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Interpreting the radius valley: the composition of small planets and the M dwarf valley

Presentation #400.04 in the session Plenary 5.

Published onJun 20, 2022
Interpreting the radius valley: the composition of small planets and the M dwarf valley

One of the most fascinating paradigm shifts in exoplanet science has been the realisation that small exoplanets come in two distinct flavours, i.e., super-Earth and sub-Neptune planets, separated by a radius valley. The valley is key to understanding the formation of small planets, as its location and slope as a function of orbital period are shaped by stellar radiation, disk properties, and planet composition. In this talk I present key new insights that can be learned from measuring the radius valley for stars of different type. In particular, I show recently published results that have determined the location and slope of the radius valley for M dwarf stars. The radius valley shifts with respect to FGK stars and I match this shift to theoretical models of photo-evaporation and core-powered mass-loss. The slope is similar to that of FGK stars, which is inconsistent to other recently published observations but consistent with theoretical models. I furthermore show how recent and forthcoming radial velocity observations of small TESS planets can be used to expand our view of the valley from period-radius space to mass-period-radius space, and that the vast majority of small planet compositions can be interpreted in the context of photo-evaporation without needing to invoke the presence of water worlds. Finally, I present some early results from refitting Kepler transit observations, revealing the radius valley is significantly deeper than previously observed and discuss the consequences of this finding for the composition of small exoplanets.

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