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Four small planets across the M-dwarf radius valley; how the longest-period well-characterised planet provides evidence for gas-depleted planet formation

Presentation #608.09 in the session Multiple-Planet Systems.

Published onApr 03, 2024
Four small planets across the M-dwarf radius valley; how the longest-period well-characterised planet provides evidence for gas-depleted planet formation

The numerousness of exoplanet systems has permitted demographic studies that have revealed large scale trends, such as the radius valley. This is a dearth of planets with radii between 1.6-2.0 REarth that varies over orbital period and stellar mass. Theoretical studies have aimed to understand the underlying physical processes carving the valley. These theories can well explain the trend for planets orbiting FGK-dwarfs, however the M-dwarf valley is thought to be caused by different underlying physical processes. Evolution by thermally-driven mass loss or gas-depleted formation have been proposed, however there are few multi-planet M-dwarf systems to test predictions. These systems are vital for our understanding as they remove any variations between stars. Systems with a longer period planets allow us refine planet formation and evolution theory further as these mechanisms are thought to vary across orbital period and planet equilibrium temperature. However, to date the rarity of these systems restricts our ability to test theory. The PLATO space telescope, to be launched in 2026, will aim to mitigate our observational bias by discovering small, long-period transiting bodies that are amenable to mass characterisation. Although currently these bodies are still rare. In this talk, I will report the discovery and characterisation of the first four-planet system spanning the M-dwarf radius valley. Via TESS, CHEOPS, and HARPS-N data, we find that this cool star hosts one super-Earth below and three planets above the radius valley. The three interior planets follow the trend of increasing radii and decreasing densities with period as predicted by planet formation theory. However, the fourth planet breaks this trend as we determine that the outer planet is smaller and denser. This landmark finding is the longest-period well-characterised, small (Rp < 2 REarth) planet around a M-dwarf and its position above radius valley predictions would suggest a gas-rich composition based on current formation and evolution theory. Our interior structure modeling confirms our density findings as the outer planet is a gas-devoid, rocky body. Our characterisation rejects thermally-driven mass-loss models for the radius valley and supports gas-depleted planet formation around M-dwarfs at longer orbital periods for the first time. This system is an exciting step down the road of characterising long period planets that will be further developed with future missions, such as PLATO, that will revolutionise our understanding of the architectures, and thus the formation and evolution, of planetary systems.

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