Download Wednesday, Oct. 8

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Phys 1810 Lecture 15:
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Total Lunar Eclipse
on Wednesday Oct
8! Mid-eclipse at
5:55 am.
READ BEFORE LECTURE:
– formation of the moon 8.8
– exoplanets Chapt 15
– The Sun Chapt 16
•
Also can read up on comet structure.
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Review Angular Momentum
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• Ang. mom. is conserved == constant.
• mass is a constant
• If decrease r, then rotation rate must
increase to keep ang. mom. constant.
(#revolutions/sec)
Planetary System Formation
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• A theory needs to explain why:
– planets orbit in a plane
– planets orbit in same direction
– Venus rotates backwards & Uranus rolls
– inner planets are small & rocky
– outer planets are large & gaseous
– asteroids are old
– Why Kuiper Belt Objects (KBO) and Oort
cloud comets are icy
Theory in 2 parts
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• Part 1: Solar Nebula Theory or Nebular
Theory
• Part 2: Condensation Theory
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Part 1: Nebular Theory
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Cygnus Region
(English and Taylor for CGPS)
1. Supernova explosion
• Pollutes interstellar medium (ISM)
with elements heavier than lithium
(needed for life).
• Compresses ISM  gas cloud
collapses under its own gravity.
(or rotation of galaxy)
Part 1: Nebular Theory
summary
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2. The solar nebula: Starts as a rotating cloud
All material orbits in same direction explains why planets
orbit in same direction.
• initially has radius of Oort cloud.
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collapse along the poles more easily than along the equator
 disk.
Helps explain why planets orbit in a plane.
•
due to conservation of angular momentum, spin of nebula
increases as cloud contracts.
Part 1: Nebular Theory
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Star formation
• Central condensation becomes proto-star
(i.e. proto-sun).
• Proto-star radiates heat due to gravitational
contraction. (Kelvin-Helmholtz contraction.)
Stage 1: Nebular Theory
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Part 2: Condensation Theory of Planet Formation
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• Planet formation occurs as star
formation continues to proceed.
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Part 2: Condensation Theory: Four Stages
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• Stage 1: Accretion
– Matter accretes onto dust grains
(electrostatic)
– Collisions cause the size to increase
 planetesimals of 300 km
diameter.
 Nebula with H and He gas and millions
of planetesimals.
Part2: Observations supporting accretion stage.
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Part 2: Condensation Theory
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Stage 2: Protoplanet formation (a, b, c)
Planetesimals are attracted to each other by
their gravity.
a) Collision & merger of planetesimals to
form protoplanets.
Why some rotate backwards or roll
b) Collisions of planetesimals & protoplanets
cause fragmentation of planetesimals.
Part 2: Condensation Theory
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Stage 2b: Protoplanet formation continued
• Most fragments are swept up by
protoplanets.
• Some fragments evolve into asteroids &
comets.
 Explains existence of asteroids & comets.
Part 2: Condensation Theory
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Stage 2:
c) Differentiation of Solar System
into inner & outer planets.
• Simultaneously build up gas
giant planets.
• Temperature (T) in the disk
drops off with distance from
the protosun.
Part 2: Condensation Theory: Differentiation
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Stage 2:
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Accretion first starts in cooler outer regions of solar nebula.
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Radius > = 5 AU
–
T sufficiently low that metals, silicates & ices can survive
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Provides lots of solids to combine into planetesimals.
 Explains why comets are “icy”.
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protoplanets rapidly build up (possibly 10*mass of the earth)
Part 2: Condensation Theory: Differentiation
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Stage 2:
• Radius <5 AU
– Hard for ice to survive & combine into
planetesimals.
 Explains why asteroids & inner planets are “rocky”.
• Protoplanets form out of rocky planetesimals.
Part 2: Condensation Theory: Differentiation during Stage
2
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Radius > = 5 AU
• T sufficiently low that
metals, silicates and
ices can survive
 Explains why comets
are “icy”.
• Massive protoplanets
rapidly build up
Radius <5 AU
• Hard for ice to survive
 Explains why asteroids
& inner planets are
“rocky”.
• Protoplanets form out
of rocky planetesimals.
Part 2: Observational Evidence of
Planetesimal Accumulation
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• Beta Pictoris
– Comet-like objects in a disk.
– Planet candidate in inner region.
Part 2: Condensation Theory
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Stage 2: Protoplanet formation
 Predominantly H & Helium gas disk, icy &
rocky planetesimals, & protoplanets in
the process of forming.
Part 2: Condensation Theory
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Stage 2c: Jovian planet formation
Occurs as planetesimals are colliding.
• largest 4 protoplanets gravitationally
sweep up large amounts of gas from
nebula.
• E.g. Jupiter’s atmosphere is 86% H &
13.8% He.
 Explains the existence of gas giants
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Part 2: Condensation Theory
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Stage 2c: Jovian planet formation
• Terrestrials protoplanets are not massive
enough to attract gas (fewer solids to
accumulate).
 Terrestrial planets are rocky
Visualization: Planetary Systems Simulations  Formation
of gas giant planets in a protoplanetary disks
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• Gravitational Instability Theory 
turbulence helps.
• Note structures in the disk & sudden
occurrence of knots of gas.
summary
Part 2: Condensation Theory
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Stage 3: Solar Nebula is cleared of gas & dust.
• Proto-sun, on the brink of nuclear fusion,
generates jets and/or stellar winds.
• Gas giants need to have formed before
this.
• Collisions of planetesimals continue
terrestrial planets continue to grow.
Part 2: Evidence of Stellar Winds in Protoplanetary Disks
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• Young stellar objects
– disks & bi-polar outflows.
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Part 2: Evidence of Stellar Winds in Young Stellar Objects
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HST observations of HH 30 by
Alan Watson (UNAM) and
collaborators.
HST observations of XZ Tauri by
John Krist (STScI and
collaborators.
• Herbig-Haro Object on left.
• T Tauri stage on right (for a pair of
stars).
Part 2: Evidence of Stellar Winds Disrupting Cloud
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Part 2: Gas clouds harbouring Protostars with Jets
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Part 2: Gas clouds harbouring Protostars with Jets
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Part 2: Condensation Theory
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Stage 3: Solar Nebula is cleared of gas & dust.
 Gas giant planets, protoplanets, &
planetesimals.
Part 2: Condensation Theory
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Stage 4: Planetesimal Ejection
• Gravitational interactions between giant
planets redistribute planetesimals.
– Jupiter & Saturn Oort Cloud.
– Neptune & Uranus  Kuiper Belt

Crash into inner planets -> atmospheres and water.
 Why planets are in centre of larger system with
a ring (Kuiper Belt) & a cloud of smaller objects
in outer parts (Oort Cloud).
Part 2: Condensation Theory
summary
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Stage 4: Planetesimal Ejection
• Due to gravitational pull of Jupiter,
planetesimals between Mars & Jupiter
cannot form a planet. Tidal force too strong
on coalescing planetesimals.
Planetary System Formation: Timeline.
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Nuclear Fusion
summary
Solar System Formation Theory:
summary
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• Part I: Nebular Theory
Planets orbiting in same direction in
a plane.
• Part 2: Condensation Theory
A couple of planets do not have
prograde rotations.
Existence & composition of comets
& asteroids.
Rocky inner planets & gas giant
outer planets.
Formation of the Moon
summary
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• Giant Impact Hypothesis
Needs to expain why moon, compared to Earth, is:
• low in volatiles
• low in iron
• same O isotope ratios as in Earth’s mantle.
Impact Scenario 1
M. Ćuk and S. T. Stewart, Science, 2012.
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Transfer of angular
momentum between
protoEarth,
protoMoon and sun
causes Earth’s spin
to slow to ~24hrs..
• Impactor: ½ size of Mars
• Iron sinks to core of protoEarth
• Luna from Earth mantle debris
summary
Impact Scenario 2
Robin M. Canup, Science, 2012.
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• Impactor: slightly smaller than protoEarth
• Iron sinks to core of protoEarth
• Luna from Earth mantle debris
Formation of the Moon
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• second stage
summary
Review of animations:
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• Forming the debris torus:
– iron stays in proto-Earth core
– impact object’s material mainly
absorbed by proto-Earth
– proto-Earth’s mantle material forms
the debris disk
•  same isotopes as Earth’s mantle &
little iron.
• impact is catastrophic vapourize
volatiles & their v > v_escape.
• silicon vapour solidifies in debris torus
• accretion and subsequent collisions build
up Luna (analogous to planet formation)
Practise Exercise:
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• Form teams.
• Pick a leader.
• Leader attempt to outline the 4 stages
of solar system formation.
• Rest of team confirm or correct the
leader’s outline.