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

– 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

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

• Tends to make the nebula expand & not form a star

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

– 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

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

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

• 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