Saturns Are Not Large Neptunes: The effect of removing inflated giants from empirical mass-radius relations

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ID: 329020
2026
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Abstract
Abstract Empirical mass-radius relations have long been used in exoplanetology to study planet demographics and to predict masses and radii for objects where only one of these is known. Inferred transitions between planet classes can also provide empirical evidence in support of theory. However, mass-radius relations fitted in two-dimensions can be complicated when additional factors influence the planetary parameters, as is the case with the radius inflation of giant planets. In this work, we present an empirical mass-radius relation derived by fitting a broken power-law to a cleaned PlanetS sample from which inflated giants have been removed. We find that when inflated giants are excluded, the preferred model contains three breakpoints and four segments, showing the emergence of a distinct Saturnian regime between 54 ± 3M⊕ < M < 258 ± 11M⊕. In this regime, planets are still growing but at a lower rate than the Neptunian regime, consistent with the onset of gravitational self-compression. We also update the irradiation threshold for giant planet inflation, $S_{\rm thr}=99 \rm S_{\oplus }$ ($1.3 \rm ergs\, s^{-1}\, cm^{-2}$); below Sthr, 90 % of giant planets have a radius excess Δlog R < 2σ.
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Authors Georgina Dransfield, Valentina Tardugno, Niamh K. O'Sullivan
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Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1762
URL
https://doi.org/10.1093/mnras/stag1762
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