TOI-2490b – the most eccentric brown dwarf transiting in the brown dwarf desert

We report the discovery of the most eccentric transiting brown dwarf in the brown dwarf desert, TOI-2490b. The brown dwarf desert is the lack of brown dwarfs around main-sequence stars within ∼ 3 au and is thought to be caused by differences in formation mechanisms between a star and planet. To date, only ∼ 40 transiting brown dwarfs have been confirmed. TOI-2490b is a 73.6 ± 2.4 M_J, 1.00 ± 0.02 R_J brown dwarf orbiting a 1.004⁺₋₀⁰.₀₂₂⁰³¹ M_⊙, 1.105⁺₋₀⁰.₀₁₂⁰¹² R_⊙ sun-like star on a 60.33 d orbit with an eccentricity of 0.77989 ± 0.00049. The discovery was detected within Transiting Exoplanet Survey Satellite sectors 5 (30 min cadence) and 32 (2 min and 20 s cadence). It was then confirmed with 31 radial velocity measurements with FEROS by the WINE collaboration and photometric observations with the Next Generation Transit Survey. Stellar modelling of the host star estimates an age of ∼ 8 Gyr, which is supported by estimations from kinematics likely placing the object within the thin disc. However, this is not consistent with model brown dwarf isochrones for the system age suggesting an inflated radius. Only one other transiting brown dwarf with an eccentricity higher than 0.6 is currently known in the brown dwarf desert. Demographic studies of brown dwarfs have suggested such high eccentricity is indicative of stellar formation mechanisms.