The standard model for giant planet formation is based on the accretion of
solids by a growing planetary embryo, followed by rapid gas accretion once the
planet exceeds a so-called critical mass. The dominant size of the accreted
solids (cm-size particles named pebbles or km to hundred km-size bodies named
planetesimals) is, however, unknown. Recently, high-precision measurements of
isotopes in meteorites provided evidence for the existence of two reservoirs in
the early Solar System. These reservoirs remained separated from ~1 until ~ 3
Myr after the beginning of the Solar System's formation. This separation is
interpreted as resulting from Jupiter growing and becoming a barrier for
material transport. In this framework, Jupiter reached ~20 Earth masses within
~1 Myr and slowly grew to ~50 Earth masses in the subsequent 2 Myr before
reaching its present-day mass. The evidence that Jupiter slowed down its growth
after reaching 20 Earth masses for at least 2 Myr is puzzling because a planet
of this mass is expected to trigger fast runaway gas accretion. Here, we use
theoretical models to describe the conditions allowing for such a slow
accretion and show that Jupiter grew in three distinct phases. First, rapid
pebble accretion brought the major part of Jupiter's core mass. Second, slow
planetesimal accretion provided the energy required to hinder runaway gas
accretion during 2 Myr. Third, runaway gas accretion proceeded. Both pebbles
and planetesimals therefore have an important role in Jupiter's formation.Comment: Published in Nature Astronomy on August 27, 201