We report on the confirmation and mass determination of a transiting planet orbiting the old and inactive G7 dwarf star HD 219666 ( M _ { \star } = 0.92 \pm 0.03 M _ { \odot } , R _ { \star } = 1.03 \pm 0.03 R _ { \odot } , \tau _ { \star } =10 \pm 2 Gyr ) .
With a mass of M _ { b } = 16.6 \pm 1.3 M _ { \oplus } , a radius of R _ { b } = 4.71 \pm 0.17 R _ { \oplus } , and an orbital period of P _ { \mathrm { orb } } \simeq 6 days , HD 219666 b is a new member of a rare class of exoplanets : the hot-Neptunes .
The Transiting Exoplanet Survey Satellite ( TESS ) observed HD 219666 ( also known as TOI-118 ) in its Sector 1 and the light curve shows four transit-like events , equally spaced in time .
We confirmed the planetary nature of the candidate by gathering precise radial-velocity measurements with the High Accuracy Radial velocity Planet Searcher ( HARPS ) at ESO 3.6m .
We used the co-added HARPS spectrum to derive the host star fundamental parameters ( T _ { eff } = 5527 \pm 65 \mathrm { K } , log g _ { \star } = 4.40 \pm 0.11 ( cgs ) , [ Fe/H ] = 0.04 \pm 0.04 dex , \log R ^ { \prime } _ { HK } = - 5.07 \pm 0.03 ) , as well as the abundances of many volatile and refractory elements .
The host star brightness ( V=9.9 ) makes it suitable for further characterisation by means of in-transit spectroscopy .
The determination of the planet orbital obliquity , along with the atmospheric metal-to-hydrogen content and thermal structure could provide us with important clues on the formation mechanisms of this class of objects .