Download Parallels: Proto-Planetary Disks and rings

Birth of Planets
17 March 2016
Origin of the Solar System
• It’s a fundamental problem in astronomy
• We must explain the characteristics of our
own system, and of planets around other stars
• Elements were made inside stars
• Stars formed from giant molecular clouds
• Planets form in a disk around the protostar
• Giant planets form from accretion onto cores
• Terrestrial planets are only the cores
Hubble image of Proto-planetary Disk
Chapter Opener
Protoplanetary Disk
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Unnumbered Figure 2 Page 229
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The disk spins
Characteristics of our Solar System
• Regular planet orbits: same plane, direction,
almost circular
• Planets are closely spaced
• Terrestrial and Jovian planets
– Small, rocky planets near sun
– Gas giant planets further out
• Many small bodies: Asteroids and comets
leftover
• Age: 4.6 billion years, known from meteorites
• Satellites and rings imitate a miniature Solar
System
From disk to planets
• Metals compose 2% of the disk, enough for
grains (tiny, solid particles) to condense… this
is the first stage
• Like raindrops or snowflakes, the grains collide
gently and stick together, growing bigger
• When about 1km across, they are called
planetesimals. Their gravity attracts others
• The planetesimals stick (accretion) eventually
forming baby planets (embryos)
Figure 8.6
Planet Formation from Planetesimals
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Figure 8.7
M
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Figure 8.14
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Frost Line
• Distance from the Sun where water condenses
• Inside it: only rocks and metal condense
• Outside it: water ice (much more abundant
because Hydrogen is the most common
element) condenses, too
• Planetesimals gather into a seed
• This solid seed is called a planetary ‘core’
Terrestrial or Jovian?
• Terrestrial (Earth-like) made of rocks and
metals inside frost-line: they are smaller,
because not enough gravity to hold gas from
the solar nebula
• Jovian (Jupiter-like) cores are bigger, mostly
ice, since it is available outside frost-line.
Maybe 10x Earth, the core gravity is strong
enough attract H and He gas, they become gas
giants
• The attracted gas forms a disk around the
giant: moons form within it, like the planets
did around the Sun
Figure 8.8
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Density waves and
propellers in a
forming disk
Figure 8.10
Impacts
Continue!
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Figure 8.11
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Figure 8.11a
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Figure 8.11b
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Figure 8.12
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Moon Formation
Unnumbered Figure 4 Page 229
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Figure 8.13
Summary
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Discussion
• How is planet formation like star formation?
• Unlike?
• Talk with those around you and give 2 answers
to each
Like and unlike
• Like
– Both form in flat disks
– They occur at the same time
– Heat from the central object warms the disk
nearby
• Unlike
– Planets form by accretion
– Frost-line separates terrestrial & Jovian
Unnumbered Figure 3 Page 229
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Figure 8.5
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The Ice-Line or Frost Line
Planet Line-up
•
•
•
•
Mercury
Venus
Earth
Mars
----- Frost-Line -----
•
•
•
•
Jupiter
Saturn
Uranus
Neptune
Stages in planet formation
• Dust grains collects the mid-plane of the
protoplanetary disk
• Grows by mutual collisions: ‘accretion’
• Planetesimals (about 1km across) grow and
collide
• Giant collisions are the final stage
• Giant planet cores are bigger outside the
frost-line: they attract gas to become gas
giants like Jupiter
• Star ignites: the gas and dust blown away
Figure 8.9
Clean up
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Leftovers
• Near the Sun (inside frost line, also known as
snow line or ice line) rocky objects become
asteroids
• Far from the Sun (outside frost line) icy
objects form the comets and Kuiper Belt
Objects
Final Cleanup!
Summary
• Stars have a life cycle, enriching space with
biogenic elements
• Stars are born when gravity causes a cloud of
gas to condense
• In the center of the disk, the star forms;
outside, planets form
• Moons from in disks around the planets
• Planets and moons may be habitable: suitable
for life to arise and persist