  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 $\gtrsim 10$ 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 (~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 $\sim 1\
M_\oplus$ 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.
