  The Kepler high-precision planetary sample has revealed a radius valley,
separating compact super-Earths from sub-Neptunes with lower density.
Super-Earths are generally assumed to be rocky planets that were probably born
in-situ, while the composition and origin of sub-Neptunes remains debated. To
provide more constraints on the formation history and composition, based on the
planetary sample of Kepler multiple planet systems, we derive the distributions
of orbital period ratios of sub-Neptune and super-Earth planet pairs and
calculate the normalised fraction of near-first-order mean motion resonances.
Using synthetic planetary systems generated by the Generation III Bern Model,
we also obtain theoretical predictions of period ratio distributions of planet
pairs of different compositions and origins. We find that actual Kepler
sub-Neptune pairs show a normalised fraction smaller (larger) than the model
predictions for water-rich (water-poor) pairs with confidence levels of about
two sigma. The derived normalised fraction of actual Kepler Super-Earth pairs
is generally consistent with that of water-poor model planet pairs but
significantly smaller than that of synthetic water-rich planet pairs. Based on
the distributions of orbital period ratios, we conclude that orbital migration
has been more important for sub-Neptunes than for super-Earths, suggesting a
partial ex situ formation of the former and an origin of the radius valley
caused in part by distinct formation pathways. However, the model comparisons
also show that sub-Neptunes in actual Kepler multiple systems are not likely to
be all water-rich/ex situ planets but a mixture of the two (in situ/ex situ)
pathways. Whereas, Kepler super-Earth planets are predominantly composed by of
water-poor planets that were born inside the ice line, likely through a series
of giant impacts without large scale migration.
