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

Abstract

The Neptunian desert and savanna have recently been found to be separated by a ridge, an overdensity of planets in the period range of $\simeq$3-5 days. These features are thought to be shaped by dynamical and atmospheric processes, but their roles are not yet well understood. Our aim was to confirm and characterize 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 $R_{\mathrm{p}}$ = $6.25\pm 0.24$ ${\mathrm{R}}_{\oplus }$ ($R_{\mathrm{p}}$ = $0.558\pm 0.021$ ${\mathrm{R}}_{\mathrm{J}}$) and a mass of $M_{\mathrm{p}}$ = $32.7\pm 5.9$ ${\mathrm{M}}_{\oplus }$ ($M_{\mathrm{p}}$ = $0.103\pm 0.018$ ${\mathrm{M}}_{\mathrm{J}}$), which corresponds to a mean density of $\ddot{I}{}_{\mathrm{p}}$ = $0.74\pm 0.16$ $\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. 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 ($Z_{\mathrm{planet}}$ = $0.76_{-0.11}^{+0.04}$). 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 = $0.0092_{-0.0066}^{+0.0184}$) 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 $\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, which supports the hypothesis of different evolutionary pathways populating the two 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.

Published in Astronomy and Astrophysics. Abstract shortened. 35 pages, 26 figures