KOBE-1: The first planetary system from the KOBE survey

Two planets likely residing in the sub-Neptune mass regime around a late K-dwarf^★

¹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC campus, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada (Madrid), Spain
² Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa
⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁶ Aix Marseille Univ, CNRS, CNES, Institut Origines, LAM, Marseille, France
⁷ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
⁸ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía SN, 18008 Granada, Spain
⁹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
¹¹ Departamento de Ingeniería Topográfica y Cartografía, E.T.S.I. en Topografía, Geodesia y Cartografía, Universidad Politécnica de Madrid, 28031 Madrid, Spain
¹² ATG Europe for European Space Agency (ESA), Camino bajo del Castillo, s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692 Madrid, Spain
¹³ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁴ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ Can Magrans s/n, 08193 Bellaterra (Barcelona), Spain
¹⁵ Institut d’Estudis Espacials de Catalunya (IEEC), C/Esteve Terradas, 1, Edifici RDIT, Campus PMT-UPC, E-08860 Castelldefels (Barcelona), Spain
¹⁶ Depto. Estadística e Investigación Operativa, Universidad de Cádiz, Avda. República Saharaui s/n, 11510 Puerto Real, Cádiz, Spain
¹⁷ Universität Göttingen, Institut für Astrophysik und Geophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

^★★ Corresponding author; olga.balsalobre@cab.inta-csic.es

Received: 16 October 2024
Accepted: 12 December 2024

Abstract

Context. K-dwarf stars are promising targets in the exploration of potentially habitable planets. Their properties, falling between G and M dwarfs, provide an optimal trade-off between the prospect of habitability and ease of detection. The KOBE experiment is a blind-search survey exploiting this niche, monitoring the radial velocity of 50 late-type K-dwarf stars. It employs the CARMENES spectrograph, with an observational strategy designed to detect planets in the habitable zone of their system.

Aims. In this work, we exploit the KOBE data set to characterize planetary signals in the K7 V star HIP 5957 (KOBE-1) and to constrain the planetary population within its habitable zone.

Methods. We used 82 CARMENES spectra over a time span of three years. We employed a generalized Lomb–Scargle periodogram to search for significant periodic signals that would be compatible with Keplerian motion on KOBE-1. We carried out a model comparison within a Bayesian framework to ensure the significance of the planetary model over alternative configurations of lower complexity. We also inspected two available TESS sectors in search of planetary signals.

Results. We identified two signals: at P_b = 8.5 d and P_c = 29.7 d. We confirmed their planetary nature through ruling out other non-planetary configurations. Their minimum masses are 8.80 ± 0.76 M_⊕ (KOBE-1 b), and 12.4 ± 1.1 M_⊕ (KOBE-1 c), corresponding to absolute masses within the planetary regime at a high certainty (>99.7%). By analyzing the sensitivity of the CARMENES time series to additional signals, we discarded planets above 8.5 M_⊕ within the habitable zone. We identified a single transit-like feature in TESS, whose origin is still uncertain, but still compatible within 1σ with a transit from planet c.

Conclusions. The KOBE-1 multi-planetary system, consisting of a relatively quiet K7-dwarf hosting two sub-Neptune-minimum- mass planets, establishes the first discovery from the KOBE experiment. We have explored future prospects for characterizing this system, concluding that Gaia DR4 will be insensitive to their astrometric signature. Meanwhile, nulling interferometry with the Large Interferometer For Exoplanets (LIFE) mission could be capable of directly imaging both planets and characterizing their atmospheres in future studies.