K2-399 b is not a planet: The Saturn that wandered through the Neptune desert is actually a hierarchical eclipsing binary
Journal article, 2024
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 846.62a-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.
Techniques: radial velocities
Planets and satellites: fundamental parameters
Techniques: photometric
Planets and satellites: detection
Planets and satellites: individual: K2-399
Author
J. Lillo-Box
Centro de Astrobiologia (CAB)
D. W. Latham
Harvard-Smithsonian Center for Astrophysics
Karen A. Collins
Harvard-Smithsonian Center for Astrophysics
D. J. Armstrong
The University of Warwick
D. Gandolfi
University of Turin
Eric L.N. Jensen
Swarthmore College
A. Castro-González
Centro de Astrobiologia (CAB)
O. Balsalobre-Ruza
Centro de Astrobiologia (CAB)
B. Montesinos
Centro de Astrobiologia (CAB)
S.G. Sousa
University of Porto
J. Aceituno
Calar Alto Astronomical Observatory
R. Schwarz
Harvard-Smithsonian Center for Astrophysics
Norio Narita
Instituto de Astrofísica de Canarias
University of Tokyo
National Institutes of Natural Sciences
A. Fukui
Instituto de Astrofísica de Canarias
University of Tokyo
J. Cabrera
German Aerospace Center (DLR)
A. Hadjigeorghiou
The University of Warwick
M. Kuzuhara
National Astronomical Observatory of Japan
National Institutes of Natural Sciences
T. Hirano
National Institutes of Natural Sciences
National Astronomical Observatory of Japan
Malcolm Fridlund
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Leiden University
A. Hatzes
Thüringer Landessternwarte Tautenburg
O. Barragán
University of Oxford
Natalie Batalha
University of California
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 689 L8Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/202451398