Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-25
Course Announcements
•
How is the sunset/sunrise observing going?
•
SW-chapter 6 posted: due Today
SW-chapter 7 posted: due Fri. Oct. 30
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 Our Solar System is only a tiny part of the
universe. There are many more like it.
 Theories of its origin must explain its
contents: planets, moons, asteroids, etc.
The Solar System
The
Ecliptic
April 17, 2002
The
Planetary
Orbits
The Inner Planets
The Outer Planets
Neptune
Saturn
Uranus
Jupiter
“Facts” that must be accounted for in
any theory of solar system formation
•Almost all the planets orbit in the same plane
•All the planets orbit in the same direction
•Almost all the planets rotate in the same direction
as they orbit
•The inner planets are rocky bodies while the outer
planets are gaseous and/or icy bodies
•Almost 99% of the mass of the solar system is in
the sun
•Most of the angular momentum of the solar system
is in the planets
 Young stars are
surrounded by
rotating disks of gas
and dust.
 The infant Sun
would also have
been surrounded by
such.
 The rest of the Solar
System formed from
that rotating disk.




Protostar: large, hot ball of gas; not a star yet.
Forms in a collapsing cloud of gas and dust.
Forms at the center, where it is densest.
When the right conditions are met, it becomes a
star.
 The rest of the mass is the protoplanetary
disk.
 The planets and other objects in the Solar
System will form from it.
 The flattened disk is a result of angular
momentum conservation.
 The cloud begins as a
diffuse spherical collection
of material.
 Parts of it are going in the
same direction.
 The angular momentum of
the system is conserved.
 Result: a spinning sphere
will become a flattened,
rotating disk.
 The collapse is slowed
perpendicular to the
rotation axis, but not
parallel to it!
 It is easier for the parts
along the rotation axis to
fall in.
 Most of the gas lands on
an accretion disk, which
continues the rotation.
 Accretion = growth by
infall.
MATH TOOLS 7.1
 Angular momentum depends on the
rotational speed of an object, its mass,
and how its mass is distributed.
 A spinning uniform sphere’s angular
momentum:
 The spinning angular momentum of a
collapsing sphere in space stays constant.
 As it collapses, it must speed up. Speed is
inversely related to the rotational period.
CONNECTIONS 7.1
 The idea of conservation of energy is a
powerful one.
 One type of energy can be converted into
another.
 Example: a hydroelectric plant using falling
water to turn a turbine to generate electricity.
 In the protoplanetary system, gravitational
potential energy is turned into kinetic energy
as an object falls closer to the center, and
then thermal energy once it hits the other disk
material.
 Within the disk, small
particles will collide and
stick.
 Small particles are
blown into larger ones
by gas motions.
 This leads to larger
particles (~ 1 km in size)
called planetesimals.
 Once they reach this
size, planetesimals
will pull more
particles onto them
by gravity, leading to
planets.
 Today’s remaining
planetesimals:
asteroids, comets.
 The inner disk is
hot.
 The gravitational
energy of the
infalling material is
converted into heat.
 Material that lands
on the inner part of
the disk has fallen
farther and has
more energy to
convert into heat.
 Particles in the outer
disk do not have as
far to fall.
 Also, the protostar at
the center is
contracting and
heating up.
 This also heats the
inner disk more than
the outer.
 The temperature difference between the inner
and outer disks causes a difference in
composition.
 Inner disk: Only materials that do not melt at
high temperatures can form or remain.
 Refractory = does not melt at high
temperature.
 The outer disk has volatile materials like ices.
 Volatile = can melt or evaporate at moderate
temperatures.