  The majority of the transiting planets discovered by the Kepler mission
(called super-Earths here, includes the so-called 'sub-Neptunes') orbit close
to their stars. As such, photoevaporation of their hydrogen envelopes etch
sharp features in an otherwise bland space spanned by planet radius and orbital
period. This, in turn, can be exploited to reveal the mass of these planets, in
addition to techniques such as radial velocity and transit-timing-variation.
Here, using updated radii for Kepler planet hosts from Gaia DR2, I show that
the photoevaporation features shift systematically to larger radius for planets
around more massive stars (ranging from M-dwarfs to F-dwarfs), corresponding to
a nearly linear scaling between planet mass and its host mass. By modelling
planet evolution under photo-evaporation, one further deduces that the masses
of super-Earths peak narrowly around $8 M_\oplus (M_*/M_\odot)$. Moreover, the
composition of their cores is likely terrestrial, and they were initially
coated with H/He envelopes a couple percent in mass. Interestingly, the masses
of these planets do not appear to depend on the metallicity values of their
host stars, while they may depend on the orbital separation weakly, possibly as
$r^{1/2}$. Taken together, the simplest interpretation of our results is that
super-Earths are at the so-called 'thermal mass', where the planet's Hill
radius is equal to the vertical scale height of the gas disk.
