Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.sai.msu.su/tnp/origin.html
Дата изменения: Unknown
Дата индексирования: Mon Oct 1 21:13:52 2012
Кодировка:
Поисковые слова: taurus
Here is a brief outline of the current theory of the events
in the early history of the solar system:
A cloud of interstellar gas and/or dust (the "solar nebula")
is disturbed and collapses under its own gravity. The disturbance
could be, for example, the shock wave from a nearby supernova.
As the cloud collapses, it heats up and compresses
in the center. It heats enough for the dust to
vaporize. The initial collapse is supposed to take
less than 100,000 years.
The center compresses enough to become a protostar
and the rest of the gas orbits/flows around it. Most
of that gas flows inward and adds to the mass of the
forming star, but the gas is rotating. The centrifugal
force from that prevents some of the gas from reaching
the forming star. Instead, it forms an "accretion disk"
around the star. The disk radiates away its energy and
cools off.
First brake point. Depending on the details, the
gas orbiting star/protostar may be unstable and
start to compress under its own gravity. That
produces a double star. If it doesn't ...
The gas cools off enough for the metal, rock and (far
enough from the forming star) ice to condense out into
tiny particles. (i.e. some of the gas turns back into dust).
The metals condense almost as soon as the accretion disk
forms (4.55-4.56 billion years ago according to isotope
measurements of certain meteors);
the rock condenses a bit later (between
4.4 and 4.55 billion years ago).
The dust particles collide with each other
and form into larger particles. This goes on
until the particles get to the size of boulders
or small asteroids.
Run away growth. Once the larger of these
particles get big enough to have a nontrivial
gravity, their growth accelerates. Their gravity
(even if it's very small) gives them an edge over
smaller particles; it pulls in more, smaller particles,
and very quickly, the large objects have accumulated
all of the solid matter close to their own orbit.
How big they get depends on their distance from
the star and the density and composition of the
protoplanetary nebula. In the solar system, the
theories say that this is large asteroid to lunar
size in the inner solar system, and one to fifteen
times the Earth's size
in the outer solar system. There
would have been a big jump in size somewhere
between the current orbits of
Mars and Jupiter:
the energy from the Sun would have kept
ice a vapor at closer distances, so the solid,
accretable matter would become much more common
beyond a critical distance from the Sun. The
accretion of these "planetesimals" is believed
to take a few hundred thousand to about twenty
million years, with the outermost taking the longest
to form.
Two things and the second brake point. How
big were those protoplanets and how quickly
did they form? At about this time, about 1 million
years after the nebula cooled, the star
would generate a very strong solar wind, which
would sweep away all of the gas left in the
protoplanetary nebula. If a protoplanet was
large enough, soon enough, its gravity would
pull in the nebular gas, and it would become
a gas giant. If not, it would remain a rocky or
icy body.
At this point, the solar system is composed
only of solid, protoplanetary bodies and gas
giants. The "planetesimals" would slowly collide
with each other and become more massive.
Eventually, after ten to a hundred million years,
you end up with ten or so planets, in stable orbits,
and that's a solar system. These planets and their surfaces
may be heavily modified by the last, big
collision they experience (e.g. the largely
metal composition of Mercury or the
Moon).
Note: this was the theory of planetary formation as it stood before
the discovery of extrasolar planets.
The discoveries don't match what the
theory predicted.
That could be an observational bias (odd solar systems may be
easier to detect from Earth) or problems with the theory (probably with subtle
points, not the basic outline.)
... Appendices
... Chronology
... Origin
... Linguistics
...
Text by Frank Crary, converted to html by
Bill Arnett; last updated:
1998 Mar 17