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Solar System Formation
And the Stuff that was Left Over
Things we need to explain
• The similarities in the motions and orbits of
the objects in the solar system
• Dichotomy in planetary morphology and
• Asteroids and comets
• Peculiarities
Orbital Patterns
• All planets orbit the
sun in the same
• Orbits nearly circular
and in same plane
• Nearly all rotate the
same direction as sun
• Satellites have
properties similar to
their host planets
Morphology Dichotomy
Terrestrial Planets
Jovian Planets
• Located in inner solar
• Small and dense
• Rocks and metals
• Few satellites
• No rings
• Located in outer solar
• Large and low density
• Lots of H compounds
• Lots of satellites
• Rings
• Uranus rotates on its side
• Venus rotates opposite to
its orbital motion
• Earth has one of the
largest satellites in inner
solar system
– Other inner planets don’t
have satellites or very small
What theory can explain all
The Nebular Theory
Orbital Motions
• Heating – because of conservation of energy,
as nebula contracts, it heats up
• Spinning – because of conservation of angular
momentum, as nebula contracts, it spins faster
and faster
• Flattening – due to collisions
Making Planets
• Start with some seed
• Accretion – the process of
more and more material
being added to the seed
• Gravity attracts more stuff
– Inside frost line: all heavy
– Outside frost line: Hydrogen
compounds can condense
Clearing out the Nebula
• Young sun lights up and starts pouring out
radiation and particles
• Radiation and solar wind blow away the
rest of the H and He
• Clearing out the gas stops the planet from
accreting more material
• Planets are now stuck with what they’ve
When did the Solar System Form?
• Radiometric dating shows
us the solar system is
about 4.5 billion years old
• Look at ratios of
radioactive isotopes
• Half life – the length of
time required for half of
the material to decay
Asteroids, Meteors, and
Oh My!
What’s the Difference?
• Asteroid
– Rocky leftover planetesimals that never made it into a
• Meteor
– Flash of light in Earth’s atmosphere
– (The weather person is a meteorologist)
• Comet
– Similar to asteroids but made of ice
– Where were they formed
• Dwarf Planet
– Big enough to be round
– Not big enough to clear its orbit
• Small rocky
• Not big enough to
have enough gravity
to make them round
• Craters show us that
they have histories
similar to planets
Asteroid Belt
• Belt between Mars
and Jupiter where
most asteroids live
• Why is it there?
Asteroid Belt
• Orbital Resonances
with Jupiter
– Creates Kirkwood
gaps in belt
– Prevented asteroids
from ever coming
together and forming a
• Meteor – flash of light caused by
something entering Earth’s atmosphere
• Meteoroid – the object in Earth’s
• Meteorite – such objects once they have
reached the ground
• Primitive Meteorites
– Unchanged since the
formation of the solar
• Processed Meteorites
– From the core of a
shattered asteroid
– Often mostly Fe
• Nucleus
– Main chunk of ice from
which the comet is
• Coma
– Cloud of gas around
nucleus caused by
sun’s heating of the
A Tale of Two Tails
• Plasma Tail
– UV light from sun
ionizes gas in coma
– Solar wind pushes
ions away from sun
• Dust Tail
– Dust-sized particles
unaffected by solar
– Pushed away by
radiation pressure
• Oort Cloud
– Goes out to 50,000 AU
– Contains trillions of
• Kuiper Belt
– 30-50 AU
• Heavy bombardment
• Comets get flung way
out by big things like
Kuiper Belt Objects
• Ice binds comet
• When all the ice is
melted away, the
comet might
Tunguska Video
• Direct Detection – Images of the planet
• Indirect Detection – precise
measurements of the host star that tell us
a planet must be orbiting it
Direct Detection
• Extremely difficult
– Star is usually too
– Need outstanding
angular resolution
• Fomalhaut and its
planet seen by HST
Indirect Methods
Gravitational Wobble:
Astrometric Technique
Doppler Technique
• Planet travels in front of star, blocking
some of stars light, which we can measure
• Movie