Earth has formed in our solar system
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Earth has formed in our solar system
We need to understand planetary formation
Constraints:• Astronomical observations• Study of meteorites• Study of planets• Theoretical modelling
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Planetary orbits are nearly circular
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Inner or terrestrial planetsSmall,dense, slow rotation, few moons
Giant or jovian planetsLarge, low density, fast rotation, numerous satellites
Compositional classes
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Moon Mars
Ancient planetary surfaces are covered with impact craters
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Stellar nebula
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Solar nebula hypothesis
• Proposed by Kant and Laplace• Gravitational collapse of a gaseous cloud
(supernova?)• During collapse, faster spin (conservation
of angular momentum) and temperature increase
• Competing forces induce flattening
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• Temperature depends on location in nebula because of cooling
• Hence location is important for planetary composition (planets closer to the sun should be more refractory)
• Interaction of large body at final stages could change composition considerably
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All stars form from clouds of gas and dust which roam our galaxy. Eventually, gravity causes the cloud to collapse; since the cloud is spinning, material falls in along the "poles" faster than it does near the "equator". This flattening results in a disk-like object.
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Material slowly makes its way into the center of this disk, forming a new star. While the star continues to grow, lumps form (solar nebula is heterogeneous) in the disk which will ultimately become planets.
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The disk eventually thins as more material falls onto the star and the protoplanets. A hole in the disk near the star forms as material is completely incorporated into the star and planets.
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Now fully formed planets exist within the hole, even as new planets are still under construction in the outer parts of the disk.
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• This stage of planetary formation is very violent: impacts of large objects shape the final planets (e.g. formation of the moon)
• There are many collisions and core formation provide enough heat to melt the planet magma ocean
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John Chambers
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Ultimately, the remaining dust clears completely, leaving a fully formed solar system like our own.
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Time frame for the formation of our solar system
Stevenson, Nature, 2008
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Stevenson 2008
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Problems with solar nebula model
• Too little angular momentum in the centre (sun)
• Planets in the wrong place • Deviations from ecliptic plane
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Unknowns
• How dense was the solar nebula• Time scales • (fast homogeneous differentiation)
(slow differentiated bodies)• Temperature• Planetary rearrangement after initial
formation• Volatiles