Revisiting the exoplanet radius valley with host stars from SWEET-Cat

TL;DR

This paper revisits the exoplanet radius valley using MAISTEP-derived stellar parameters from SWEET-Cat to achieve precise planet radii and robust demographic trends. By analyzing 1,221 main-sequence hosts (1,405 planets) and recomputing radii from radius ratios with Gaia-based luminosities, the authors confirm a partially filled valley near $2\,R_{\oplus}$ and quantify its dependence on orbital period, incident flux, stellar mass, and age. The results yield consistent 2D and 4D trends, with the valley shifting to larger radii and flattening over gigayear timescales, supporting a dominant role for core-powered mass loss, while not ruling out photoevaporation. The work underscores the importance of precise stellar ages and radii for exoplanet demographics and points to future age determinations from missions like PLATO to further discriminate atmospheric-loss mechanisms.

Abstract

The radius valley,a deficit of planets near 2 $\mathrm{R_{\oplus}}$, was observed among exoplanets of radius $\lesssim$ 5 $\mathrm{R_{\oplus}}$ with periods $<$ 100 days by NASA's $Kepler$ mission. It separates super-Earths (rocky, $\lesssim 1.9$ $\mathrm{R_{\oplus}}$) from sub-Neptunes (volatile-rich, $\gtrsim 2$ $\mathrm{R_{\oplus}}$) and may arise from formation conditions or atmospheric loss. Disentangling these mechanisms has led to numerous studies of population-level trends, although the resulting interpretations remain sensitive to sample selection and the robustness of host-star parameters. We re-examine its existence, depth, and dependence on period, flux, stellar mass, and age. Using SWEET-Cat and MAISTEP tool, we derived stellar parameters for 1,221 main-sequence stars (1,405 planets), with effective temperatures 4400--7500 K and radii 0.62--2.75 $\mathrm{R_{\odot}}$, achieving 2\% precision in radius and mass. Planetary radii were recomputed from radius ratios, yielding 5\% median uncertainty. The valley is partially filled near 2 $\mathrm{R_{\oplus}}$ and depends on period, flux, and stellar mass, with slopes $-0.12^{+0.02}_{-0.01}$, $0.10^{+0.02}_{-0.03}$, and $0.19^{+0.09}_{-0.07}$. Sub-Neptunes show a stronger stellar mass-dependent trend than super-Earths ($0.17^{+0.04}_{-0.04}$ vs $0.11^{+0.05}_{-0.05}$). With stellar age, the super-Earth/sub-Neptune ratio rises from $0.51^{+0.11}_{-0.08}$ ($<3$ Gyr) to $0.64^{+0.11}_{-0.11}$ ($\gtrsim3$ Gyr), and the valley becomes shallower and shifts to larger radii. A 4D fit shows consistent slopes with 2D analyses and a weaker age trend ($0.07^{+0.03}_{-0.04}$). These results suggest prolonged atmospheric loss, which is consistent with a core-powered mass loss scenario and emphasize the need for improved determinations, a goal expected to be achieved by future missions like PLATO.

Figures (13)

• Figure 1: Kiel diagram showing 779 selected planet host stars overlaid on the main-sequence model grid (gray points). The marker shows median uncertainties of 0.05 dex in log $g$ and 60 K in $T_{\text{eff}}$.
• Figure 2: Top panel: Histograms of planet radii from the literature (black) and our revised values (orange), using 0.33 R$_{\oplus}$ bin width. Dashed curves represent kernel density estimates (KDEs) computed with 0.33 R$_{\oplus}$ bandwidth. Lower panel: KDE distributions of radius uncertainties, with vertical dashed lines indicating the medians: 0.05 (this work) and 0.10 (literature values from the NASA Exoplanet Archive).
• Figure 3: Planet radius against orbital period. Small circles represent individual planet detections. Contours show the two-dimensional KDE of the planet population. The orange line is the best-fit trend, with a shaded band indicating the 68% confidence interval from bootstrap sampling. The error bar indicate the median uncertainty in planet radius.
• Figure 4: Same as Fig. \ref{['pl_rade_orbper']}, except for planet size against incident flux. See text for details
• Figure 5: Same as Fig. \ref{['pl_rade_orbper']}, but for planet size as a function of host star mass. See text for details.