The Solar System

CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 409

Th rough these processes, the theory proposes, the nebula

became fi lled with trillions of solid particles ranging in size from

pebbles to tiny planets. As the largest began to exceed 100 km in

diameter, a third process began to aff ect them, and a new stage

in planet building began, the formation of protoplanets.

The Growth of Protoplanets

Th e coalescing of planetesimals eventually formed protoplanets,

the name for massive objects destined to become planets. As

these larger bodies grew, a new process helped them grow faster

and altered their physical structure.

If planetesimals had collided at orbital velocities, it is

unlikely that they would have stuck together. A typical orbital

velocity in the solar system is about 10 km/s (22,000 mph).

Head-on collisions at this velocity would vaporize the material.

However, the planetesimals were all moving in the same direc-

tion in the nebular plane and didn’t collide head on. Instead,

they merely “rubbed shoulders,” so to speak, at low relative

velocities. Such gentle collisions would have been more likely to

combine planetesimals than to shatter them.

Some adhesive eff ects probably helped. Sticky coatings

and electrostatic charges on the surfaces of the smaller plane-

tesimals probably aided formation of larger bodies. Collisions

would have fragmented some of the surface rock; but, if the

planetesimals were large enough, their gravity could have held

on to some fragments to form a layer of soil composed of

planetesimals, which eventually build the planets. Th e study of
planet building is the study of these processes: condensation,
accretion, and gravitational collapse, each of which will be
described in detail in this section.
According to the solar nebula theory, planetary development
in the solar nebula began with the growth of dust grains. Th ese
specks of matter, whatever their composition, grew from micro-
scopic size fi rst by condensation, then by accretion.
A particle grows by condensation when it adds matter one
atom or molecule at a time from a surrounding gas. Snowfl akes,
for example, grow by condensation in Earth’s atmosphere. In the
solar nebula, dust grains were continuously bombarded by atoms
of gas, and some of those stuck to the grains. A microscopic grain
capturing a layer of gas molecules on its surface increases its mass
by a much larger fraction than a gigantic boulder capturing a
single layer of molecules. Th at is why condensation can increase
the mass of a small grain rapidly, but, as the grain grows larger,
condensation becomes less eff ective.
Th e second process is accretion, the sticking together of
solid particles. You may have seen accretion in action if you have
walked through a snowstorm with big, fl uff y fl akes. If you caught
one of those “fl akes” on your glove and looked closely, you saw
that it was actually made up of many tiny, individual fl akes that
had collided as they fell and accreted to form larger particles. In
the solar nebula, the dust grains were, on average, no more than
a few centimeters apart, so they collided frequently and could
accrete into larger particles.
When the particles grew to sizes larger than a centimeter,
they would have been subject to new processes that tended to
concentrate them. One important eff ect was that the growing
solid objects would have collected into the plane of the solar
nebula. Small dust grains could not fall into the plane because
the turbulent motions of the gas kept them stirred up, but
larger objects had more mass, and gas motions could not have
prevented them from settling into the plane of the spinning
nebula. Astronomers calculate this would have concentrated
the larger solid particles into a relatively thin layer about 0.01
AU thick that would have made further growth more rapid.
Th ere is no clear distinction between a very large grain and a
very small planetesimal, but you might consider an object to
be a planetesimal when its diameter approaches a kilometer
(0.6 mi) or so (■ Figure 19-8).
Th is concentration of large particles and planetesimals into
the plane of the nebula is analogous to the fl attening of a forming
galaxy, and a process also found in galaxies may have become
important once the plane of planetesimals formed. Computer
models show that the rotating disk of particles should have been
gravitationally unstable and would have been disturbed by spiral
density waves resembling the much larger ones found in spiral
galaxies. Th ose waves could have further concentrated the plan-
etesimals and helped them coalesce into objects up to 100 km
(60 mi) in diameter.

Visual-wavelength image

■ Figure 19-8

What did the planetesimals look like? You can get a clue from this photo

of the 5-km-wide nucleus of Comet Wild 2 (pronounced Vildt-two). Whether

rocky or icy, the planetesimals must have been small, irregular bodies,

scarred by craters from collisions with other planetesimals. (NASA)