WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star

We present the discovery by the WASP-South survey of WASP-121 b, a new remarkable short-period transiting hot Jupiter. The planet has a mass of |1. 183-₀. ₀₆₂^+0. 064|MJup, a radius of 1. 865 ± 0. 044 RJup, and transits every |1. 274\, 9255-₀. ₀₀₀\, ₀₀₂₅^+0. 000\, 0020| days an active F6-type main-sequence star (V = 10. 4, |1. 353-₀. ₀₇₉^+0. 080| M⊙, 1. 458 ± 0. 030 R⊙, Teff = 6460 ± 140 K). A notable property of WASP-121 b is that its orbital semimajor axis is only ∼1. 15 times larger than its Roche limit, which suggests that the planet is close to tidal disruption. Furthermore, its large size and extreme irradiation (∼7. 1 109 erg s−1 cm−2) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAnsiting Planets and PlanetesImals Small Telescope, we indeed detect its emission in the z′-band at better than ∼4σ, the measured occultation depth being 603 ± 130 ppm. Finally, from a measurement of the Rossiter–McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of |257^{. } 8-₅^{. 5}^+5{^{. } 3}|⁠. This result may suggest a significant misalignment between the spin axis of the host star and the orbital plane of the planet. If confirmed, this high misalignment would favour a migration of the planet involving strong dynamical events with a third body.