Download - 8. Solar System Origins
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8. Solar System Origins• Chemical composition of the galaxy• The solar nebula• Planetary accretion• Extrasolar planets
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Our Galaxy’s Chemical Composition• Basic physical processes
– “Big Bang” produced hydrogen & helium– Stellar processes produce heavier elements
• Observed abundances– Hydrogen ~71% the mass of the Milky Way– Helium ~27% the mass of the Milky Way– Others ~ 2% the mass of the Milky Way
• Elements as heavy as iron form in stellar interiors• Elements heavier than iron form in stellar deaths
• Implications– A supernova “seeded” Solar System development
• It provided abundant high-mass elements• It provided a strong compression mechanism
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Solar System Chemical Composition
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Coalescence of Planetesimals
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Abundance of the Lighter Elements
Note: The Y-axis uses a logarithmic scale
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The Solar Nebula• Basic observation
– All planets orbit the Sun in the same direction• Extremely unlikely by pure chance
• Basic implication– A slowly-rotating nebula became the Solar System
• Its rate of rotation increased as its diameter decreased• Basic physical process
– Kelvin-Helmholtz contractionGravity Pressure
• As a nebula contracts, it rotates faster– Conservation of angular momentum
Spinning skater• Kinetic energy is converted into heat energy• Accretion of mass increases pressure• Temperature & pressure enough to initiate nuclear fusion
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Conservation of Angular Momentum
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Formation of Any Solar System• Presence of a nebula (gas & dust cloud)
– Typically ~ 1.0 light year in diameter– Typically ~ 99% gas & ~1% dust– Typically ~ 10 kelvins temperature
• A compression mechanism begins contraction– Solar wind from a nearby OB star association– Shock wave from a nearby supernova
• Three prominent forces– Gravity Inversely proportional to d2
• Tends to make the nebula contract & form a star– Pressure Directly proportional to TK
• Tends to make the nebula expand & not form a star– Magnetism Briefly prominent in earliest stages
• Tends to make the nebula expand & not form a star
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More Solar System Formation Stages• Central protostar forms first, then the planets
– H begins fusing into He => Solar wind gets strong– This quickly blows remaining gas & dust away
• Circumstellar disks– Many are observed in our part of the Milky Way
• Overwhelming emphasis on stars like our Sun– Many appear as new stars with disks of gas & dust
• Potentially dominant planets– Jupiter >2.5 the mass of all other planets combined– Many exoplanets are more massive than Jupiter
• Knowledge is limited by present state of technology
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The Birth of a Solar System
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Formation of Planetary Systems
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Planetary Accretion• Basic physical process
– Countless tiny particles in nearly identical orbits– Extremely high probability of collisions
• High energy impacts: Particles move farther apart
• Low energy impacts: Particles stay gravitationally bound
– Smaller particles become bigger particles• ~109 asteroid-size planetesimals form by
accretion• ~102 Moon-size protoplanets form by
accretion• ~101 planet-size objects form by
accretion• Critical factor
– Impacts of larger objects generate more heat• Terrestrial protoplanets are [almost] completely molten• “Chemical” differentiation occurs
– Lowest density materials rise to the surfaceCrust
– Highest density materials sink to the center Core
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Microscopic Electrostatic Accretion
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Condensation Temperature• Basic physical process
– Point source radiant energy flux from varies µ 1/D2
• Ten times the distance
One percent the energy flux– Any distant star is essentially a point source
• The concept applies to all forming & existing stars– At some distance, it is cold enough for solids to form
• This distance is relatively close for rocks– Much closer to the Sun than the planet Mercury
• This distance is relatively far for ices– Slightly closer to the Sun than the planet Jupiter
– This produces two types of planets• High density solid planets
Terrestrial planets• Low density gaseous planets
Jovian planets
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Two Different Formation Processes
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Condensation In the Solar System
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The Center of the Orion Nebula
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Mass Loss By a Young Star In Vela
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Exoplanet Detection Methods
http://www.rssd.esa.int/SA-general/Projects/Staff/perryman/planet-figure.pdf
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Extrasolar Planets: 13 Sept. 2002• Basic facts
– No clear consensus regarding a definition• Usually only objects <13 MassJup & orbiting stars
– Objects > 13 MassJup are considered “brown dwarfs”– Objects < 13 MassJup are considered anomalies
• Orbiting a massive object fusing H into He– A star in its “normal lifetime”
• Summary facts– 88 extrasolar planetary systems– 101 extrasolar planets– 11 multiple–planet systems
• Unusual twist– A few “planetary systems” may be “star spots”
• Magnetic storms comparable to sunspots on our Sun
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Exoplanets Confirmed by 2007• 18 July 2003
– 117 extrasolar planets– 102 extrasolar planetary systems– 13 extrasolar multiple–planet systems
• 4 July 2005– 161 extrasolar planets– 137 extrasolar planetary systems– 18 extrasolar multiple–planet systems
• 19 September 2007– 252 extrasolar planets– 145 extrasolar planetary systems– 26 extrasolar multiple–planet systems
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Extrasolar Planets Encyclopaedia• 27 January 2010
– 429 planets– 363 planetary systems– 45 multiple planet systems
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Extrasolar Planets: Size Distribution
MassJup
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Most Recent Confirmed Exoplanets• 29 January 2013
– 863 extrasolar planets– 678 extrasolar planetary systems– 129 extrasolar multiple–planet systems– 2,233 unconfirmed Kepler candidates
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Exoplanets: 17 September 2013
http://exoplanets.org/
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Exoplanets: Orbital Distribution
http://exoplanets.org/multi_panel.gif
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Exoplanets: Star Iron Content
http://exoplanets.org/fe_bargraph_public.jpg
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Star Gliese 86: Radial Velocity Data
• Doppler shift data reveal an extrasolar planet– An orbital period of ~ 15.8 days– A mass of ~ 5 . MJupiter
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Possible First Exoplanet Photo
http://www.gemini.edu/images/stories/press_release/pr2008-6/fig1.jpg
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• Galactic chemical composition– ~98% hydrogen + helium– ~ 2% all other elements
• Solar System formation– Solar nebula– Compression mechanism– Gravity, pressure & magnetism– Protostar with circumstellar disk
• Planetary accretion– Concept of condensation temperature
• Rock & ices can form• Extrasolar planets
– 863 confirmed– 2,233 Kepler candidates
Important Concepts