412 PART 4^ |^ THE SOLAR SYSTEM
to consider catastrophic events. Uranus rotates on its side. Th is
might have been caused by an off -center collision with a massive
planetesimal when the planet was nearly formed. Two hypotheses
have been proposed to explain the backward rotation of Venus.
Th eoretical models suggest that the sun can produce tides in the
thick atmosphere of Venus that could have eventually reversed
the planet’s rotation—an evolutionary hypothesis. It is also pos-
sible that the rotation of Venus was altered by an off -center
impact late in the planet’s formation, and that is a catastrophic
hypothesis. Both may be true.
Th e second item in Table 19-1, the division of the planets
into Terrestrial and Jovian worlds, can be understood through the
condensation sequence. Th e Terrestrial planets formed in the
inner part of the solar nebula, where the temperature was high
and only compounds such as the metals and silicates could con-
dense to form solid particles. Th at produced the small, dense
Terrestrial planets. In contrast, the Jovian planets formed in the
outer solar nebula, where the lower temperature allowed the gas
to form large amounts of ices, perhaps three times more ices than
silicates. Th at allowed the Jovian planets to grow rapidly and
became massive, low-density worlds. Also, Jupiter and Saturn are
so massive they were able to grow by drawing in cool gas by gravi-
tational collapse from the solar nebula. Th e Terrestrial planets
could not do this because they never became massive enough.
Th e heat of formation (the energy released by in-falling mat-
ter) was tremendous for these massive planets. Jupiter must have
grown hot enough to glow with a luminosity of about 1 percent
that of the present sun, although it never got hot enough to gen-
erate nuclear energy as a star would. Nevertheless, Jupiter is still
hot inside. In fact, both Jupiter and Saturn radiate more heat than
they absorb from the sun, so they are evidently still cooling.
A glance at the solar system suggests that you should expect
to fi nd a planet between Mars and Jupiter at the present location
of the asteroid belt. Mathematical models indicate that the rea-
son asteroids are there rather than a planet is that Jupiter grew
into such a massive body that it was able to gravitationally dis-
turb the motion of nearby planetesimals. Th e bodies that could
have formed a planet just inward from Jupiter’s orbit instead col-
lided at high speeds and shattered rather than combining, were
thrown into the sun, or were ejected from the solar system. Th e
asteroids seen today are the last remains of those rocky
planetesimals.
Th e comets, in contrast, are evidently the last of the icy
planetesimals. Some may have formed in the outer solar nebula
beyond Neptune and Pluto, but many probably formed among
the Jovian planets where ices could condense easily. Mathematical
models show that the massive Jovian planets could have ejected
some of these icy planetesimals into the far outer solar system. In
a later chapter, you will see evidence that some comets are icy
bodies coming from those distant locations, falling back into the
inner solar system.Mathematical models of the solar nebula have been com-
puted using specially built computers running programs that
take weeks to fi nish a calculation. Th e results show that the rotat-
ing gas and dust of the solar nebula could have become unstable
and formed Jovian planets by skipping straight to the step of
gravitational collapse. Th at is, massive planets may have been
able to form by direct collapse of gas without fi rst forming a
dense core by condensation and accretion of solid material.
Jupiters and Saturns can form in these direct collapse models in
only a few hundred years. If the Jovian planets formed in this
way, they could have formed before the solar nebula disappeared,
even if the nebula was eroded quickly by neighboring massive
hot stars.
Th is new insight into the formation of the outer planets
may also help explain a puzzle about the formation of Uranus
and Neptune. Th ose planets are so far from the sun that accre-
tion could not have built them rapidly. Th e gas and dust of the
solar nebula must have been sparse out there, and Uranus and
Neptune orbit so slowly they would not have swept up mate-
rial very rapidly. Th e conventional view is that they grew by
accretion so slowly that they never became quite massive
enough to begin accelerated growth by gravitational collapse.
In fact, it is hard to understand how they could have reached
even their present sizes if they started growing by accretion so
far from the sun. Th eoretical calculations show that they might
instead have formed closer to the sun, in the region of Jupiter
and Saturn, and then could have been shifted outward by
gravitational interactions with the bigger planets. In any case,
the formation of Uranus and Neptune is part of the Jovian
problem.
Th e traditional solar nebula theory proposes that the planets
formed by accreting a core and then, if they became massive
enough, accelerated growth by gravitational collapse. Th e pro-
posed modifi cation to the theory suggests that the outer planets
could have skipped the core accretion phase.
Explaining the Characteristics of the
Solar System
Now you have learned enough to put all the pieces of the puzzle
together and explain the distinguishing characteristics of the
solar system in Table 19-1.
Th e disk shape of the solar system is inherited from the
motion of material in the solar nebula. Th e sun and planets and
moons mostly revolve and rotate in the same direction because
they formed from the same rotating gas cloud. Th e orbits of the
planets lie in the same plane because the rotating solar nebula
collapsed into a disk, and the planets formed in that disk.
Th e solar nebula hypothesis is evolutionary in that it calls on
continuing processes to gradually build the planets. To explain
the odd rotations of Venus and Uranus, however, you may need