HD 77946 RV curves

HD 77946 is an F5 (M*=1.17M_{sun}_, R*=1.31R_{sun}_) star, which hosts a transiting planet recently discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), classified as TOI-1778 b. Using TESS photometry, high-resolution spectroscopic data from HARPS-N, and photometry from CHEOPS, we measure the radius and mass from the transit and radial velocity observations, and find that the planet, HD 77946 b, orbits with period Pb=6.527282_-0.000020_^+0.000015^d, has a mass of Mb=8.38+/-1.32M_{Earth}_, and a radius of Rb=2.705_-0.081_^+0.086^R_{Earth}_.

From the combination of mass and radius measurements, and the stellar chemical composition, the planet properties suggest that HD 77946 b is a sub-Neptune with a ~1 per cent H/He atmosphere. However, a degeneracy still exists between water-world and silicate/iron-hydrogen models, and even though interior structure modelling of this planet favours a sub-Neptune with a H/He layer that makes up a significant fraction of its radius, a water-world composition cannot be ruled out, as with Teq= 1248^+40^_-38_K, water may be in a supercritical state.