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WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its youngage ($<$1 Gyr), the star presents an anomalously low stellar activity level:the measured logR'$_{\rm HK}$ activity parameter lies slightly below the basallevel; there is no significant time-variability in the logR'$_{\rm HK}$ value;there is no detection of the star in the X-rays. We present results of far-UVobservations of WASP-18 obtained with COS on board of Hubble Space Telescopeaimed at explaining this anomaly. From the star's spectral energy distribution,we infer the extinction (E(B-V) $\approx$ 0.01 mag) and then the interstellarmedium (ISM) column density for a number of ions, concluding that ISMabsorption is not the origin of the anomaly. We measure the flux of the fourstellar emission features detected in the COS spectrum (CII, CIII, CIV, SiIV).Comparing the CII/CIV flux ratio measured for WASP-18 with that derived fromspectra of nearby stars with known age, we see that the far-UV spectrum ofWASP-18 resembles that of old ($>$5 Gyr), inactive stars, in stark contrastwith its young age. We conclude that WASP-18 has an intrinsically low activitylevel, possibly caused by star-planet tidal interaction, as suggested byprevious studies. Re-scaling the solar irradiance reference spectrum to matchthe flux of the SiIV line, yields an XUV integrated flux at the planet orbit of10.2 erg cm$^{-2}$ s$^{-1}$. We employ the rescaled XUV solar fluxes to modelsof the planetary upper atmosphere, deriving an extremely low thermal mass-lossrate of 10$^{-20}$ $M_{\rm J}$ Gyr$^{-1}$. For such high-mass planets, thermalescape is not energy limited, but driven by Jeans escape.

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System