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8.1-Search for Origins, 8.2-Birth of the Solar System, 8.3 The Formation of Planets

Astronomy & Astrophysics
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8.1 The Search for origins
-4 criteria for the success of a solar system formation theory
1. must explain the patterns of motion
2. must explain why planets fall into two major categories
3. must explain the existence of huge numbers of asteroids and comets and way these objects reside
primarily in the regions we cal the asteroid belt, the Kuiper belt and the Oort cloud
4. must explain all the general patterns while at the same time making allowances for exceptions to
the general rules
-nebular hypothesis: assumed that our solar system was formed from the gravitational collapse of an
interstellar cloud of gas
-parts of it was proved to be wrong later on, but much information has accumulated in favour of the
nebular hypothesis that it achieved the status of a scientific theory
8.2 The Birth of the Solar System
-nebular says that our solar system was born from a cloud of gas that collapsed under its own gravity Æ
solar nebular
-the gas came from 1. galactic recycling and 2. other gas clouds
Galactic Recycling
-universe born in the Big Bang
-hydrogen and helium were the only chemical elements present then
-all heavier elements were produced by stars, through nuclear fusion or explosions
-when stars die, they release much of their content back into space, including newly produced heavy
-this material can then be recycled into new generations of stars
-therefore, the gas that made up the solar nebular was the result of billions of years of galactic
recycling that occurred before our solar system was born
-the sun and the planets were born from this gas
-the Earth and other terrestrial worlds ended up being made primarily from the heavier elements
mixed within
Other Gas Cloud
-supports that our solar system was born from an interstellar cloud of gas because stars that appear to
be in the process of formation today are always found within interstellar clouds
-each star in the process of formation is surrounded by relatively dense gas that appears to be
collapsing under its own gravity, suggesting that is it forming within its own solar nebula
-solar nebula probably began as a large, roughly spherical cloud of very cold and very low-density gas
-was probably so spread out that gravity alone may not have been able to pull it together to start its
-the collapse may have been triggered by a cataclysmic event (shock wave from the explosion of a
-once the collapse started, the law of gravity kept it going
-the mass of cloud remained the same as it shrank, and the strength of gravity increased as the
diameter of the cloud decreased
-solar nebula should have become a spinning, flattened disk surround a central star
Heating, Spinning and Flattening
-three important processes that altered its density, temperature and shape
-large, diffuse cloud Æ much smaller spinning disk
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