AST101H1 Lecture : 8.4-The Aftermath of Planet Formation, 8.5-The Age of the Solar System
2 Aug 2010
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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
energy
-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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AST101H1 Full Course Notes
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Document Summary
Although the solar wind blew away excess gas, many planetesimals remained scattered between the newly formed planets. 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) 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.
