Letter to the Editor

Transiting planet candidate from K2 with the longest period

¹ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
e-mail: Helen.Giles@unige.ch
² Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
⁵ Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, UK
⁶ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
⁷ Brorfelde Observatory, Observator Gyldenkernes Vej 7, 4340 Tølløse, Denmark

Received: 5 June 2018
Accepted: 22 June 2018

Abstract

Context. We present the transit and follow-up of a single transit event from Campaign 14 of K2, EPIC248847494b, which has a duration of 54 h and a 0.18% depth.

Aims. Using photometric tools and conducting radial velocity follow-up, we vet and characterise this very strong candidate.

Methods. Owing to the long, unknown period, standard follow-up methods needed to be adapted. The transit was fitted using Namaste, and the radial velocity slope was measured and compared to a grid of planet-like orbits with varying masses and periods. These used stellar parameters measured from spectra and the distance as measured by Gaia.

Results. Orbiting around a sub-giant star with a radius of 2.70 ± 0.12 R_Sol, the planet has a radius of 1.11_−0.07^+0.07 R_Jup and a period of 3650₋₁₁₃₀⁺¹²⁸⁰ days. The radial velocity measurements constrain the mass to be lower than 13 M_Jup, which implies a planet-like object.

Conclusions. We have found a planet at 4.5 AU from a single-transit event. After a full radial velocity follow-up campaign, if confirmed, it will be the longest-period transiting planet discovered.