Abstract: Recent observations and analysis of low mass (<10 M?), exoplanets have found that rocky planets only have radii up to 1.5-2 R?. Two general hypotheses exist for the cause of the dichotomy between rocky and gas-enveloped planets (or possible water worlds): either low mass planets do not necessarily form thick atmospheres of a few wt. %, or the thick atmospheres on these planets easily escape driven by x-ray and extreme ultraviolet (XUV) emissions from young parent stars. Here we show that a cu...