AY 101 Chapter Notes - Chapter 9: Kuiper Belt, Oort Cloud, Terrestrial Planet
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The Big Picture
In this chapter we concluded our study of the solar system by focusing on its smallest objects, ﬁnding that
these objects can have big consequences. Keep in mind the following “big picture” ideas:
•Asteroids, comets, and meteorites may well be small compared to planets, but they provide much of the
evidence that has helped us understand how the solar system formed.
•The small bodies of the solar system are subject to the gravitational inﬂuences of the largest. The jovian
planets shaped the asteroid belt, the Kuiper belt, and the Oort cloud, and they continue to nudge objects
onto collision courses with the planets.
•Pluto, once considered a “misﬁt” among the planets, is now recognized as just one of many moderately
sized objects in the Kuiper belt. In terms of composition, the dwarf planets Pluto and Eris-along with
other similar objects-are essentially comets of unusually large size.
•Collisions not only bring meteorites and leave impact craters but also can profoundly affect life on
Earth. An impact probably wiped out the dinosaurs, and future impacts pose a threat that we cannot
Summary of Key Concepts
What are asteroids like?
•Asteroids are rocky leftovers from the era of planetary formation. Most are small, and despite their
enormous numbers their total mass is less than that of any terrestrial planet.
Why is there an asteroid belt?
•The asteroid belt is all that remains of the swarm of planetesimals that once lay between Mars and
Jupiter. Orbital resonances nudged orbits in this region, leading to collisions and gradually ejecting
material, so that the region lost most of its original mass. Resonances continue to shape asteroid orbits
and lead to occasional collisions today.
How are meteorites related to asteroids?
•Most meteorites are pieces of asteroids. Primitive meteorites are essentially unchanged since the birth
of the solar system. Processed meteorites are fragments of larger asteroids that underwent
How do comets get their tails?
•Comets are icy leftovers from the era of planet formation, and most orbit far from the Sun. If a comet
approaches the Sun, its nucleus heats up and its ice vaporizes into gas. The escaping gases carry along
some dust, forming a coma and two tails: a plasma tail of ionized gas and a dust tail. Larger particles
can also escape, causing meteor showers on Earth.
Where do comets come from?
•Comets come from two reservoirs: the Kuiper belt and the Oort cloud. The Kuiper belt comets still
reside in the region beyond Neptune in which they formed. The Oort cloud comets formed between the
jovian planets and were kicked out to a great distance by gravitational encounters with those planets.
How big can a comet be?
•In the Kuiper belt, icy planetesimals were able to grow to hundreds or thousands of kilometers in size.
The recently discovered Eris is the largest known of these objects and Pluto is the second largest.
What are large objects of the Kuiper belt like?
•Like smaller comets, these objects are ice-rich in composition. They orbit the Sun roughly between the
orbit of Neptune and twice that distance from the Sun. Their orbits tend to be more elliptical and more
inclined to the ecliptic plane than those of the terrestrial and jovian planets. Many share orbital
resonances with Neptune. A few, including Pluto, have moons.
Did an impact kill the dinosaurs?
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