The Kepler –36 planetary system consists of two exoplanets at similar separations ( 0.115 & 0.128 AU ) , which have dramatically different densities .
The inner planet has a density consistent with an Earth-like composition , while the outer planet is extremely low-density , such that it must contain a voluminous H/He envelope .
Such a density difference would pose a problem for any formation mechanism if their current densities were representative of their composition at formation .
However , both planets are at close enough separations to have undergone significant evaporation in the past .
We constrain the core-mass , core composition , initial envelope-mass , and initial cooling-time of each planet using evaporation models conditioned on their present-day masses and radii , as inferred from Kepler photometry and transit timing analysis .
The inner planet is consistent with being an evaporatively stripped core , while the outer planet has retained some of its initial envelope due to its higher core-mass .
Therefore , both planets could have had a similar formation pathway , with the inner planet having an initial envelope-mass fraction of \lesssim 10 \% and core-mass of \sim 4.4 M _ { \oplus } , while the outer had an initial envelope-mass fraction of order 15-30 % and core-mass \sim 7.3 M _ { \oplus } .
Finally , our results indicate that the outer planet had a long ( \gtrsim 30 Myr ) initial cooling-time , much longer than would naively be predicted from simple timescale arguments .
The long initial cooling-time could be evidence for a dramatic early cooling episode such as the recently proposed “ boil-off ” process .