Department of Physics

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Browsing Department of Physics by Author "Achim Weiss"

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    Revisiting the exoplanet radius valley with host stars from SWEET-Cat
    (Astronomy & Astrophysics manuscript, 2026-01-21) Kamulali, Juma; Vardan Adibekyan; Benard Nsamba; Sergio G. Sousa; Tiago. L. Campante; Achim Weiss; Bridget Kabugho; Nuno Moedas; Nuno C. Santos; Otto Trust
    Context. The radius valley, a deficit in the number of planets with radii around 2 R⊕, was observed among exoplanets with sizes ≲ 5 R⊕ and orbital periods < 100 days by NASA’s Kepler mission. This feature separates two distinct populations: super-Earths (rocky planets with radii ≲ 1.9 R⊕) and sub-Neptunes (planets with substantial volatile envelopes and radii ≳ 2 R⊕). The valley has been proposed to stem from either planet formation conditions or evolutionary atmospheric loss processes. 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. Aims. To re-examine the existence and depth of the radius valley, and how its location varies with orbital period, incident flux, stellar mass, and stellar age. Methods. We derived robust fundamental stellar parameters of 1,221 main-sequence stars (hosting 1,405 confirmed planets) from the SWEET-Cat database using a grid-based machine-learning tool (MAISTEP), which incorporates effective temperatures and metallicities from spectroscopy, as well as Gaia-based luminosities. Our analysis covers stars with effective temperatures between 4400 – 7500 K (FGK spectral types) and estimated radii between 0.62 – 2.75 R⊙. We attained an average uncertainty of 2% in stellar radius and 2% in mass. Combining the updated stellar radii with planet-to-star radius ratios from the NASA Exoplanet Archive, we recomputed the planetary radii achieving an average uncertainty of 5%. Results. Our findings confirm a partially filled planet radius valley near 2 R⊕. The valley depends on orbital period, incident flux, and stellar mass, with slopes of −0.12+0.02−0.01, 0.10+0.02−0.03, 0.19+0.09−0.07, respectively. We also find a stronger mass-dependent trend in average sizes of sub-Neptunes than super-Earths of slopes 0.17+0.04−0.04 and 0.11+0.05−0.05, respectively. With stellar age, the super-Earth/sub-Neptune number ratio increases from 0.51+0.11−0.08 (< 3 Gyr) to 0.64+0.11−0.11 (≥ 3 Gyr). In addition, the valley becomes shallower and shifts to larger radii, indicating age-dependent evolution in planet sizes. A four-dimensional (planet radius, orbital period, stellar mass, and stellar age) linear fit of the valley produces slopes in orbital period and stellar mass that are consistent with the results of the two-dimensional analyses, and a weaker slope of 0.07+0.03−0.04 in stellar age. Conclusions. The valley’s shift and shallowing over gigayear timescales point to prolonged atmospheric loss, which is consistent with a core-powered mass-loss scenario. Our findings also highlight the importance of stellar age in the interpretation of exoplanet demographics and motivate improved age determinations, as expected from future missions like PLATO.