für Umweltforschung an der Universität zu Köln, Germany; (2) Dipartimento di Fisica, Università degli Studi di Torino, Italy; (3) Center for Space and Habitability, University of Bern, Switzerland; (4) ETH Zurich, Department of Physics, Switzerland; (5) Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Toru´n, Poland

We present the discovery of a multi-planet system that comprises three mini-Neptunes transiting the bright (V=9.56) late F-type star TOI-261. Based on the analysis of two TESS sectors, the system was initially found to host two transiting sub-Neptune-sized planets (TOI-261 b and c) on relatively short period orbits (P_b ≈ 3.4 days and P_c ≈ 10.2 days). We conducted an intensive radial velocity follow-up campaign of TOI-261 using the high-precision HARPS and PFS spectrographs.

We conducted an intensive radial-velocity follow-up campaign of TOI-261 using the high-precision HARPS and PFS spectrographs, which allowed us to spectroscopically confirm the two planets and measure their masses. Our HARPS Doppler observations led to the serendipitous discovery of a possible third sub-Neptune-mass planet on a longer period orbit (TOI-261 d). A reanalysis of the two TESS sector light curves led us to identify two single transit events (one per sector) likely originating from the HARPS-detected planet. We subsequently confirmed its transits through dedicated observations with the CHEOPS space telescope, yielding an orbital period of P_d ≈ 22.9 days.

This results in a well characterized system of three nearly equal sub-Neptunes lining up at the ridge of the Neptune desert and populating the so called “Neptune-savanna”. This rare system gives the opportunity to learn more about the origin and the evolution of Neptune size planets. We performed an internal structure modelling, an atmospheric stability and evolution analysis and a dynamical orbit analysis which we will present. The three sub-Neptunes are also promising targets for further atmospheric analysis with JWST.