The structure and formation of
the Solar System
By Mark Richardson
The structure of a solar system
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Star(s): the gravitational centre(s) of the system. Undergoing nuclear fusion.
Planets: Bodies of adequate size orbiting a sun
Dwarf Planets: Small versions of planets – perhaps captured, not formed.
Moons: Bodies orbiting planets and dwarf planets
Asteroids: Smallest bodies in the system made mostly of rock (refractory material)
Comets: Smallest bodies in the system made of significant ice (volatiles)
Origin of the solar system
• In the galaxy there
are gas clouds.
• Inside the cloud there
is:
– Pressure
– Gravity
• These forces are
typically in balance.
The beginning of the beginning
• ~5-6 Billion years ago
this balance got
disturbed!
– Possibly a nearby
supernova etc.
• Gravity won!
• The gas cloud then
collapses – most
towards the centre.
Why a disk?
• Some gas on the
edge of the cloud
feels a gravitational
pull towards the
centre
• Assume the cloud has
some initial spin
Why a disk?
• Written with
components
Why a disk?
• Now as things get closer to
the spin axis, they speed
up:
– Conservation of Angular
Momentum
– Figure Skater
• The faster you do a turn in
a car – the stronger the
force you feel
Why a disk?
• How is that important?
• The component of gravity
acting towards the spin
axis is serving the
purpose of your door in a
turning car
– it’s what’s making you turn
– It is NOT making you move
towards the axis
• Only the other component
will actually move you!
The planetary disk
• More complicated
view:
• Link: Formation Movie
• Eventually most
material cannot move
closer to the forming
sun.
The layout of things
• The original cloud had
some
inhomogeneities.
• These have
accumulated more
mass than the
average bit of the
cloud (the largest of
these is typically at
the centre)
• These form the first
planetesimals!
What’s next … and the controversy
• At this point it is believed that the
planetesimals collide with one another, due
to gravity.
• These eventually become the foundation for
our planets
• Some theories show this process to take
almost the age of the solar system itself (see
the Origin and Evolution of the Solar Sytem – Woolfson) for
planets to form, and that the sun should be
rotating much faster.
Possible Solutions
• The sun may rotate slowly because it lost
angular momentum through magnetic fields
and jets – research is still ongoing.
• Interesting solutions to the planet growth
problem have been suggested, although both
the nature of the problem and these solutions
need to be studied further.
Inner System: Planetismals
• As the protosun heats up it
emits a wind of photons that
push the lightest compounds
in the disk to the outer
region.
• This leaves the inner system
almost free of gas – this is
why the inner planets are
rocky planets.
• Atmospheres subsequently
form by a combination of
outgassing and comet
impacts.
Inner System: Planetismals
Constitution of the planets
• The planets form
through the continuous
accretion of planetoids.
• The impacts of large
planetoids can
completely liquefy the
material.
• This allows denser
materials, such as iron,
to sink to the centre,
explaining the
constitution of material.
Outer System
• The same general process takes place in the outer system, only
there are more gases and ice.
• This allows for significant atmospheres, and larger planets.
• These larger planets are more able to capture smaller objects in
a stable orbit – moons and rings
The Sun
• During this process the
protosun has continued
to accrete more material
• This increases the
pressure on material in
the core of the protosun
which in turn increases
the temperature.
• At ~1 million K the
protosun can begin
Hydrogen burning – it is
now a sun
The big picture
• We have seen a theory explaining the creation of
a star and planets and other materials from an
initially disturbed gas cloud.
• It is the most accepted theory – but not without its
inconsistencies.
• Some last minute problems and most likely
explanations:
Last minute problems
• Venus and Uranus don’t rotate the same direction as
everything else.
• Venus is flipped 180o while Uranus is flipped roughly
90o.
• Everything should be rotating the same if they came
out of the same cloud.
• Only explanation is a large impact made the whole
planet rotate.
• This would only be possible if very early objects of
comparable mass hit each planet.
Leftovers
• All the material in the early solar system is not used up
when planets are formed.
• Left over stuff is still flying around out there – these
asteroids, meteoroids and comets still make impacts.
• The asteroid belt is a large collection of asteroids. A theory,
which is most favoured, is that the proximity to Jupiter tore
apart a planet that was forming there.
• Starting at about Pluto’s orbit, the Kuiper belt extends much
farther out and houses most of the comets in the solar
system.
• Beyond this is a cloud called the Oort cloud. This is the
farthest extent of the solar system. This has very small
objects in it, as well as some larger ones.
Sources and Acknowledgments
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Solar Views: www.solarviews.com, 1997-2007, Copyright Calvin J. Hamilton.
Wikipedia
Astronomy.org: http://www.astronomy.org/astronomy-survival/solform.html, 19962008, Copyright GGary A. Becker
Astronomy Online:
http://astronomyonline.org/SolarSystem/SolarSystemFormation.asp, Copyright
2004-2009
Mad Star: http://www.astro.wisc.edu/~townsend/static.php?ref=diploma-1, 2009
Copyright Rich Townsend
• A special thanks to Ian Short, Mary Lou Whitehorne,
And You!