Uncovering the ultimate planet impostor

An eclipsing brown dwarf in a hierarchical triple with two evolved stars^★

¹ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus 28692 Villanueva de la Cañada (Madrid), Spain
e-mail: Jorge.Lillo@cab.inta-csic.es
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Region Metropolitana, Chile
³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Depto. de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁵ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁶ Spanish Virtual Observatory, Madrid, Spain
⁷ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁸ School of Physics and Astronomy, University of Birmingham, Edgbaston, B15 2TT, UK
⁹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
¹⁰ Department of Astronomy, Yale University, 52 Hillhouse Ave., New Haven, CT 06511, USA
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, BUS-2410, Belgium

Received: 23 April 2021
Accepted: 8 June 2021

Abstract

Context. Exoplanet searches through space-based photometric time series have shown to be very efficient in recent years. However, follow-up efforts on the detected planet candidates have been demonstrated to be critical to uncover the true nature of the transiting objects.

Aims. In this paper we show a detailed analysis of one of those false positives hidden as planetary signals. In this case, the candidate KOI-3886.01 showed clear evidence of a planetary nature from various techniques. Indeed, the properties of the fake planet set it among the most interesting and promising for the study of planetary evolution as the star leaves the main sequence.

Methods. To unveil the true nature of this system, we present a complete set of observational techniques including high-spatial resolution imaging, high-precision photometric time series (showing eclipses, phase curve variations, and asteroseismology signals), high-resolution spectroscopy, and derived radial velocities to unveil the true nature of this planet candidate.

Results. We find that KOI-3886.01 is an interesting false positive case: a hierarchical triple system composed by a ~K2 III giant star (KOI-3886 A) accompanied by a close-in eclipsing binary formed by a subgiant ~G4 IV star (KOI-3886 B) and a brown dwarf (KOI-3886 C). In particular, KOI-3886 C is one of the most irradiated brown dwarfs known to date, showing the largest radius in this substellar regime. It is also the first eclipsing brown dwarf known around an evolved star.

Conclusions. In this paper we highlight the relevance of complete sets of follow-up observations to extrasolar planets detected by the transit technique using large-pixel photometers such as Kepler and TESS and, in the future, PLATO. In particular, multi-color high-spatial resolution imaging was the first hint toward ruling out the planet scenario in this system.