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 ~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

Solar System Chemical Composition

Coalescence of Planetesimals

Abundance of the Lighter Elements Note: The Y-axis uses a logarithmic scale

The Solar Nebula Basic observation Basic implication 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 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 & form a star Pressure Directly proportional to TK Tends to make the nebula expand & not form a star Magnetism Briefly prominent in earliest stages

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

The Birth of a Solar System

Formation of Planetary Systems

Planetary Accretion Basic physical process Critical factor 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 Crust Highest density materials sink to the center Core

Microscopic Electrostatic Accretion

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

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 http://www.rssd.esa.int/SA-general/Projects/Staff/perryman/planet-figure.pdf

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

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

Extrasolar Planets Encyclopaedia 27 January 2010 429 planets 363 planetary systems 45 multiple planet systems 22

Extrasolar Planets: Size Distribution MassJup

Most Recent Confirmed Exoplanets 29 January 2013 863 extrasolar planets 678 extrasolar planetary systems 129 extrasolar multiple–planet systems 2,233 unconfirmed Kepler candidates

Exoplanets: 17 September 2013 http://exoplanets.org/

Exoplanets: Orbital Distribution http://exoplanets.org/multi_panel.gif

Exoplanets: Star Iron Content http://exoplanets.org/fe_bargraph_public.jpg

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

Possible First Exoplanet Photo http://www.gemini.edu/images/stories/press_release/pr2008-6/fig1.jpg

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