updated Mar 2, 2005, jeb PRELIMINARY
- 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
- Angular Momentum Problem
Any model must explain what we observe:
- Each planet is
relatively isolated
- each planet is about twice as far from Sun as its inner neighbor
- Planetary orbits slightly elliptical -
nearly circular
- All the planets' orbits lie roughly in same plane
- Sun's rotational equator lies nearly in this plane
- Planets orbit the Sun in the same direction as the Sun rotates
- Sun and most planets rotate in same direction
- Tilt of rotation axis with respect to orbit usually small
- Most moons revolve around their parent planets in the same direction as the
planets rotate on their axes
- Planets differ in composition
-
Asteroid belt
- concentration of very old bodies which differ in chemical and geologic properties from planets and moons
- primitive, unevolved material
-
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
-
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.
- planet distances obey the
Titius-Bode "Law"
- descriptive law that has no theoretical justification
- Neptune serious exception
- 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
- 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 help cool nebula and acts as condensation nuclei
- -> explains the observables
Steps in the evolutionary process of the condensation theory
-
Very large (about 100,000 AU) gas cloud fragment 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
- Centrifugal effects
- outer parts of nebula
flatten into a disk
- central part of nebula forms Sun
- planets will eventually form in disk and the Sun is part of the disk
- 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
- 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%
- 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
- Gas and dust particles in
parallel, circular orbits
- small eddies collide at low velocities
- condensation nuclei stick together by gravity and electrostatic forces (accretion)
- 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)
- 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
- Alternative theory -
gravitational instability theory
- outer portions of cloud collapse
- jovian planets would have smaller rocky cores
- Icy planetesimals near Jupiter and Saturn flung out of solar system.
- those near Uranus and Neptune flung to large orbits (
Oort cloud)
- Giant-Planet Migration (Jupiter moves inward)
- 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.
- Proto-sun core gets to about 10 million degrees Kelvin
-
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
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
low rotation rate
- 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
- What is Angular Momentum
- Angular Momentum = Mass x Speed x Distance
- Mass (Sun) = about 1000 x Mass (Planets)
- Ang. Mom.(Sun) = 0.3% of Solar System Ang. Mom. (Jupiter has 60%)
- WHY DOES THE SUN COMPRISE MOST OF THE MASS OF THE SOLAR SYSTEM, BUT LITTLE OF THE ANGULAR MOMENTUM?
- The simple answer:
- Sun is rotating too slowly
- But why?
- Where did the Sun's angular momentum go?
-
Solar Wind blew it away?
- Escaped with matter to interstellar space?
- carried off with ejected planetesimals
- There are candidate explanations, but the correct one is not yet
determined
While conceding that they do not know which explanation is correct, astronomers believe the angular
momentum problem has a solution and the condensation model is basically correct
- 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.
- About
136 planets have been identified
- Note the larger planets are easier to detect, so Earth-sized planets will
not show up (yet)
-
51 Pegasi, example of a detected planetary system
- Alternative discovery method -
extrasolar transit
of the detected extrasolar planets
- Masses comparable to Jupiter (1/3 - 10 x MJUPITER)
- Many orbits are smaller than Jupiter or Saturn (many very close - 0.1 AU - to Star)
- Many orbits are highly eccentric, compared to Jupiter and Saturn
Orbits of extrasolar planets residing beyond 0.15 AU
- Observations are biased toward heavier planets
- larger wobbles and larger effects from transits
- What has been found does not resemble the planets of the Solar System
but so far no sensitivity to light planets
- In the future, improved sensitivity will allow search for Earth-like
planets
- period of cloud contraction is far too long for use to "watch"
- our computer calculations predict it
- we can observe it at various stages of evolution
- cloud contraction
- cloud fragments
- protostars
- protostellar winds
- 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:
- Rapid progress in discovery of extrasolar planets
