Jupiter's Formation
Jupiter is much more massive than all of the
planets in the inner solar system combined.
It is also mainly made of hydrogen and helium,
like our Sun, while the
inner planets are rocky. Despite its general
chemical composition, Jupiter does have a
rocky core, about 3% of its total mass.
This core is similar in composition to, but 10
times larger than, our
Earth. These differences
largely arise from the position of Jupiter's
orbit in the solar system.
Like all of the outer planets, Jupiter formed
from a primordial nebula that also gave
birth to all other objects in the solar system
(including the Sun). The disk of dust that
surrounded the Sun about 4.5 billion years ago
was heated by our star, and the temperature
reached by the dust particles was a function
of their distance from the Sun. Microscopic
dust particles gradually collided and formed
larger ones, a process that ultimately led to
the formation of spherical bodies of
asteroid size, called planetesimals.
Collisions between two planetesimals often
resulted in mutual destruction, but more often
led to the formation of larger planetesimals.
Planets were the end result of this slow
accretion process. The newly formed planets
then mopped up the remaining dust and were
showered by the impact of asteroids. The most
recent of such impacts was the comet
Shoemaker-Levy 9, whose fragments hit Jupiter
in 1994. The inner planets formed from dust
rich in silicates and iron, but poor in
volatile compounds like water, ammonia and
methane. These compounds were very important
components of the solar nebula in outer
regions of the solar system.
Figure 5.1: The impact sites of comet Shoemaker-
Levy
Jupiter formed at a distance from the Sun
where the temperature of the primordial nebula
was low enough for water ice not to evaporate.
Hence, Jupiter in its formative stages had a
lot more material at its disposal than did the
inner planets. At that distance the dust
comprised silicates, iron, water ice and
smaller quantities of organic molecules like
hydrocarbons. Not only was the raw
material more abundant, but ice was also a
good 'glue,' helping the accretion process.
Jupiter grew so much that its gravitational
attraction was able to capture hydrogen and
helium--which where abundant, but in gaseous
form. The only other planet that able to
accomplish this capture efficiently was
Saturn.
