Bring:
Density demonstrations - bottles with coins, water, sand
[Pluto-Charon on a stick….]
•
•
•
•
eraser on string
density bottles
spinning table
glass dish w/pepper
The Planets at a Glance
Small
Inner
Rocky
Planets
Misfit
Planets
Giant
Outer
Gas
Planets
Size, Density,
Composition
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Terrestrial (silicate) planets
Venus
Earth
Mars
Mercury
Moon
Io
Ganymede
• Consist mainly of silicates ((Fe,Mg)SiO4) and iron (plus FeS)
• Mercury is iron-rich, perhaps because it lost its mantle during
a giant impact (more on this later)
• Volatile elements (H2O,CO2 etc.) uncommon in the inner
solar system because of the initially hot nebular conditions
• Some volatiles may have been supplied later by comets
• Satellites like Ganymede have similar structures but have an
ice layer on top (volatiles are more common in the outer
Gas and Water Giants
90% H/He
75% H/He
10% H/He
10% H/He
• Jupiter and Saturn consist
mainly of He/H with a rockice core of ~10 Earth
masses
• Their cores grew fast
enough that they captured
the nebular gas before it
was blown off
• Uranus and Neptune are
primarily ices
(CH4,H2O,NH3 etc.)
covered with a thick He/H
atmosphere
• Their cores grew more
slowly and captured less
gas
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Note how they spin!
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Orbital Inclinations
Main Belt
Asteroids
Near
Earth
Objects
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Trojan
Asteroids
Jupiter
Hubble’s Best Pictures
Smoothed and modeled images
Kuiper Belt Object Detection
Digital cameras and computers make this much, much easier…..
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http://cfa-www.harvard.edu/iau/lists/TNOs.html
Uranus
Saturn
The
Kuiper
Belt
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Jupiter
1,342 as of
1 October 2007
Neptune
2003 UB313
Now
calle
d Eris
Xena
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Images 1.5 hrs
apart Oct 21,
2003.
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Eris aka XENA
• 557 year orbit
• a=68 AU
• Diameter ~3000 km
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2003 EL61 & UB313
have moons
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48 Kuiper Belt Objects have
moons.
Why is the presence of a
moon VERY, VERY useful?
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Keeping Neptune fixed and
watching paths of other
planets for millions of years
Pluto now
But orbital calculations show
that Pluto and Neptune’s orbits
are in a 3:2 resonance - they
dance together - but never get
close.
Neptune
fixed
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Three
Swarms
1. Asteroids
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2. Kuiper
Belt Objects
3. Oort
cloud
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Orbit of
Sedna
P=10,000 yr
a=450 AU
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Our solar system
1.
2.
3.
4.
Patterns of motions
2 types of planets
Asteroids and comets
Exceptions
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How does a solar
system form from a
cloud of gas?
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Formation: Sources of Evidence
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Star-forming
regions
Chemistry of
source material
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Our solar system
1. Patterns of motions
2. 2 types of planets
3. Asteroids and
comets
4. Exceptions
Other solar
systems
Similarities and
differences
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As of Oct 6 2007
• 186 systems
• 218 planets
http://www.princeton.
edu/~willman/planetary_systems/
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Collapse of the Solar Nebula
• Formation of the Sun seems a good place to start.
• Theories of star formation are based on observing
millions of stars of different ages.
• Start with a nebula of gas and dust.
– Nebula = noun = "cloud" (plural = nebulae)
– Nebular = adjective = "cloud-like"
Section could have been called
Collapse of Nebular Solar Nebula.
How Big Was Solar Nebula?
• ~1% efficiency (guess)
• start with ~100 Mass of Sun = 1032 kg
• If Temperature of cloud ~1000K
– density ~ 10-12 kg m-3
– R~2,500 AU
•If Temperature of cloud ~10K
– density ~ 10-18 kg m-3
– R~250,000 AU
We are not addressing the how/why of this - take-home
message = initial cloud was LARGE & had low density
What was the composition
of solar nebula?
WHEN did it collapse?
H2O, NH3, CH4
Water, Ammonia, Methane
Hydrogen compounds
Ignore inert gases
He, Ne, Ar
Elements
made in star
explosions super novae
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Interstellar Organic Molecules - lots of them!
C H O N…
Collapse of the Solar Nebula
•
•
•
Spins up
Forms a disk
Heats up
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Conservation of
Angular
Momentum
MVR = Constant
Where did the
angular momentum
come from???
Small random motions averaging out to a
tiny bulk motion
- this bulk motion is then “amplified” (due
to conservation of angular momentum) as
the cloud collapses
Why a Disk?
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As the cloud collapses (due
to gravity) the gases, dust
and stuff orbit the central
mass On the timescale of an orbit
gravity still balances the
centrifugal force - the disk is
not formed by being “flung
out into a disk”. Nor does
gravity “pull the material into
a disk”. These are common
misconceptions.
Go with the flow or crash to oblivion - Extreme
Conformism!
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Explains how
everything ends up
orbiting - and spinningthe same way
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• 98% of material - hydrogen & helium - does not
condense - anywhere - stay as gases.
• Inside frostline only refractory materials condense rocks & metals
• Outside frostline volatiles also condense - WAM AND rocks & metals too.
Refractory =
melts/evaporates at
higher temperatures,
tends to be a solid at
reasonable
temperatures
Volatile =
melts/evaporates at
lower temperatures,
tends to be a gas at
reasonable
temperatures
An Artist’s Impression
The young Sun
solid planetesimals
gas/dust
nebula
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Accretion of Planetesimals
• Many smaller objects collected into just a few
large ones
• The bigger get bigger - Oligarchic growth
Hyperion
•
•
Moon of Saturn
Size: 180 x 140 x
112 km
Typical
planetestimal??
Core material for
giant planet?
http://ciclops.org/view_event.php?id=37
http://saturn.jpl.nasa.gov
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Accretion of Planetesimals
• Many smaller objects collected into just a few
large ones
• REALLY big planetesimals (~20 Mearth)
gravitationally pull in hydrogen - the most
abundant gas - and become GIANT.
The Giant Planets
Hydrogen envelopes over cores
of rock, metals and Water,
Ammonia, Methane
Why Only 2 Types of Planets?
1. Cosmic Abundance of Elements - H, O, N, C
2. Temperature Colder Farther from Sun
 Abundance ices condense beyond frost line
 Snowballs -> bigger snowballs
 Giant snowballs have enough gravity to hold H
- most abundant element - > giant planets
 Small amounts of rock & metal-> terrestrial
planets
 Ice dwarfs, comets, asteroids = leftovers
Dust grains
Timescale Summary
Runaway growth
~Moon-size
(planetesimal)
~0.1 Myr
Orderly growth
~Mars-size
(embryo)
~1 Myr
Late-stage accretion
(Giant impacts. Gas loss?)
~Earth-size
(planet)
~10-100 Myr
• Nebula collapse <1 Million Years - fast!!
• Planetesimal formation < 1MY
• Jupiter, Saturn <2 MY
• Terrestrial Planets <4 MY
• Uranus & Neptune??
•Accretion is SLOW in the outer solar system
Less material
Material orbits the sun slowly
Few collisions, slower accretion
Too slow for Uranus & Neptune to have formed
in their current locations
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3 swarms of small bodies: Asteroid Belt, Kuiper
Belt, Oort Cloud of comets
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Q u ickT im e ﾪ a n d a
d e co m p re s s o r
a re n e e d e d to s e e th is p ic tu re .
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Oldest Meteorite
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Allende - fell to Earth
near Chihuahua,
Mexico at 1:05am on
February 8, 1969.
Age: 4.5 BY old
How do we know it’s that old?
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Fission of atomic nucleus
+ bits
Potassium-40
"mother"
Argon-40
"daughter"
Probability of "splitting up":
Expect half the material to decay in 1.25 billion years
Half-Life = 1.25 billion years
Isotopic decay
Half-Life = 1.25
billion years
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How do we date rocks?!
Measure the ratio:
Mother Isotope
Daughter Isotope
Potassium - 40
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Argon - 40
Half-Life = 1.25
billion years
• Mineral grains (zircon)
• Uranium/Lead (U/Pb)
ratios suggest
• age ~4.4 billion years
• sedimentary rocks in
west-central Australia.
4.4 Billion Years Old
Oldest Earth
Rocks
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Oldest Solar System Rocks
• The oldest dated moon rocks, however, have ages between 4.4 - 4.5
billion years and provide a minimum age for the formation of Moon
• Meteorites - therefore Solar System formed - 4.53 and 4.58 billion
years ago
• http://pubs.usgs.gov/gip/geotime/radiometric.html#table
4.54 BY to <1% accuracy
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Where is everything?
Note logarithmic scales!
V E
Me
Ma
Terrestrial planets
J
S
U
N P
KB
Gas giants Ice giants
1 AU is the mean Sun-Earth distance = 150 million km
Nearest star (Proxima Centauri) is 4.2 LY=265,000 AU
Me
V E Ma
Inner solar system
1.5 AU
Note
log scales!5 A
U
AU
0
3
Outer solar system
Basic data
Sun
ρ
(days)
(1024kg)
Radius
(km)
24.7
2x106
695950
1.41
Distance
(AU)
Porbital
(yrs)
Protation
-
-
Mass
(g cm-3)
Mercury
0.38
0.2458.6
0.33
2437
5.43
Venus
0.72
0.62243.0R
4.87
6052
5.24
Earth
1.00
1.001.00
5.97
6371
5.52
Mars
1.52
1.881.03
0.64
3390
3.93
Jupiter
5.20
11.860.41
1899
71492
1.33
Saturn
9.57
29.600.44
568
60268
0.68
Uranus
19.19
84.060.72R
86.6
24973
1.32
Neptune
30.07
165.90.67
102.4
24764
1.64
Pluto
39.54
248.66.39R
0.013
1152
2.05
See e.g. Lodders and Fegley, Planetary Scientist’s Companion
What does the Solar System consist of?
• The Sun
99.85% of the mass
(78% H, 20% He)
• Nine Eight Planets
• Satellites
• A bunch of other junk (comets,
asteroids, Kuiper Belt Objects etc.)
Three kinds of planets . . .
• Nebular material can be divided into “gas”
(mainly H/He), “ice” (CH4,H2O,NH3 etc.) and
“rock” (including metals)
• Planets tend to be dominated by one of these
three end-members
• Proportions of gas/ice/rock are roughly
100/1/0.1
• The compounds which actually Gas-rich
condense will depend on the local
nebular conditions (temperature)
Rock-rich
• E.g. volatile species will only
be stable beyond a “snow line”.
This is why the inner planets are
Ice-rich
rock-rich and the outer planets
gas- and ice-rich
Solar System Formation - Overview
• Some event (e.g. nearby supernova) triggers gravitational
collapse of a cloud (nebula) of dust and gas
• As the nebula collapses, it forms a spinning disk (due to
conservation of angular momentum)
• The collapse releases gravitational energy, which heats the
centre; this central hot portion forms a star
• The outer, cooler particles suffer repeated collisions,
building planet-sized bodies from dust grains (accretion)
• Young stellar activity (T-Tauri phase) blows off any
remaining gas and leaves an embryonic solar system
• These argument suggest that the planets and the Sun
should all have (more or less) the same composition
• Comets and meteorites are important because they are
relatively pristine remnants of the original nebula
Complications
• 1) Timing of gas loss
– Presence of gas tends to cause planets to spiral inwards, hence
timing of gas loss is important
– Since outer planets can accrete gas if they get large enough, the
relative timescales of planetary growth and gas loss are also
important
• 2) Jupiter formation
– Jupiter is so massive that it significantly perturbs the nearby area
e.g. it scattered so much material from the asteroid belt that a
planet never formed there
– Jupiter scattering is the major source of the most distant bodies
in the solar system (Oort cloud)
– It must have formed early, while the nebular gas was still
present. How?
Observations (1)
• Early stages of solar system formation can be imaged directly –
dust disks have large surface area, radiate effectively in the IR
• Unfortunately, once planets form, the IR signal disappears, so
until very recently we couldn’t detect planets (~150 so far)
• Timescale of clearing of nebula (~1-10 Myr) is known because
young stellar ages are easy to determine from mass/luminosity
relationship.
Thick disk
This is a Hubble image of a young solar
system. You can see the vertical green
plasma jet which is guided by the star’s
magnetic field. The white zones are gas
and dust, being illuminated from inside by
the young star. The dark central zone is
where the dust is so optically thick that the
light is not being transmitted.
Observations (2)
• We can use the
present-day observed
planetary masses
and compositions to
reconstruct how
much mass was there
initially – the
minimum mass solar
nebula
• This gives us a constraint on the initial nebula conditions e.g.
how rapidly did its density fall off with distance?
• The picture gets more complicated if the planets have moved . .
• The observed change in planetary compositions with distance
gives us another clue – silicates and iron close to the Sun,
volatile elements more common further out
Cartoon of Nebular Processes
Disk cools by radiation
Polar jets
Dust grains
•
•
Infalling
material
Nebula disk
(dust/gas)
Cold,
Hot,
low ρ
high
ρ
Stellar magnetic field
(sweeps innermost disk clear,
reduces stellar spin rate)
Scale height increases radially (why?)
Temperatures decrease radially – consequence of lower
irradiation, and lower surface density and optical depth leading
to more efficient cooling
What is the nebular composition?
•
•
Why do we care? It will control what the planets are made of!
How do we know?
–
–
–
•
Composition of the Sun (photosphere)
Primitive meteorites (see below)
(Remote sensing of other solar systems - not yet very useful)
An important result is that the solar photosphere and the primitive meteorites
give very similar answers: this gives us confidence that our estimates of
nebular composition are correct
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Kepler’s 3 Laws of
Planetary Motion
1. Planets move on elliptical orbits with
the Sun at one focus
2. Planets move faster when closer to
the Sun, slower when farther from the
Sun
3. (Orbital Period)2 = (Semi-major axis)3
=
P2
a3
Years
A.U.
Sequence of events
• 1. Nebular disk
formation
• 2. Initial coagulation
(~10km, ~104 yrs)
• 3. Runaway growth (to
Moon size, ~105 yrs)
• 4. Orderly growth (to
Mars size, ~106 yrs),
gas loss (?)
• 5. Late-stage collisions
(~107-8 yrs)
Debris
Swarm
1
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The
Asteroid
Belt
100s of thousands of asteroids
100s Near Earth Objects
100s Trojans ± 60° of Jupiter
http://cfa-www.harvard.edu/iau/Animations/Animations.html
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• Total mass of
asteroids ~mass of
Moon
• "families" of
asteroids have similar
color, spectra & orbits
Note: distance scale is in factors of 10!
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Sizes of objects
not to scale
Leftovers: Three Kinds of Small Bodies
1. Jupiter’s gravity stirs up rocky
planetesimals between Mars & Jupiter:
the Asteroid Belt
2. Jovian planets stir up orbits of icy
planetesimals in their vicinity, flings ‘em
out into the Oort Cloud.
3. Icy planetesimals slowly form from
nebula outside Neptune’s orbit: The
Kuiper Belt
4. All these objects rain down on the
planets: making impact craters &
bringing ‘good stuff’