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Short-period sub-Neptunes with substantial volatile envelopes are among themost common type of known exoplanets. However, recent studies of the Keplerpopulation have suggested a dearth of sub-Neptunes on highly irradiated orbits,where they are vulnerable to atmospheric photoevaporation. Physically, weexpect this "photoevaporation desert" to depend on the total lifetime X-ray andextreme ultraviolet flux, the main drivers of atmospheric escape. In this work,we study the demographics of sub-Neptunes as a function of lifetime exposure tohigh energy radiation and host star mass. We find that for a given present dayinsolation, planets orbiting a 0.3 $M_{sun}$ star experience $\sim$100 $\times$more X-ray flux over their lifetimes versus a 1.2 $M_{sun}$ star. Defining thephotoevaporation desert as a region consistent with zero occurrence at 2$\sigma$, the onset of the desert happens for integrated X-ray fluxes greaterthan 1.43 $\times 10^{22}$ erg/cm$^2$ to 8.23 $\times 10^{20}$ erg/cm$^2$ as afunction of planetary radii for 1.8 -- 4 $R_{\oplus}$. We also compare thelocation of the photoevaporation desert for different stellar types. We findmuch greater variability in the desert onset in bolometric flux space comparedto integrated X-ray flux space, suggestive of photoevaporation driven by steadystate stellar X-ray emissions as the dominant control on desert location.Finally, we report tentative evidence for the sub-Neptune valley, first seenaround Sun-like stars, for M & K dwarfs. The discovery of additional planetsaround low-mass stars from surveys such as the TESS mission will enabledetailed exploration of these trends.

The sub-Neptune desert and its dependence on stellar type: Controlled by lifetime X-ray irradiation