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WASP-121 b: a hot Jupiter in a polar orbit and close to tidal disruption

We present the discovery by the WASP-South survey, in close collaboration with the Euler and TRAPPIST telescopes, of WASP-121 b, a new remarkable short-period transiting hot Jupiter, whose planetary nature has been statistically validated by the PASTIS software. The planet has a mass of $1.183_{-0.062}^{+0.064}$ $M_{\mathrm{Jup}}$, a radius of 1.865 $\pm$ 0.044 $R_{\mathrm{Jup}}$, and transits every $1.2749255_{-0.0000025}^{+0.0000020}$ days an active F6-type main-sequence star ($V$=10.4, $1.353_{-0.079}^{+0.080}$ $M_{\odot}$, 1.458 $\pm$ 0.030 $R_{\odot}$, $T_{\mathrm{eff}}$ = 6460 $\pm$ 140 K). A notable property of WASP-121 b is that its orbital semi-major axis is only $\sim$1.15 times larger than its Roche limit, which suggests that the planet might be close to tidal disruption. Furthermore, its large size and extreme irradiation ($\sim$$7.1\:10^{9}$ erg $\mathrm{s}^{-1} \mathrm{cm}^{-2}$) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAPPIST telescope, we indeed detect its emission in the $z'$-band at better than $\sim$4$σ$, the measured occultation depth being 603 $\pm$ 130 ppm. Finally, from a measurement of the Rossiter-McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of $257.8_{-5.5}^{+5.3}$ deg. This result indicates a significant misalignment between the spin axis of the host star and the orbital plane of the planet, the planet being in a nearly polar orbit. Such a high misalignment suggests a migration of the planet involving strong dynamical events with a third body.