EPIC 201702477b: A TRANSITING BROWN DWARF from K2 in A 41 DAY ORBIT

Abstract

We report the discovery of EPIC 201702477b, a transiting brown dwarf in a long period (40.73691 ±0.00037 day) and eccentric (e = 0.2281 ±0.0026) orbit. This system was initially reported as a planetary candidate based on two transit events seen in K2 Campaign 1 photometry and later validated as an exoplanet candidate. We confirm the transit and refine the ephemeris with two subsequent ground-based detections of the transit using the Las Cumbres Observatory Global Telescope 1 m telescope network. We rule out any transit timing variations above the level of ∼30 s. Using high precision radial velocity measurements from HARPS and SOPHIE we identify the transiting companion as a brown dwarf with a mass, radius, and bulk density of 66.9 ±1.7 M J, 0.757 ±0.065 R J, and 191 ±51 g cm-3 respectively. EPIC 201702477b is the smallest radius brown dwarf yet discovered, with a mass just below the H-burning limit. It has the highest density of any planet, substellar mass object, or main-sequence star discovered so far. We find evidence in the set of known transiting brown dwarfs for two populations of objects - high mass brown dwarfs and low mass brown dwarfs. The higher-mass population have radii in very close agreement to theoretical models, and show a lower-mass limit around 60 M J. This may be the signature of mass-dependent ejection of systems during the formation process.