  We construct models of the structural evolution of super-Earth- and
mini-Neptune-type exoplanets with hydrogen-helium envelopes, incorporating
radiative cooling and XUV-driven mass loss. We conduct a parameter study of
these models, focusing on initial mass, radius, and envelope mass fractions, as
well as orbital distance, metallicity, and the specific prescription for mass
loss. From these calculations, we investigate how the observed masses and radii
of exoplanets today relate to the distribution of their initial conditions.
Orbital distance and initial envelope mass fraction are the most important
factors determining planetary evolution, particular radius evolution. Initial
mass also becomes important below a "turnoff mass," which varies with orbital
distance, with mass-radius curves being approximately flat for higher masses.
Initial radius is the least important parameter we study, with very little
difference between the hot start and cold start limits after an age of 100 Myr.
Model sets with no mass loss fail to produce results consistent with
observations, but a plausible range of mass loss scenarios is allowed. In
addition, we present scenarios for the formation of the Kepler-11 planets. Our
best fit to observations Kepler-11b and Kepler-11c involves formation beyond
the snow line, after which they moved inward, circularized, and underwent a
reduced degree mass loss.
