Planetary System Formation
"The created world is but a small parenthesis in eternity."
— Thomas Browne
|Protoplanetary disks edge on. [more]|
lanetary system formation coincides with the process of
in which our
belongs to the generation of stars created 4.6 billion years
ago, when our galaxy was roughly half its present age. A cloud of
interstellar gas, dust, and ices containing several generations of material
collapsed to form the nebula from which the Sun and the rest of our solar
system grew. This collapse may have been triggered by a nearby supernova.
Cosmologists believe that because the material in the nebula was rotating to
some degree, not all of the nebular material fell directly into the central
mass that would become the Sun. Instead, some of the material was confined
to a flat, spinning disk, called a protoplanetary disk, around a young Sun.
As time went on, the grains and ices in the disk bumped into and stuck to
one another forming macroscopic objects with sizes on of order
0.01-10 meters, all orbiting in the same direction
and same plane analogus to the
rings around Saturn
. As the objects grew
larger, their gravitational forces increased, attracting more matter from
the disk and gradually building kilometer-sized bodies called
planetesimals. These planetesimals further collided and either shattered
into fragments or merged producing larger objects.
The gravitational pull of the largest planetesimals
produced rapid growth to the size of small planets
and formed the nuclei of the planets
we know them today.
|Thin disk around Beta Pictoris. [more]
||HD 141569: Gap in stellar dust disk. [more]
||HR 4796A: Dust ring around star. [more]
These three images show dust rings around a newly formed stars. Beta
Pictoris' disk is slightly warped possibly due to the gravitational
pull of a planet. The other two images also show gaps, bright and
dark areas similar to Saturn's rings.
Some planetesimals in the outer solar system became large enough to
accrete gas forming the giant planets Juipter,
Saturn, Uranus, and
Neptune. Because of the higher temperatures
in the inner solar system, accretion of ice and gas was inhibited so
the planetesimals grew into what is known as the rocky terrestrial planets.
Planetary growth slowed down significantly once a gap was cleared within its
orbit. But even today planets continue to grow by small amounts as they
sweep up micrometeor dust particles or are impacted every few million years by larger
asteroids or comets such as the dramatic impact of comet
Shoemaker-Levy with Jupiter.
Planetesimals that became modest in size but did not merge to form larger
asteroids and comets.
The asteroid belt may be result of fragmentation of planetisimals that
were prevented from growing larger by the close proximity of Jupiter's gravitational pull.
Other planetesimals were tossed about into random orbits
from gravitational interaction with the larger planets. The
Oort Cloud was formed early in the history
of the solar system through gravitational interaction of
planetisimals with Uranus and Neptune. (Close encounters
with Saturn and Jupiter would have ejected the objects
out of the solar system.)
These planetisimals were thrown outwards close to the
Solar System escape velocity. The Oort cloud
consists of some 1,000,000,000,000 long-period comets
that extends out to tens of thousands of AU, half way to our
closest stellar neighbors.
Comets found within Kuiper Belt
may be early remnants of the Sun's protoplanetary disk.
Additional debris left over from the earliest phases of solar system
formation includes small grains of sand and small