Transcript Astro-Lecture-Ch06 - Physics and Astronomy

Chapter 6
Lecture Outline
The Birth and Evolution
of Planetary Systems
21st CENTURY ASTRONOMY
THIRD EDITION
Hester | Smith | Blumenthal | Kay | Voss
Stars Form and Planets Are Born
• Our Solar System is only a tiny part of the universe
• Theories of its origin must explain the contents:
planets, moons, asteroids, etc.
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Stars Form and Planets are Born
• Young stars are
surrounded by rotating
disks of gas and dust.
• The Sun should also
have been surrounded.
• The rest of the Solar
System formed from
that rotating disk
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Protostellar Sun
• Protostar: large, hot ball of gas; not a star yet
• Forms in a collapsing cloud of gas and dust
• Star will form at the very center, where it is densest
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Protoplanetary Disk
• The rest of the mass is the protoplanetary disk.
• The planets and other objects in the Solar System
will form from it.
• Flattened disk is due to angular momentum.
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Angular Momentum
• The angular momentum of a
system will be conserved.
• Result: a spinning sphere will
become a flattened, rotating disk.
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Angular Momentum
• The collapse is slowed
perpendicular to the rotation
axis, but not parallel to it!
• Most of the gas lands on an
accretion disk.
• Accretion = growth by infall.
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Growth of Particles
• Within the disk, small
particles will collide and stick.
• Small particles are blown into
larger ones by gas motions
• This leads to larger particles,
about 1 km, called
planetesimals.
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Growth of Particles
• Once they reach this
size, planetesimals will
pull more particles onto
them by gravity, leading
to planets.
• Today’s remaining
planetesimals:
asteroids, comets.
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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.
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The Inner Disk Is Hot
• 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.
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The Inner Disk
• The temperature difference between the inner and
outer disks causes a difference in composition.
• In the inner disk, only materials that do not melt
at high temperatures can form or remain.
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The Inner Disk
• Refractory = does not melt at high temperature.
• The outer disk has volatile materials like ices.
• Volatile = can melt or evaporate at moderate
temperatures.
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Gathering Atmospheres
• Planets can gather gasses from the disk.
• This makes the primary atmosphere.
• Low-mass planets cannot hold on to their primary
atmospheres
• Some low-mass planets later emit gasses from their
interiors (e.g., from volcanoes), producing a
secondary atmosphere.
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Loss of Atmospheres
• Low-mass planets would lose their primary
atmospheres.
• Recall that temperature measures the speed of
motion of gas atoms.
• Low-mass planets have low escape velocities.
• Some gas atoms can go fast enough to escape
small (or hot) planets.
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Our Solar System
• The 4 inner planets are rocky.
• The 4 outer planets are gaseous giants.
• Asteroids and comets are leftover planetesimals,
while moons formed from the giant planets’
accretion disks.
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Our Solar System Is Not Special
• The physical processes that led to the Solar System
should be commonplace.
• We can see young stars with disks.
• Planet: a body that orbits a star and has a mass
less than 13 Jupiters.
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Extrasolar Planets
• We have found more than 330 extrasolar planets,
or “exoplanets.”
• Four main techniques to find these planets:
•
•
•
•
Spectroscopic radial velocity method
Transit method
Microlensing method
Direct imaging
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Spectroscopic Radial Velocity
• Gravity is a mutual force, so both stars and planets
orbit one another.
• Motion can be detected by Doppler shifts.
• Some stars have periodic velocity changes and
therefore they are orbited by planets.
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Transit and Microlensing
• A planet passing in front
of a star (transiting) can
decrease the total
brightness of the star.
• Microlensing makes a
star temporarily brighter
through a planet’s
gravity focusing its light.
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Direct Imaging
• It is very difficult to directly see a faint planet in the
bright glow of its star, but it can be done.
• 11 planets have been identified this way so far.
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Known Planetary Systems
• Most planetary systems we have found do not
resemble ours.
• Many known planets have 1 to 10 times the mass
of Jupiter.
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Known Planetary Systems
• Some of these orbit close to their stars and are
called hot Jupiters.
• It is easier to find these very large planets due to
the greater “wobble” they cause for their stars.
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Known Planetary Systems
• These hot
Jupiters most
likely formed far
from their stars,
and then
migrated inward.
• These
discoveries help
us realize that
we are just a
part of the vast
universe.
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Concept Quiz  Hot Protostars
We know that stars have different temperatures. Consider a
newly forming star that was much hotter than the proto-Sun.
What would we expect about its planets?
A. The planets orbit at random angles around the star.
B. Rocky planets might be formed over a wider range of
distances than in our Solar System.
C. The star would be “naked,” without a surrounding disk.
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Concept Quiz  Other Planets
Which of these is not a reason why we can find planets around
other stars?
A.
B.
C.
D.
The planet’s gravity causes the star to orbit.
We can take images and directly see the planets.
We can detect radio signals from life on the planets.
A star’s light could be affected by its planet.
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Concept Quiz  Protoplanets
Which of the following statements is false?
A. Planetary systems begin as a disk of material around a
protostar.
B. Planetesimals accrete material to become planets.
C. All the planetesimals in our Solar System have become
planets.
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Solar System Formation
Click the above picture to launch the animation
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Visual Analogy: Traffic Circle
Click the above picture to launch the animation
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This concludes the Lecture PowerPoint presentation
for Chapter 6
For more learning resources,
please visit the StudySpace website
for 21st Century Astronomy at
http://wwnorton.com/studyspace
21st CENTURY ASTRONOMY
THIRD EDITION
Hester | Smith | Blumenthal | Kay | Voss
©2010 W.W. Norton & Company, Inc.
©2010 W.W. Norton & Company, Inc.