WASP-193b: An extremely low-density super-Neptune

Abstract Gas giants transiting bright nearby stars are key objects for our understanding of planetary system formation and evolution mechanisms. This paper presents a new transiting exoplanet discovered by the WASP-South transit survey, WASP-193b. WASP-193b orbits its Vmag = 12.0 F9 main-sequence host star every 6.25d. Our analyses found that WASP-193b has a mass of Mp = 0.146 ± 0.033 M_Jup, a radius of R_p = 1.58 ± 0.14R_Jup, translating into an extremely low density of rho_p = 0.049 ± 0.015g/cm3. The planet was confirmed photometrically by the 0.6-m TRAPPIST-South and 1.0-m SPECULOOS-South telescopes and spectroscopically by the ESO-3.6-m/HARPS and Euler-1.2-m/CORALIE spectrographs. The combination of its large transit depth (dF≈1.4%), its super-low density, its high-equilibrium temperature (T_eq = 1275 ± 40K), and the infrared brightness of its host star (magnitude Kmag=10.7) makes WASP-193b an exquisite target for a detailed atmospheric characterization by transmission spectroscopy.