  It has been proposed that the observed systems of hot super-Earths formed in
situ from high-mass disks. By fitting a disk profile to the entire population
of Kepler planet candidates, Chiang & Laughlin (2013) constructed a
"minimum-mass extrasolar nebula" with surface density profile Sigma r^-1.6.
Here we use multiple-planet systems to show that it is inconsistent to assume a
universal disk profile. Systems with 3-6 low-mass planets (or planet
candidates) produce a diversity of minimum-mass disks with surface density
profiles ranging from Sigma r^-3.2 to Sigma r^0.5 (5th-95th percentile). By
simulating the transit detection of populations of synthetic planetary systems
designed to match the properties of observed super-Earth systems, we show that
a universal disk profile is statistically excluded at high confidence. Rather,
the underlying distribution of minimum-mass disks is characterized by a broad
range of surface density slopes. Models of gaseous disks can only explain a
narrow range of slopes (roughly between r^0 and r^-1.5). Yet accretion of
terrestrial planets in a gas-free environment preserves the initial radial
distribution of building blocks. The known systems of hot super-Earths must
therefore not represent the structure of their parent gas disks and can not
have predominantly formed in situ. We instead interpret the diversity of disk
slopes as the imprint of a process that re-arranged the solids relative to the
gas in the inner parts of protoplanetary disks. A plausible mechanism is inward
type 1 migration of Mars- to Earth-mass planetary embryos, perhaps followed by
a final assembly phase.
