  A new piece of evidence supporting the photoevaporation-driven evolution
model for low-mass, close-in exoplanets was recently presented by the
California-Kepler-Survey. The radius distribution of the Kepler planets is
shown to be bimodal, with a ``valley' separating two peaks at 1.3 and 2.6
Rearth. Such an ``evaporation-valley' had been predicted by numerical models
previously. Here, we develop a minimal model to demonstrate that this valley
results from the following fact: the timescale for envelope erosion is the
longest for those planets with hydrogen/helium-rich envelopes that, while only
a few percent in weight, double its radius. The timescale falls for envelopes
lighter than this because the planet's radius remains largely constant for
tenuous envelopes. The timescale also drops for heavier envelopes because the
planet swells up faster than the addition of envelope mass. Photoevaporation,
therefore, herds planets into either bare cores ~1.3 Rearth, or those with
double the core's radius (~2.6 Rearth). This process mostly occurs during the
first 100 Myrs when the stars' high energy flux are high and nearly constant.
The observed radius distribution further requires that the Kepler planets are
clustered around 3 Mearth in mass, are born with H/He envelopes more than a few
percent in mass, and that their cores are similar to the Earth in composition.
Such envelopes must have been accreted before the dispersal of the gas disks,
while the core composition indicates formation inside the ice-line. Lastly, the
photoevaporation model fails to account for bare planets beyond ~30-60 days, if
these planets are abundant, they may point to a significant second channel for
planet formation, resembling the Solar-System terrestrial planets.
