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8. Solar System Origins • Chemical composition of the galaxy • The solar nebula • Planetary accretion • Extrasolar planets Our Galaxy’s Chemical Composition • Basic physical processes – “Big Bang” produced hydrogen & helium – Stellar processes produce heavier elements • Observed abundances – Hydrogen – Helium – Others ~71% the mass of the Milky Way ~27% the mass of the Milky Way ~ 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 Solar System Chemical Composition Coalescence of Planetesimals Abundance of the Lighter Elements 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 Note: The Y-axis uses a logarithmic scale – Kelvin-Helmholtz contraction Gravity ⇔ 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 Conservation of Angular Momentum 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 & – Pressure Directly form a star 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 More Solar System Formation Stages The Birth of a Solar System • 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 Formation of Planetary Systems 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 surface – Highest density materials sink to the center Crust Core Microscopic Electrostatic Accretion Condensation Temperature • Basic physical process – Point source radiant energy flux from varies ∝ 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 • Low density gaseous planets Terrestrial planets Jovian planets Two Different Formation Processes Condensation In the Solar System The Center of the Orion Nebula Mass Loss By a Young Star In Vela Exoplanet Detection Methods 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” http://www.rssd.esa.int/SA-general/Projects/Staff/perryman/planet-figure.pdf Exoplanets Confirmed by 2007 • 18 July 2003 • Magnetic storms comparable to sunspots on our Sun Extrasolar Planets Encyclopaedia • 27 January 2010 – 117 extrasolar planets – 102 extrasolar planetary systems – 13 extrasolar multiple–planet systems – 429 planets – 363 planetary systems – 45 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 Extrasolar Planets: Size Distribution Most Recent Confirmed Exoplanets • 29 January 2013 – 863 extrasolar planets – 678 extrasolar planetary systems – 129 extrasolar multiple–planet systems – 2,233 unconfirmed Kepler candidates MassJup Exoplanets: 17 September 2013 Exoplanets: Orbital Distribution http://exoplanets.org/ http://exoplanets.org/multi_panel.gif Exoplanets: Star Iron Content Star Gliese 86: Radial Velocity Data • Doppler shift data reveal an extrasolar planet – An orbital period of ~ 15.8 days – A http://exoplanets.org/fe_bargraph_public.jpg Possible First Exoplanet Photo mass of ~ 5 . MJupiter Important Concepts • 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 http://www.gemini.edu/images/stories/press_release/pr2008-6/fig1.jpg
