  Sub-Neptune planets formed in the protoplanetary disk accreted
hydrogen-helium (H,He) envelopes. Planet formation models of sub-Neptunes
formed by pebble accretion result in small rocky cores surrounded by polluted
H,He envelopes where most of the rock (silicate) is in vapor form at the end of
the formation phase. This vapor is expected to condense and rain-out as the
planet cools. In this Letter we examine the timescale for the rainout and its
effect on the thermal evolution. We calculate the thermal and structural
evolution of a 10 Earth masses planet formed by pebble accretion, taking into
account material redistribution from silicate rainout (condensation and
settling) and from convective mixing. We find that the duration of the rainout
in sub-Neptunes is on Gyr timescale and varies with envelope mass: planets with
envelopes below 0.75 Earth mass rainout into a core-envelope structure in less
than 1 Gyr, while planets in excess of 0.75 Earth mass of H,He preserve some of
their envelope pollution for billions of years. The energy released by the
rainout inflates the radius with respect to planets that start out from a plain
core-envelope structure. This inflation would result in estimates of the H,He
contents of observed exoplanets based on the standard core-envelope structure
to be too high.We identify a number of planets in the exoplanet census where
rainout may operate, which would result in their H,He contents to be
overestimated by up to a factor two. Future accurate age measurements by the
PLATO mission may allow the identification of planets formed with polluted
envelopes.
