Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903

The drastic growth in the amount of exoplanets discovered around cool stars has facilitated paradigm shifts in planet formation and evolution. Observational demographic studies motivate theoretical models aiming to understand underlying physical processes. The M-dwarf radius valley can be described by two processes; evolution by thermally-driven mass loss or formation in a gas-depleted environment. Host star irradiation or internal planet heating would strip planetary atmospheres altering their atmospheres. Exoplanets might form in a reduced proto-planetary disk which would alter their rocky interior structures. Vitally, these models predict different physical planetary properties. Thus, by characterising the interiors of exoplanets orbiting M-dwarfs we can directly test formation and evolution theory. However there are few multi-planet M-dwarf systems to test predictions, severely restricting our understanding of the hidden histories of these worlds. I will report the characterisation of the first four-planet system spanning the radius valley around a Galactic thick disk M-dwarf. We find that this cool star hosts one super-Earth below and two planets above the radius valley, with a distant fourth world. The three inner planets follow the trend of decreasing densities as predicted by planet formation theory. However, the fourth planet breaks this trend as it is a smaller, denser, and essentially gas-devoid, rocky body. This landmark finding rejects thermally-driven mass-loss for the radius valley and supports gas-depleted planet formation around M-dwarfs at longer orbital periods for the first time. This result would strongly imply that the outer planet formed later than the inner worlds and thus provide an unprecedented view into planet formation timing. Intriguingly, the upcoming PLATO mission will directly test this formation pathway by expanding this work to the outer regions of Sun-like stars and revolutionise our understanding of the formation and evolution of planetary systems. Published in Wilson et al. 2026. Science, Volume 392, Issue 6795, id.eadl2348.