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) M-Jup, a radius of 1.865 +/- 0.044 R-Jup, and transits every 1.2749255(-0.000 0025)(+0.000 0020) days an active F6-type main-sequence star (V = 10.4, 1.353(-0.079)(+0.080) M-circle dot, 1.458 +/- 0.030 R-circle dot, T-eff = 6460 +/- 140 K). A notable property of WASP-121 b is that its orbital semimajor axis is only similar to 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 (similar to 7.1 10(9) 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 similar to 4 sigma, 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 degrees.8(-5 degrees.5)(+5 degrees.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.