The Formation of Planetary Systems
The Solar System and Beyond

May 25, 2011, jeb

OUTLINE Search for the Theory of Formation of Solar System The Observables The Condensation Model Catastrophes Search for Extrasolar Planets Is Our Solar System Unusual? Conclusions

Search for the Theory of Formation of Solar System

• Theory must explain observed details
  • details we have been studying all term
• Condensation with Accretion Theory
  • favored by most astronomers
  • evolutionary theory
  • Solar System developed by gradual and natural steps
• Catastrophes
  • some observables must result from special events




The Observables

Any model must explain what we observe:


1. Each planet is relatively isolated
   • each planet is about twice as far from Sun as its inner neighbor (table)
   
   
2. Planetary orbits slightly elliptical - nearly circular
   
   
3. All the planets' orbits lie roughly in same plane
   • Sun's rotational equator lies nearly in this plane
   
   
4. Planets orbit the Sun in the same direction as the Sun rotates
   
   
5. Sun and most planets rotate in same direction
   • Tilt of rotation axis with respect to orbit usually small
     • Venus and Uranus exceptions
   
   
6. Most moons revolve around their parent planets in the same direction as the planets rotate on their axes
   
   
7. Planets differ in composition
   • composition varies roughly with distance from Sun
   • dense, metal-rich planets in the inner part
   • giant, hydrogen-rich planets in the outer part
   
   
8. Asteroid belt
   • concentration of very old bodies which differ in chemical and geologic properties from planets and moons
     • primitive, unevolved material
   
   
9. Kuiper Belt, a collection of asteroid-sized icy bodies orbiting beyond Neptune
   • Pluto and some moons may well be members and former members of this class of objects
   
   
10. Oort cloud of comets
    • icy fragments not contained in ecliptic plane


• Additional facts
  • planets contain about 90% of solar system's angular momentum
  • planet and asteroid rotation rates are similar (5-15 hours) unless tides slow them down.
  • despite their differences the bodies of the solar system seem to form a common family.
    • seem to have originated at the same time
    • few indications exist of later captures from other stars or interstellar space




Planetary Irregularities

• There are many facts which need not be explained by the formation theory
  • came about through the evolutionary history
  • Mercury's 3:2 spin-orbit coupling
  • Venus' runaway greenhouse effect
  • Moon's synchronous rotation
  • emergence of life on Earth
  • apparent absence of life on Mars
  • Kirkwood gaps in the Asteroid Belt
  • Rings and atmosphere of the Jovian planets
  • and many others




The Condensation Model

• Early attempts to explain the origin of this system
  • the nebular theory
    • French philosopher Rene Descartes
    • German philosopher Immanuel Kant
    • French astronomer/mathematician Pierre Simon de Laplace
  • the nebular theory
    • a cloud of gas broke into rings
    • condensed to form planets.
      • an evolutionary theory
      • with a series of gradual & natural steps
  • doubts about the stability of such rings
  • scientists then considered catastrophic theories
    • accidental or unlikely celestial events such as close encounter of the sun with a star? Such encounters are quite rare, and the hot, tidally disrupted gases would dissipate rather than condense to planets
    
    
• So - > is there a way to fix the nebular theory?
  • condensation theory - based on evolutionary processes
    • the preferred scientific model for the formation of the Solar System
    • add effects of interstellar dust to the nebular theory
    • dust helps cool nebula and acts as condensation nuclei
    • -> explains the observables




Steps in the evolutionary process of the condensation theory


1. Gas cloud with dust collapses.
   • Collapse initiated by external force (another passing cloud, or exploding star)
   • As it collapses its slight rotation increases
     • conservation of angular momentum
   • Note - dust is a key element
     • speeds cooling, allowing collapse
     • nuclei for condensation, growing into planetismals
   
   
2. Centrifugal effects
   • outer parts of nebula flatten into a disk
   • central part of nebula heats up and forms Sun
   • planets will eventually form in disk and the Sun is part of the disk
   
   
3. Gas molecules and dust grains in circular orbits
   • those on noncircular orbits collide with particles and eventually dampen noncircular motion
   • large scale motion is parallel, circular orbits
   
   
4. Collapsing gas and dust heats through collisions
   • around 3000 K so everything in gaseous form
   • Hydrogen (about 90%) and Helium (about 10%) make up most of nebula
   • silicates and Iron compounds about 1%
   
   
5. Nebula cools
   • outer parts cooling off more than inner parts (that are close to hot proto-sun).
   • metal stuff can condense (freeze) at high temperatures while volatile stuff condenses at lower temps
   • local temperature and density depends on distance from proto-sun.
   • at Jupiter temperature cool enough to freeze water
   • further out ammonia and methane freezing out
   • chondrules of material with highest freezing temp form and become incorporated in material of lower freezing temperature
   • planets will also differentiate later on:
     • heavy metals in core - lighter near surface
   
   
6. Gas and dust particles in parallel, circular orbits
   • small eddies collide at low velocities
   • stick together by gravity and electrostatic forces
   • coalescing particles form bodies rotating in same direction as revolution with similar rotation rates (planetesimals)
   • gravity tends to divide nebula into ring-shaped zones (later form planets)
   
   
7. Massive planetesimals pull in nearby nebula (core-accretion theory)
   • some can form mini-solar nebulae to form moons
   • Jupiter and Saturn have a lot of water ice mass
     • -> sweep up a lot of Hydrogen and Helium
   • Uranus and Neptune less so
   
   
8. Icy planetesimals interact over period of hundreds of millions of years with Jupiter, Saturn, Uranus, and Neptune
   • many eventually flung out of solar system
   • some to large orbits in the Oort cloud
   
   
9. Early Sun has magnetic field and spews out ions
   • ions dragged along by rotating magnetic field
   • dragging ions brake the Sun
   • also accretion disks like solar nebula
     • tend to transfer angular momentum outward.
   
   
10. Proto-sun core gets to about 10 million degrees Kelvin
    • starts fusion - Sun turns on
    • T-Tauri winds sweep out rest of nebula that was not already incorporated into the planets.




Interplanetary Debris

• Schematic Time-line of the Solar System Formation
• Asteroid Belt
  • inner planetesimals that escaped capture were boosted beyond Mars
  • Jupiter's huge gravitational field keeps them from coalescing.
• Comets and the Kuiper Belt
  • Planetesimal Ejection
    • After giant planets formed, they "kicked" planetesimals to very large radii
    • Many, perhaps as many as 15%, of the KBOs are in 3:2 resonance with Neptune
      • like Pluto
      • plutinos




Catastrophes

The Condensation Theory is not able to explain all of the observed features of the Solar System, only the major ones

Remaining detailed features are explained by collisions ("catastrophes") late in the Solar System's formation



• Mercury's large nickel-iron core
  • collision of partially differentiated planets
    • cores merge and mantles blown off
  
  
• Venus slow, retrograde rotation rate
  • two large bodies merged
  
  
• Earth-Moon system
  • collision of Mars-sized object with young Earth
  
  
• North-South asymmetry of Mars
  • collision of large planetesimal
  
  
• Tilted rotation axis of Uranus
  • grazing collision with planetismal or merger of two smaller planets
  
  
• Uranus' moon Miranda - nearly destroyed
  • collision with planetismal
  
  
• Retrograde orbit of Triton and elongated orbit of Nereid (Neptune's moons)
  • interactions of Neptune with planetismals
    • may also explain Pluto-Charon-Hydra-Nix system



Search for Extrasolar Planets

Hundreds of planetary systems beyond the Solar System have now been discovered
• Existence of planets can be detected by wobble in star
• Numerous nearby stars have been examined carefully for evidence of planets
  • This search covers many Sun-like stars within a few hundred light-years
  • About 5% of these stars are found to show evidence for planets.
  • More than 400 planets have been identified
  • Note the larger planets are easier to detect, so Earth-sized planets are hard to spot
• 51 Pegasi, example of a detected planetary system
  
  
• Alternative discovery method - extrasolar transit

Planetary Properties of the detected extrasolar planets


1. Many masses comparable to Jupiter (1/3 - 10 x M_JUPITER)
   • Only a few of smaller mass
2. Many orbits are smaller than Jupiter or Saturn (many very close - 0.1 AU - to Star)
   • hot Jupiters
3. Many orbits are highly eccentric compared to Jupiter and Saturn


Orbits of extrasolar planets residing beyond 0.15 AU

European CoRoT

(for Convection, Rotation, and planetary Transits)
launched in December 2006
designed to study 120,000 Sun-like stars
In 2009, discovery of the first Earth-like planet was announced
• but it turns out not to be so "Earth-like"
  • Rocky, yes
  • But about 5 times as massive as the Earth,
  • 60 times closer to its star than Earth to Sun,
  • Surface temperature greater than 4000 Fahrenheit,
  • And likely a volcanic wasteland

NASA's Kepler Mission

launched in March 2009
studies similar number of stars as CoRoT

• also employs planetary transits
6-year lifetime expected
Potential to detect hundreds of planetary transits

possibly including dozens of Earth-sized bodies
January 2011 - Kepler's first rocky planet - Kepler10b
Kepler discoveries
Kepler11f





Extrasolar Planets Encyclopedia




Is Our Solar System Unusual?

• Observations are biased toward heavier planets
  • larger wobbles and larger effects from transits
• Most of the planets found so far do not resemble the planets of the Solar System
  but so far little sensitivity to light planets
• In the future the sensitivity in searching for Earth-like planets will improve
• Habitable zones
  • a key requirement for life is likely the existence of liquid water
  • this means the surface must be roughly in the range 0-100 degrees Centigrade
  • the surface temperature depends on the distance of a planet from its parent star and the intrinsic brightness of the star
  • Habitable Zones



Conclusions

• Favored Theory of Formation of Solar System
  • Condensation theory
    • advanced nebular theory
    • key new ingredient: interstellar dust
    • evolutionary theory
      • Solar System developed by gradual and natural steps
  • Condensation theory explains many observed details of the Solar System
    • details we have been studying all term
  • Some observables must have resulted from special events:
    • catastrophes
  • Rapid progress in discovery of extrasolar planets

Other Sources on The Origin of the Solar System

• StarDate Guide
• The Nine Planets
• Other Planetary Systems