Letter to the Editor

K2-399 b is not a planet

The Saturn that wandered through the Neptune desert is actually a hierarchical eclipsing binary^⋆

¹ Centro de Astrobiología (CAB), CSIC-INTA, ESAC campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, UK
⁴ Center for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, UK
⁵ Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro Giuria, 1, 10125 Torino, Italy
⁶ Dept. of Physics & Astronomy, Swarthmore College, Swarthmore, PA 19081, USA
⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Centro Astronómico Hispano en Andalucía, Sierra de los Filabres sn, 04550 Gérgal Almería, Spain
⁹ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁰ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹² Institut für Planetenforschung, Deusches Zentrum für Luft- und Raumfahrt, Rutherfordstr. 2, 12489 Berlin, Germany
¹³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory 43992, Sweden
¹⁵ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁶ Thüringer Landessternwarte Sternwarte 5, D-07778 Tautenburg, Germany
¹⁷ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹⁸ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95060, USA

Received: 5 July 2024
Accepted: 21 August 2024

Abstract

Context. The transit technique has been very efficient over the past decades in detecting planet-candidate signals. The so-called statistical validation approach has become a popular way of verifying a candidate’s planetary nature. However, the incomplete consideration of false-positive scenarios and data quality can lead to misinterpretation of the results.

Aims. In this work, we revise the planetary status of K2-399 b, a validated planet with an estimated false-positive probability of 0.078% located in the middle of the so-called Neptunian desert, and hence a potential key target for atmospheric prospects.

Methods. We used radial velocity data from the CARMENES, HARPS, and TRES spectrographs, as well as ground-based multiband transit photometry provided by LCOGT MuSCAT3 and broad band photometry to test the planetary scenario.

Results. Our analysis of the available data does not support the existence of this (otherwise key) planet, and instead points to a scenario composed of an early G-dwarf orbited –with a period of a 846.62_−0.28^+0.22 days– by a pair of eclipsing M-dwarfs (hence a hierarchical eclipsing binary) likely in the mid-type domain. We thus demote K2-399 b as a planet.

Conclusions. We conclude that the validation process, while very useful to prioritize follow-up efforts, must always be conducted with careful attention to data quality while ensuring that all possible scenarios have been properly tested to get reliable results. We also encourage developers of validation algorithms to ensure the accuracy of a priori probabilities for different stellar scenarios that can lead to this kind of false validation. We further encourage the use of follow-up observations when possible (such as radial velocity and/or multiband light curves) to confirm the planetary nature of detected transiting signals rather than only relying on validation tools.