Download Astronomy of the Solar System

10/5/2011
Origins and Formation of the Solar System
Theory of Formation
• What must this theory explain?
– Motions of solar system objects
– Division between small, rocky terrestrial planets
and massive, gas-giant Jovian planets
– Presence of asteroid belt, Kuiper Belt, Oort cloud,
etc.
– Exceptions to the above (Earth’s Moon, tilt of
Uranus, etc.)
• Theory shouldn’t be specific to our solar
system.
Our Solar System
Our Solar System
• Sun
• Planets
– Moons
– Dwarf Planets
• Asteroids
• Comets
The Early Universe
• Big Bang
– ~92% Hydrogen
– ~8% Helium
– Very little of anything else
• First galaxies and stars form 150 million
years after big bang.
– Massive stars produce heavy elements
– Supernovae seed heavy elements into ISM
• New stars form with greater abundances of
heavy elements. (Sun is Population III)
Different Theories
• Nebular Theory: Solar system formed
through the gravitational collapse of a large
cloud of gas.
– Immanuel Kant, 1755 and Pierre-Simon Laplace,
1795
• Close Encounter Hypothesis: Planets are
debris from a close encounter between the
Sun and another star.
– Doesn’t account for orbital motions or
Terrestrial/Jovian division
Nebular Theory
Nebular Theory
Proplyds in the Orion Nebula
Collapse
• Large, irregular cloud of gas
– Initially stable
• Outside stimulus starts collapse
– Collision between clouds
– Shock from nearby supernova
– Etc.
• Begins runaway gravitational collapse
– Gravity pulls the same in all directions
Heating, Spinning, Flattening
• Solar nebula starts as a large, diffuse cloud
• Ends up as a small, flat, spinning disk
– How?
• Heating
– Collisions between inwardly falling particles
convert kinetic energy to random motions (heat).
• Spinning
– Conservation of angular momentum
• Flattening
– Collisions between particles flatten the disk.
Testing the Model
• Our model should work for other solar
systems too, so we should search for
evidence of heating, spinning and flattening
elsewhere.
• Protostellar Disks and Herbig-Haro
Objects
• Computer simulation
• Accretion Disks
Protostellar Disks
Herbig-Haro Objects
Computer Simulation
Formation of Planets
• What do we have now?
– Hot, spinning disk, most matter at center (Sun)
– Protoplanetary Disk
• Composition of protoplanetary disk most
likely homogeneous
– Why the division between terrestrial and jovian
planets?
• Gravity too weak to form planets
• Temperature is the key.
Condensation
• Condensation: the process by which solid
or liquid particles form in a gas
• Inner regions of the protoplanetary disk are
warmer
• Protoplanetary Disk is made of four different
materials
– Hydrogen and Helium gas (98%)
– Hydrogen compounds (e.g. water, methane,
ammonia) (1.4%)
– Minerals and metals (0.6%)
Condensation
• Different materials condense at different
temperatures
– Hydrogen and Helium: never condense in interstellar space
– Hydrogen compounds: begin to condense around
150 K
– Minerals and Metals: begin to condense around
500 K – 1500 K
• Heavier materials condensed out of the disk
nearer to the Sun, lighter materials could
only condense outside the frost line.
Frost Line
• Inside the frost line, only rocky and metallic
materials can condense.
– Mostly metals near Mercury’s orbit
– Metals and minerals between the orbits of Venus
and Mars
– Water and carbon-rich materials form near the
asteroid belt (between Mars and Jupiter)
– Hydrogen compounds outside the frost line
(between Mars and Jupiter)
• More hydrogen compounds than rocky
materials
Accretion
• How did these small seeds that condensed
out of the protoplanetary disk grow into
planets?
– Accretion
• Seeds are on nearly circular orbits
– Collisions are gentle
– Particles stick together through electrostatic
forces
• Seeds grow into Planetesimals
– Pieces of planets
Accretion
• Planetesimals grow rapidly at first
– Hundreds of kilometers in a few million years
• Larger planetesimals have gravitational
effect on smaller particles and each other
– Create crossing orbits
– Increases collisions
• Collisions between planetesimals are
destructive more often than not
– Only the largest avoid being shattered and
continue to grow
Evidence for Accretion
• Meteorites
– Rocky with embedded metallic grains
– Meteorites from asteroid belt contain more
carbon and water-containing minerals.
Jovian Planets
• Condensation proceeded the same as in the
inner regions of the solar system
– Ices (condensed hydrogen compounds) mean
more material is available
• Larger planetesimals have stronger gravity
– Pull in gases that terrestrial planets cannot
Halting Accretion
• What stopped the formation of the planets?
• Solar Wind
– Most of the material in the solar nebula never
became part of any planet
• Gas-giants stopped flow of gas into the inner
solar system
• Eventually all material has either been
accreted or removed from the solar system.
Halting Accretion
Asteroids and Comets
• How did asteroids and comets form?
– Location tells the story
• Asteroids
– Located mainly between Jupiter and Mars
– No stable orbits
• Comets
– Spend most of their time in the outer solar
system
– Icy
– Highly elliptical orbits
Age of the Solar System
• The Solar Nebula Theory describes how the
solar system formed.
• Radiometric dating can describe when the
solar system formed.
– Allows us to tell when solid objects last solidified.
• Some atoms are known as radioactive
isotopes.
– Can change into other elements through
radioactive decay
Age of the Solar System
• Radioactive isotopes have a characteristic
half-life
– Half-life is the amount of time it takes one half of
some radioactive material to undergo radioactive
decay
• By measuring the ratios of radioactive
isotopes and their products, we can roughly
determine the age of an object.
– Must make some assumptions.
• Moon rocks are about 4.44 billion years old
– Oldest meteorites are about 4.5 billion years old
Overview
•
•
•
•
Solar nebula collapsed under its own gravity
Heating, spinning, flattening
Sun forms at center of disk
Heavier materials condense out of
protoplanetary disk, lighter materials outside
of the frost line only
• Planetesimals form and begin to accrete
materials
• Accretion stops when there is no more
material
Next Time…
The Sun
Quiz 4
1)
The shape of the Sun’s spectrum is roughly described by a
a) Blackbody spectrum
b) Absorption spectrum
c) Emission spectrum
d) Radiative spectrum
2) What is do we call the bending of light rays in matter due to
the change in the speed of light?
a) Reflection
b) Refraction
c) Absorption
d) Diffraction