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Observational studies of the occurrence rate of close-in planets around Sun-like stars have revealed a bimodality in the distribution of planetary radii. The radius valley likely marks the transition between rocky planets and larger planets that host extended gaseous envelopes. The slope of the radius valley in period/radius space encodes the mechanism by which close-in planets accrete and retain their primordial H/He envelopes. I present the results of our complementary study of the population of close-in planets orbiting 17,393 low mass stars from Kepler and K2, around which a significantly different radius valley slope is measured compared to around Sun-like stars. This slope evolution may indicate that the dominant physical mechanism for sculpting the valley may switch from thermally driven atmospheric mass loss around Sun-like stars to gas-poor terrestrial planet formation around mid-to-late M dwarfs. Furthermore, I show that the central radius of the valley shifts to smaller planet radii with decreasing stellar mass, a trend that can also be used to distinguish between competing radius valley emergence models. Understanding the prominence of the photoevaporation process across the main sequence and establishing whether or not we are witnessing the emergence of a separate channel of planet formation in the low stellar mass regime can be addressed with two investments: firstly by observing the 12,324 mid-to-late M dwarfs accessible in the TESS extended mission to calculate the occurrence rate of close-in planets, and secondly, by obtaining precise radial velocity masses of planets whose expected bulk compositions differ between model predictions from photoevaporation and from gas-poor formation.

Evolution of the Radius Valley from Sun-like to Low Mass Stars (Exoplanets III; July 28, 2020)