The Origin of the Solar System 49
the disk around Jupiter’s orbit, balancing viscous forces that
would cause gas to flow back into the gap. Numerical simu-
lations show that generally gaps are not cleared completely,
and some gas continues to cross a gap and accrete onto
a planet. However, the accretion rate declines as a planet
becomes more massive.
Uranus and Neptune are referred to as ice giant plan-
ets because they contain large amounts of materials such
as water and methane that form ices at low temperatures.
They contain some hydrogen and helium, but they did not
acquire the huge gaseous envelopes that Jupiter and Saturn
possess. This suggests the nebula gas had largely dispersed
in the region where Uranus and Neptune were forming
before they became massive enough to undergo rapid gas
accretion. This may be because they formed in the outer
regions of the protoplanetary disk, where embryo growth
rates were slowest. It is also possible that the nebula dis-
persed more quickly in some regions than others. In partic-
ular, the outer regions of the nebula may have disappeared
at an early stage as the gas escaped the solar system due to
photoevaporation by ultraviolet radiation.
The presence of a gap modifies planetary migration.
Planets massive enough to open a gap still generate spiral
density waves in the gas beyond the gap, but these waves are
located further away from the planet as a result, so migration
is slower. As a planet with a gap migrates inward, gas tends
to pile up at the inner edge of the gap and become rarified
at the outer edge, slowing migration as a result. The migra-
tion of the planet now becomes tied to the inward viscous
accretion of the gas toward the star. The planet, its gap, and
the nebular gas all move inward at the same rate, given by
da
dt=− 1. 5 α(
cs
vkep) 2
vkep (16)whereα=νvkep/(acs^2 ) andνis the viscosity of the nebular
gas. This is calledtype-II migration. Type-II migration
slows when a planet’s mass becomes comparable to that
of the nebula, and migration ceases as the nebular gas
disperses.
Giant planets in the solar system experienced another
kind of migration as they interacted gravitationally with
planetesimals moving on orbits between the giant planets
and in the primordial Kuiper Belt. One consequence of
this process was the formation of the Oort cloud of comets.
Once Jupiter approached its current mass, many planetes-
imals that came close to the planet would have been flung
far beyond the outer edge of the protoplanetary disk. Some
were ejected from the solar system altogether, but others
remained weakly bound to the Sun. Over time, gravitational
interactions with molecular clouds, other nearby stars, and
the galactic disk circularized the orbits of these objects so
they no longer passed through the planetary system. Many
of these objects are still present orbiting far from the Sun
in the Oort cloud. The ultimate source of angular momen-
tum for these objects came at the expense of Jupiter’s orbit,
which shrank accordingly. Saturn, Uranus, and Neptune
ejected some planetesimals, but they also perturbed inward
many objects, which were then ejected by Jupiter. As a re-
sult, Saturn, Uranus, and Neptune probably moved outward
rather than inward.
As Neptune migrated outward, it interacted dynamically
with the primordial Kuiper Belt of comets orbiting in the
very outer region of the nebula. Some of these comets were
ejected from the solar system or perturbed inward toward
Jupiter. Others were perturbed onto highly eccentric orbits
with periods of hundreds or thousands of years, and now
form the scattered disk,a region that extends out beyond the
Kuiper Belt but whose objects are gradually being removed
by close encounters with Neptune. A sizable fraction of
the objects in the region beyond Neptune were trapped
in external mean-motion resonances and migrated outward
with the planet. Pluto, currently located in the 3:2 mean-
motion resonance with Neptune, probably represents one
of these objects.
As the giant planets migrated, it is possible that they
passed through orbital resonances with one another. In
particular, if Jupiter and Saturn passed through the 2:1
mean-motion resonance, their orbital eccentricities would
have increased significantly, with important consequences
throughout the solar system. The eccentricities of Uranus
and Neptune would have briefly become large until they
were damped by dynamical friction with the primordial
Kuiper Belt. Many comets would have been perturbed into
the inner solar system as a result. In addition, the changing
orbits of the giant planets would have perturbed many main-
belt asteroids into unstable resonances, also leading to a flux
of asteroids into orbits crossing the inner planets. Currently,
it is unclear whether Jupiter and Saturn passed through the
2:1 resonance, or when this may have happened. It has been
proposed that passage through this resonance was respon-
sible for the late heavy bombardment of the inner planets,
which occurred 600–700 Ma after the start of the solar sys-
tem and left a clear record of impacts on the Moon, Mars,
and Mercury.9. Planetary Satellites
Earth’s moon possesses a number of unusual features. It
has a low density compared to the inner planets, and it
has only a very small core. The Moon is highly depleted
in volatile materials such as water. In addition, the Earth–
Moon system has a large amount of angular momentum
per unit mass. If they were combined into a single body, the
object would rotate once every 4 hours! All these features
can be understood if the Moon formed as the result of an
oblique impact between Earth and another large, differen-
tiated body, sometimes referred to as Theia, late in Earth’s
formation.
Numerical simulations of this giant impact show that
much of Theia’s core would have sunk through Earth’s