* Astronomy

Title: WASP-121 b: a hot Jupiter in a polar orbit and close to tidal disruption
Author: L. Delrez, A. Santerne, J.-M. Almenara, D. R. Anderson, A. Collier-Cameron, R. F. Díaz, M. Gillon, C. Hellier, E. Jehin, M. Lendl, P. F. L. Maxted, M. Neveu-VanMalle, F. Pepe, D. Pollacco, D. Queloz, D. Ségransan, B. Smalley, A. M. S. Smith, A. H. M. J. Triaud, S. Udry, V. Van Grootel, R. G. West

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} Jupiter masses, a radius of 1.865 ± 0.044 Jupiter radii, 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} solar masses, 1.458 ± 0.030 solar radii, Teff = 6460 ± 140 K). A notable property of WASP-121 b is that its orbital semi-major axis is only ~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 (~7.1 x 10⁹ erg s^-1cm^-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 ~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 257.8_{-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.

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