HARPS-N and the sub-Neptune HD 77946 b

The principal investigator of this project, L. Palethorpe, together with her team, used high-precision radial velocity measurements from HARPS-N together with transit observations from NASA's TESS satellite and ESA's CHEOPS satellite to characterise a sub-Neptune around the star HD 77946 (Mstar = 1.17 Msun , Rstar = 1.31 Rsun). They found that HD 77946 b has a mass of 8.38 ± 1.32 Earth masses and a radius of 2.71 ± 0.09 Earth radii, placing it into the region of mass-radius space where there is confusion about its possible composition.

Using the radial velocity and transit methods simultaneously, it is possible to estimate both the mass and radius of an exoplanet. This can then tell us about the planet's internal composition and the possible presence of an atmosphere. However, there is a region of mass-radius space where there is still compositional confusion. Exoplanets with masses of around 10 Earth masses and radii between 2.5 and 3 Earth radii could be sub-Neptunes with rocky cores and low-mass hydrogen/helium atmospheres, or they could be water-worlds, with water making up a significant fraction of their mass. Therefore, precise characterisation of exoplanets in this region of mass-radius space is crucial to understand not only their composition, but also their origin.

The analysis of the composition of HD 77946 b suggests that it has a rocky core surrounded by a hydrogen/helium atmosphere that is about 1% of its mass, but makes up about 1/4 of its radius.

However, HD 77946 b is also very hot, which means that we can't rule out that it is a water world with water, in a super-critical state, making up 20-50% of its mass. One way to resolve this degeneracy would be to use transmission spectroscopy to determine the atmospheric composition. Although not easy, HD 77946 b is one of the best targets around a hot star, with a precise mass and radius, for such observations.

Being able to confirm, or to rule out, the existence of water worlds around stars like the Sun could have strong implications for our understanding of exoplanet demographics and the origin of our own Solar System.

Figure: M-R diagram for HD 77946 b. HD 77946 b is indicated by the star and other confirmed planets with masses and radii measured to be better than 20% and 10% respectively, taken from the Extrasolar Planets Encyclopaedia, are also shown. See the paper for more details.

Link to the paper: https://ui.adsabs.harvard.edu/abs/2024MNRAS.529.3323P/abstract