The TOI-2427 system: Two close-in planets orbiting a late K-dwarf star^★

¹ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Department Planetary Science, 50674 Cologne, Germany
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
³ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁷ Astronomical Science Program, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Astrophysics Group, Keele University, Keele ST5 5BG, UK
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA
¹¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹³ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
¹⁴ McDonald Observatory and Center for Planetary Systems Habitability, The University of Texas, Austin, TX, USA
¹⁵ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

^★★ Corresponding author: h.schmerling@uni-koeln.de

Received: 15 October 2024
Accepted: 22 May 2025

Abstract

Using high-precision photometry, NASA’s TESS space mission has discovered many intriguing transiting planet candidates. These discoveries require ground-based follow-up observations, including high-precision Doppler spectroscopy, to rule out false positive scenarios and measure the mass of the transiting planets. In this study, we present an intensive Doppler follow-up campaign of the TESS object of interest TOI-2427, carried out with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph to determine the mass of the previously validated transiting planet (TOI-2427 b) and search for additional orbiting companions. By analyzing TESS transit photometry alongside our HARPS radial velocity measurements, we spectroscopically confirmed the transiting planet TOI-2427 b, which orbits its host star every ∼ 1.3 d. We also discovered the presence of a second non-transiting planetary companion with an orbital period of ∼ 5.15 d, which is very close to four times the orbital period of the inner transiting planet. We found that TOI-2427 b is a short-period, high-density super-Earth with a mass of M_b = 5.69_−0.50^+0.51 M_⊕ and a radius of R_b = 1.64_−0.11^+0.12 R_⊕, implying a mean density of ρ_b = 7.1_−0.4^+0.8 g cm⁻³. Its interior seems to be composed of a predominantly iron core and a silicate mantle and crust. Despite its high density, it is unlikely that TOI-2427 b can sustain any atmosphere composed of lighter gases; however, it could still retain heavier gases. The outer non-transiting planet TOI-2427 c has a minimum mass of M_c sin i_c = 6.46_−0.78^+0.79 M_⊕. Assuming that TOI-2427 b and c are coplanar, a statistical analysis suggests that planets with a mass of ∼ 6.5 M_⊕ tend to have radii around 2.7_−0.8^+1.1 R_⊕. This would place TOI-2457 c near the sub-Neptune regime, while also leaving open the possibility of it being a super-Earth.