TOI-5005 b: A super-Neptune in the savanna near the ridge

The Neptunian desert and savanna have been recently found to be separated by a ridge, an overdensity of planets in the ≃3-5 days period range. These features are thought to be shaped by dynamical and atmospheric processes. However, their relative roles are not yet well understood. We intend to confirm and characterise the super-Neptune TESS candidate TOI-5005.01, which orbits a moderately bright (V = 11.8) solar-type star (G2 V) with an orbital period of 6.3 days. We confirm TOI-5005 b to be a transiting super-Neptune with a radius of Rₚ = 6.25 ± 0.24 R⊕ (Rₚ = 0.558 ± 0.021 Rⱼ) and a mass of Mₚ = 32.7 ± 5.9 M⊕ (Mₚ = 0.103 ± 0.018 Mⱼ), which corresponds to a mean density of ρₚ = $0.74 ± 0.16 g cm⁻³. Our internal structure modelling indicates that the overall metal mass fraction is well constrained to a value slightly lower than that of Neptune and Uranus (Zₚₗₐₙₑₜ = 0.76⁺⁰.⁰⁴₋₀.₁₁). We also estimated the present-day atmospheric mass-loss rate of TOI-5005 b but found contrasting predictions depending on the choice of photoevaporation model. At a population level, we find statistical evidence (p-value = 0.0092⁺⁰.⁰¹⁸⁴₋₀.₀₀₆₆) that planets in the savanna such as TOI-5005 b tend to show lower densities than planets in the ridge, with a dividing line around 1 g cm⁻³, which supports the hypothesis of different evolutionary pathways populating both regimes. TOI-5005 b is located in a key region of the period-radius space to study the transition between the Neptunian ridge and the savanna. It orbits the brightest star of all such planets, which makes it a target of interest for atmospheric and orbital architecture observations that will bring a clearer picture of its overall evolution.