Abstract
The compact multi-transiting planet systems discovered by Kepler
challenge planet formation theories. Formation in situ from disks with
radial mass surface density, $\Sigma$, profiles similar to the minimum mass
solar nebula (MMSN) but boosted in normalization by factors $\gtrsim10$ has
been suggested. We propose that a more natural way to create these planets in
the inner disk is formation sequentially from the inside-out via creation of
successive gravitationally unstable rings fed from a continuous stream of small
($\sim$cm--m size) "pebbles", drifting inwards via gas drag. Pebbles collect at
the pressure maximum associated with the transition from a magneto-rotational
instability (MRI)-inactive ("dead zone") region to an inner MRI-active zone. A
pebble ring builds up until it either becomes gravitationally unstable to form
an $\sim$1 $M_\Earth$ planet directly or induces gradual planet formation via
core accretion. The planet may undergo Type I migration into the active region,
allowing a new pebble ring and planet to form behind it. Alternatively if
migration is inefficient, the planet may continue to accrete from the disk
until it becomes massive enough to isolate itself from the accretion flow. A
variety of densities may result depending on the relative importance of
residual gas accretion as the planet approaches its isolation mass. The process
can repeat with a new pebble ring gathering at the new pressure maximum
associated with the retreating dead zone boundary. Our simple analytical model
for this scenario of inside-out planet formation yields planetary masses,
relative mass scalings with orbital radius, and minimum orbital separations
consistent with those seen by Kepler. It provides an explanation of how
massive planets can form with tightly-packed and well-aligned system
architectures, starting from typical protoplanetary disk properties.
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