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
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 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
• 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 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
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
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