A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
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Tuesday 9th
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A fading radius valley towards M-dwarfs, a persistent density valley across stellar types
The radius valley separating super-Earths from mini-Neptunes is a fundamental benchmark for theories of planet formation and evolution. Observations show that the location of the radius valley decreases with decreasing stellar mass and with increasing orbital period. Here, we build from our previous pebble-based formation model, which unveiled the radius valley as a separator between rocky- and water-worlds. We here expand our models for stellar masses ranging from 0.1 to 1.5 Msun . We find that the location of the radius valley is in excellent agreement with observations. We also find very good agreement with the dependence of the radius valley on orbital period, both for FGK- and M-dwarfs. Additionally, we note that the radius valley gets filled towards low stellar masses (0.1-0.4 Msun). This is the result of orbital migration occurring at lower planet mass for less massive stars, which allows for low-mass water-worlds to reach the inner regions of the system, blurring the separation in mass (and size) between rocky- and water-worlds.
As with Sun-like stars, pebble accretion leaves its imprint on the overall exoplanet population as a depletion of planets with intermediate compositions (0 − 20% water mass fraction), carving a planet-depleted diagonal band in the mass-radius diagram. This band is better visualised when plotting the planet mean density in terms of Earth-like composition, making the valley emerge for all stellar masses.