  The sample of known exoplanets is strongly biased to masses larger than the
ones of the giant gaseous planets of the solar system. Recently, the discovery
of two extrasolar planets of considerably lower masses around the nearby stars
GJ 436 and $\rho$ Cancri was reported. They are like our outermost icy giants,
Uranus and Neptune, but in contrast, these new planets are orbiting at only
some hundredth of the Earth-Sun distance from their host stars, raising several
new questions about their origin and constitution. Here we report numerical
simulations of planetary accretion that show, for the first time through N-body
integrations that the formation of compact systems of Neptune-like planets
close to the hosts stars could be a common by-product of planetary formation.
We found a regime of planetary accretion, in which orbital migration
accumulates protoplanets in a narrow region around the inner edge of the
nebula, where they collide each other giving rise to Neptune-like planets. Our
results suggest that, if a protoplanetary solar environment is common in the
galaxy, the discovery of a vast population of this sort of 'hot cores' should
be expected in the near future.
