Transcript originofsolarsystem

The Origin of the Solar
System
In the beginning, we started out looking like this, just
a huge cloud of gas in space….
Solar Nebula Theory
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A rotating cloud of gas
contracts and
flattens….
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to form a thin disk of
gas and dust around
the forming sun at the
center.
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Planets grow from gas
and dust in the disk
and are left behind
when the disk clears.
Dust Disks Around Stars
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Very cold, low density disks observed (in the infrared)
around stars.
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Debris left over from comets or collisions between small
bodies (like asteroids).
Evidence of planetary systems which have already formed.
Dust Disks Around Stars
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Very cold, low density disks observed (infrared) around
stars.
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Debris left over from comets or collisions between small
bodies (like asteroids).
Evidence of planetary systems which have already formed.
Dense disks of gas and dust observed (visible & radio)
orbiting young stars.
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Stellar systems are too young for planets to have formed yet.
Probable sites of ongoing planetary formation.
Examples
of the Dust
Disks
around
stars
First Important
step in Planet
formation
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Planet Building:
the Condensation of Solids
Different materials condense from the gas cloud onto
grains of elements (atoms of different gasses) at different
temperatures.
The temperature due to the Sun varied with distance, so
different materials condensed at different distances from
the Sun.
Close to the Sun (1200-1500K): metal oxides and pure
metals.
Farther out (~700-1200K): silicates and rocky material.
Outer regions (~50-200K): ices (water, methane &
ammonia).
Planet Building:
the Formation of Planetesimals
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Planetesimals – small bodies on the order of kilometers
in size.
Condensation – atoms of gas hit dust grains and stick,
adding mass to the particle.
Accretion – solid particles collide and stick to one
another.
Once particles were massive enough, the settled down
into a disk rotating around the protosun (its not quite a
star yet).
Second Important
step in Planet
formation
Accretion Taking Place
Planet Building:
the Growth of Protoplanets
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As planetesimals grew, they became more massive, and
therefore had stronger gravitational fields.
At a certain point, they were able to gravitationally hold
an atmosphere.
Planet Building
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Planetesimals
contain both rock
and metal.
A planet grows
slowly from the
uniform
particles.
The resulting
planet is of
uniform
composition.
Heat from
radioactive decay
causes
differentiation.
The resulting
planet has a
metal core and
low-density
crust.
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The first
planetesimals
contain mostly
metals.
Later the
planetesimals
contain mostly
rock.
A rock mantle
forms around the
iron core.
Heat from rapid
formation can
melt the planet.
The resulting
planet has a
metal core and
low-density
crust.
Planet-building processes
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Dust grains stick together  planetesimals
Planetesimals stick together  protoplanets
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Terrestrial:
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metallic / rocky
but small – not much material
Jovian:
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LOTS OF ICES, so quickly grew more massive
When ~15 x Earth’s mass, gravity strong enough to attract lots of H/He
from solar nebula
got really really big – but not dense
The planets eventually formed and
differentiated into: Terrestrial vs.
Jovian Planets
Terrestrial Planets
Jovian Planets
Small size, low mass
Large and massive
Dense, rocky solid surfaces
Low density, huge gaseous
atmospheres
Heavy gas atmospheres (N2, O2, CO2) Lighter elements, H and He
Slow rotators
Faster rotators, differential rotation
Few satellites (3)
Many moons (over 60)
Close to the Sun (within 1.5 AU)
Farther away (from 5.2 to 30 AU)
No ring system
Planetary rings
Four stages of terrestrial
planetary development
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1. Differentiation
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early planet was molten
heavy elements sunk, light elements rose
On Earth:
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Dense metal core
Less dense rocky mantle
Low-density rocky crust
(outgassing made primitive
atmosphere – more on that later)
Four stages of
terrestrial planetary
development
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2. Cratering
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“heavy bombardment” period (first 0.5 billion years)
many impacts with rogue planetesimals
craters made (some huge)
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many craters later covered by ocean or erased by erosion)
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Four stages of
terrestrial planetary
development
3. Flooding
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lava from below
rain from atmosphere
On Earth:
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made oceans
Four stages of
terrestrial planetary
development
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4. Slow surface evolution
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On Earth:
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wind / water erosion
plate tectonics: moving sections of crust
Clearing of solar nebula
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Sun pushed away remaining debris
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radiation pressure (light)
solar wind (particles)
Planets
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swept up debris (craters)
ejected debris
Clearing the Solar Nebula
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Around 4.6 billion years ago, the cloud of gas (the solar
nebula) vanished due to four effects:
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Radiation Pressure – light from the Sun exerted pressure on
the particles, pushing them out of the solar system.
The Solar Wind – a flow of atoms from the Sun’s upper
atmosphere also helped push particles out of the solar system.
As planets moved through their orbits, they swept up any
material in their paths.
Gravitational effects due to massive planets ejected particles
out of the solar system.
Stellar Debris
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Asteroids – rocky objects, mostly found between Mars
and Jupiter (in the Astreroid Belt ~ 2.8 AU).
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Comets – small icy bodies (dirty snowballs).
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Range in size up to 100 km in diameter.
Irregularly shaped, and cratered.
Remnants of planet formation.
Large elliptical orbits can bring comets in close to the Sun.
Recent studies suggest they are at least 50% rock and dust.
Meteoroids – specks of dust and rock which encounter
Earth’s atmosphere and either burn up or fall to the
ground. (Most only about 1g in mass).
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Meteors – Flash across the sky as the meteoroid burns up.
Meteorite – remnant of a meteoroid that reaches the ground.
A Comet
Up close and personal
with an asteroid
Stellar Motions Due to Planets
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Technically, planets don’t orbit around a star, but around
the common center of mass.
If planets are massive enough, the center of mass is not
located at the center of the star, and the star orbits
around this point as well.
This motion can be detected through Doppler shifts in
the star’s spectrum.
Using Radioactive Dating, We’ve
Discovered:
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Approximately the same
age:
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Earth rocks
Moon rocks
Martian meteorites
asteroidal meteorites
~ 4.6 billion years
Determined by radioactive
dating:
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compare original amount of
radioactive element with an
amount present now
“half-life”: time it takes for ½ of radioact.
elem. to decay into non-radioact. elem.
Explaining the Solar System
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Terrestrial: small, dense, low mass
Jovian: large, low density, high mass
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Terrestrial: heavy gas atmospheres
Jovian: lighter elements
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Jovian planets can gravitationally hold onto lighter gas
Terrestrial: few satellites, no ring system
Jovian: many satellites, planetary rings
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Condensation sequence and accretion
Jovian planets gravitationally stronger
Existence of comets and asteroids
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Leftover material from the formation of the solar system.
Evidence of Extrasolar Planets
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Two methods which suggest the existence of extrasolar
planets:
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Detection of dust which accompanies planets around stars.
Detection of stellar motions due to the presence of orbiting
planets.
Known Extrasolar Planets
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Most known extrasolar planets are high-mass and lowperiod planets. (Selection effect)
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High-mass: the greater the mass, the greater the wobble
produced in the star’s motion.
Low-period: the lower the period, the shorter the period over
which the wobble occurs.
How can high-mass, low-period planets form?
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In dense disks, friction may slow the planet’s down, causing
them to spiral inward.