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 ^ { \prime } -band at better than \sim 4 \sigma , 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 .