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8.4-The Aftermath of Planet Formation, 8.5-The Age of the Solar System

Astronomy & Astrophysics
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8.4 The Aftermath of Planet Formation
Asteroids and Comets
-although the solar wind blew away excess gas, many planetesimals remained scattered between the
newly formed planets
-became asteroids and comets
-asteroids: rocky leftover planetesimals of the inner solar system
-no planet formed between Mars and Jupiter (Asteroid Belt)
-comets: ice-rich leftover planetesimals of the outer solar system
-initially populated the entire outer solar system, beyond Neptune
-these large comets became the comets of the Kuiper Belt
-Pluto is sometimes considered as a large comet of the Kuiper
-there were comets that roamed between the jovian planets, suffered close gravitational encounters
with these planets
-caused most of these comets to be kicked into orbits that carried them far from the Sun
-their new orbits no longer display the original orderly motion of the disk (Oort Cloud)
-these comets are undetectable with present technology
Explaining Exceptions to the Rules
Heavy Bombardment
-the asteroids and comets that exists today is probably a very small fraction of the vast numbers of
leftover planetesimals that roamed the younger solar system
-the vast majority of these collisions occurred in the first few hundred million years of our solar
system·s history, during the period we call the heavy bombardment
-every world in our solar system must have been pelted by impacts during the heavy bombardment
-some of these impacts left scars that we can still see today as impact craters
-water, along with other hydrogen compounds must have been brought to Earth and the other
terrestrial planets by the impact of planetesimals that formed farther from the Sun
Captured Moons
-the moons that orbit in the wrong direction or have large inclinations to the planet·s equator, are
probably leftover planetesimals that were captured into orbit around a planet
-not easy for a planet to capture a moon
-an object cannot switch from an unbound orbit to a bound orbit, unless it somehow loses orbital
-for jovian planets, captures probably occurred when passing planetesimals lost energy to drag in the
extended and relatively dense gas
-this capture process doesn·t explain our own Moon
-much too large to have been captured by a small planet like Earth
-Moon was not formed simultaneously with Earth
-if they were, they would have been accreted from planetesimals of the same type, and should have
approximately the same composition and density
-moon·s density is considerably lower than Earth·s
Giant Impacts
-when one of these planet-sized planetesimals collided with a planet, it caused a giant impact that
could have significantly altered planet·s fate
-a giant impact is the leading hypothesis for explaining the origin of our moon
-may also explain many of the other exceptions to the general trends
-may be responsible for tilting the axes of many planets
-may be responsible for tipping Uranus on its side
-Pluto·s moon Charon, may have formed in a giant impact
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