CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 411
fi nished within 10 million years but could have continued to
grow for another 20 million years or so.
Th e solar nebula theory has been very successful in explain-
ing the formation of the solar system. But there are some prob-
lems, and the Jovian planets are the troublemakers.The Jovian Problem
New information about the star formation process makes it hard
to explain the formation of the Jovian parents, and this has
caused astronomers to expand and revise the theory of planet
formation (■ Figure 19-10).
Th e new information is that gas and dust disks around
newborn stars don’t last long. Earlier you saw images of dusty
gas disks around the young stars in the Orion star-forming
region (Figure 19-2 and Chapter 11). Th ose disks are being
evaporated by intense ultraviolet radiation from hot O and B
stars forming nearby. Astronomers have calculated that nearly
all stars form in clusters containing O and B stars, so this
evaporation may happen to most disks. Even if a disk did not
evaporate quickly, the gravitational infl uence of the crowded
stars in a cluster could strip away the outer parts of the disk.
Th ose are troublesome observations because they seem to indi-
cate that disks can’t last longer than about 7 million years at
most, and many evaporate within the astronomically very
short span of 100,000 years or so. Th at’s not long enough to
grow a Jovian planet by the combination of condensation,
accretion, and gravitational collapse proposed in the standard
solar nebular theory.
Yet, Jovian planets are common. In the fi nal section of this
chapter, you will see evidence that astronomers have found plan-
ets orbiting other stars, and almost all of those planets have the
mass of Jovian planets. Th ere may also be many Terrestrial plan-
ets orbiting those stars that are too small for astronomers to
detect at present, but the important point is that there are lots of
Jovian planets around.the planet’s interior during diff erentiation. Th e creation of a
planetary atmosphere from a planet’s interior is called outgas-
sing. Given the location of Earth in the solar nebula, gases
released from its interior during diff erentiation would not
have included as much water as Earth now has. So, some astron-
omers think that Earth’s water and even some of its present
atmosphere and biosphere accumulated late in the formation of
the planet as Earth swept up volatile-rich planetesimals. Th ese
icy planetesimals would have formed in the cool outer parts of
the solar nebula and could have been scattered toward the
Terrestrial planets by encounters with the Jovian planets, creat-
ing a comet bombardment.
According to the solar nebula theory, the Jovian planets
could begin growing by the same processes that built the
Terrestrial planets. However, in the inner solar nebula, only met-
als and silicates could form solids, so the Terrestrial planets grew
slowly. In contrast, the outer solar nebula contained not just solid
bits of metals and silicates but also plentiful ices. Astronomers
calculate that the Jovian planets would have grown faster than
the Terrestrial planets and quickly become massive enough to
begin even faster growth by gravitational collapse, drawing in
large amounts of gas from the solar nebula. Th e Terrestrial planet
zone did not include ice particles, so those planets developed
relatively slowly and never became massive enough to grow fur-
ther by gravitational collapse.
Th e Jovian planets must have reached their present size in no
more than about 10 million years, before the sun become hot
and luminous enough to blow away the remaining gas in the
solar nebula, removing the raw material for further Jovian
growth. As you will learn in the next section, disturbances from
outside the forming solar system may have reduced the time
available for Jovian planet formation even more severely. Th e
Terrestrial planets, in comparison, grew from solids and not from
the gas, so they could have continued to grow by accretion from
solid debris left behind after the gas was removed. Mathematical
models indicate that the Terrestrial planets were at least half
■ Figure 19-10
The Jovian worlds pose a
problem for modern astrono-
mers. Planet-forming nebulae
are blown away in only a few
million years by nearby lumi-
nous stars, so Jovian planets
must form more quickly than
initial calculations predicted.
Newer research suggests that
accretion followed by gravi-
tational collapse can build
Jovian planets in about a mil-
lion years. Under certain con-
ditions, direct gravitational
collapse may form some large
planets in just thousands of
years.