Circumstellar discs likely have a short window when they are self-gravitating
and prone to the effects of disc instability, but during this time the seeds of
planet formation can be sown. It has long been argued that disc fragmentation
can form large gas giant planets at wide orbital separations, but its place in
the planet formation paradigm is hindered by a tendency to form especially
large gas giants or brown dwarfs. We instead suggest that planet formation can
occur early in massive discs, through the gravitational collapse of dust which
can form the seeds of giant planets. This is different from the usual picture
of self-gravitating discs, in which planet formation is considered through the
gravitational collapse of the gas disc into a gas giant precursor. It is
familiar in the sense that the core is formed first, and gas is accreted
thereafter, as is the case in the core accretion scenario. However, by forming
a ∼1M⊕​ seed from the gravitational collapse of dust within a
self-gravitating disc there exists the potential to overcome traditional growth
barriers and form a planet within a few times 105 years. The accretion of
pebbles is most efficient with centimetre-sized dust, but the accretion of
millimetre sizes can also result in formation within a Myr. Thus, if dust can
grow to these sizes, planetary seeds formed within very young, massive discs
could drastically reduce the timescale of planet formation and potentially
explain the observed ring and gap structures in young discs.Comment: MNRAS accepted. 15 pages, 12 figure