  The Kepler mission has released over 4496 planetary candidates, among which
3483 planets have been confirmed as of April 2017. The statistical results of
the planets show that there are two peaks around 1.5 and 2.0 in the
distribution of orbital period ratios. The observations indicate that a plenty
of planet pairs could have firstly been captured into mean motion resonances
(MMRs) in planetary formation. Subsequently, these planets depart from exact
resonant locations to be near MMRs configurations. Through type I migration,
two low-mass planets have a tendency to be trapped into first-order MMRs (2:1
or 3:2 MMRs), however two scenarios of mass accretion of planets and potential
outward migration play an important role in reshaping their final orbital
configurations. Under the scenario of mass accretion, the planet pairs can
cross 2:1 MMRs and then enter into 3:2 MMRs, especially for the inner pairs.
With such formation scenario, the possibility that two planets are locked into
3:2 MMRs can increase if they are formed in a flat disk. Moreover, the outward
migration can make planets have a high likelihood to be trapped into 3:2 MMRs.
We perform additional runs to investigate the mass relationship for those
planets in three-planet systems, and we show that two peaks near 1.5 and 2.0
for the period ratios of two planets can be easily reproduced through our
formation scenario. We further show that the systems in chain resonances (e.g.,
4:2:1, 3:2:1, 6:3:2 and 9:6:4 MMRs), have been observed in our simulations.
This mechanism can be applicable to understand the formation of systems of
Kepler-48, Kepler-53, Kepler-100, Kepler-192, Kepler-297, Kepler-399, and
Kepler-450.
