miles into space.This intense activity made the Sun radiate more heat, allow-
ing it to cool and return to its normal size.
During the Sun’s early developmental stages, it was ringed by a proto-
planetary disk composed of several bands of coarse particles, called planetesi-
mals that accreted from grains of dust cast off by a supernova. Some 100
trillion planetesimals orbited the Sun during the solar system’s early stages of
development (Fig. 2). As they continued to grow, the small rocky chunks
swung around the infant Sun in highly elliptical orbits along the same plane
called the ecliptic.
The constant collisions among planetesimals built larger bodies, some of
which gre w to more than 50 miles wide. However, most of the planetary mass
still resided in the small planetesimals.The presence of a large amount of gas in
the solar nebula slowed the planetesimals,enabling them to coalesce into plan-
ets. The planetesimals in orbit between Mars and Jupiter were unable to com-
bine into a planet due to Jupiter’s strong gravitational attraction and instead
formed a belt of asteroids, many of which were several hundreds miles wide.Figure 2The formation
of the solar system from
planetesimals in the solar
nebula.
Historical Geology
1 The solar nebula forms from a supernova 2 The solar nebula begins to rotate3 The solar nebula begins to segregate into
Sun and planets4 The planets sweep up remaining planetesimals
in their orbits