# HD219666b: A hot-Neptune from TESS Sector 1

## TL;DR

This paper confirms and characterizes a hot-Neptune exoplanet, HD219666b, orbiting an old G7 star, using TESS photometry and HARPS radial velocities, providing insights into its physical properties and potential formation history.

## Contribution

First detailed mass and radius measurement of HD219666b, a hot-Neptune, using TESS and HARPS data, highlighting its suitability for atmospheric studies.

## Key findings

- HD219666b has a mass of 16.6 +/- 1.3 Earth masses.
- The planet's radius is 4.71 +/- 0.17 Earth radii.
- The host star is old, inactive, and bright, ideal for further atmospheric characterization.

## Abstract

We report on the confirmation and mass determination of a transiting planet orbiting the old and inactive G7 dwarf star HD219666 (Mstar = 0.92 +/- 0.03 MSun, Rstar = 1.03 +/- 0.03 RSun, tau_star = 10 +/- 2 Gyr). With a mass of Mb = 16.6 +/- 1.3 MEarth, a radius of Rb = 4.71 +/- 0.17 REarth, and an orbital period of P ~ 6 days, HD219666b is a new member of a rare class of exoplanets: the hot-Neptunes. The Transiting Exoplanet Survey Satellite (TESS) observed HD219666 (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 HARPS@ESO3.6m. We used the co-added HARPS spectrum to derive the host star fundamental parameters (Teff = 5527 +/- 65 K, log g = 4.40 +/- 0.11 (cgs), [Fe/H]= 0.04 +/- 0.04 dex, log R'HK = -5.07 +/- 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.