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

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 Pb = 8.5 d and Pc = 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.