  Gas giants transiting bright nearby stars provide crucial insights into
planetary system formation and evolution mechanisms. Most of these planets
exhibit certain average characteristics, serving as benchmarks for our
understanding of planetary systems. However, outliers like the planet we
present in this study, WASP-193b, offer unique opportunities to explore
unconventional formation and evolution processes. This planet completes an
orbit around its Vmag=12.2 F9 main-sequence host star every 6.25 d. Our
analyses found that WASP-193b has a mass of Mp=0.139+/-0.029 MJup and a radius
of Rp=1.464+/-0.058 RJup, translating into an extremely low density of rho_p =
0.059+/-0.014 g/cm^3, at least one order of magnitude less than standard gas
giants like Jupiter. Typical gas giants such as Jupiter have densities that
range between 0.2 and 2 g/cm^3. The combination of its large transit depth
(dF~1.4%), its extremely-low density, its high-equilibrium temperature (Teq =
1254+/-31 K), and the infrared brightness of its host star (magnitude
Kmag=10.7) makes WASP-193b an exquisite target for characterization by
transmission spectroscopy (transmission spectroscopy metric: TSM~600). One
single JWST transit observation would yield detailed insights into its
atmospheric properties and planetary mass, providing a unique window to explore
the mechanisms behind its exceptionally low density and shed light on giant
planets' diverse nature.
