Nebula  Hypothesis:

q  Emanuael Swedenborg (1734); Kant (1754); Laplace (1796)

q Laplace:

o   Mathematical physicist (Laplace equation, Laplace transform)

o   Modern theory of probability (Today called Bayesian Statistics)

o   Postulated the existence of black holes

o   Stability ofo solar system

o   Shape of the Earth

o   Collapsing cloud. Conservation of angular momentum leads to the formation of a  gaseous disk; disk fragments to form planets

o   Laplace went in state to Napoleon to accept a copy of his work, and the following account of the interview is well authenticated, and so characteristic of all the parties concerned that I quote it in full. Someone had told Napoleon that the book contained no mention of the name of God; Napoleon, who was fond of putting embarrassing questions, received it with the remark, 'M. Laplace, they tell me you have written this large book on the system of the universe, and have never even mentioned its Creator.' Laplace, who, though the most supple of politicians, was as stiff as a martyr on every point of his philosophy, drew himself up and answered bluntly, Je n'avais pas besoin de cette hypothŹse-lą. ("I had no need of that hypothesis.") Napoleon, greatly amused, told this reply to Lagrange, who exclaimed, Ah! c'est une belle hypothŹse; ća explique beaucoup de choses. ("Ah, it is a fine hypothesis; it explains many things.")

q Safranov, Wetherill

Solar Nebula Disk Model

q Molecular clouds core collapse under their own self-gravity

q Collapsing core produce proto-stars with accretion disk

o   Large number of proto-stars seen in Molecular clouds

o   Accretion disks observed

o   Angular momentum transport: key physics

q Dust in disk settles to the plane.  Agglomerates to form kilometer size planetesimals (area of active research)

q Planetismals merge to form rocky planet cores

o   “runaway” growth: oligarchic formation

q Outer planets accrete gas to form Jupiters and Saturns

o   “Critical mass” of 10 Earth masses (seen in Kepler?)

q Pre-1990s predictions: no close-in Jupiters

q Planetary migration:

o   “Hot Jupiters” did not form in-situ (temperatures too high for grain condensation to form cores)

o   Planet migration rates are estimated to be very rapid.  Some estimates suggest that most planets that formed migrated inwards and were swallowed by their host star

q Subsequent dynamical evolution of solar system: planet migration and ejection

q Direct formation of Jupiters through disk instability

o   Chemistry of Jupiter differs from Sun

q Difficult problem: planet formation, stellar and disk evolution, and migration are all happening on the same time-scale

Open questions:

q Do all stars form planets?

q Are there different mechanisms for rocky and gaseous planets?

q Do the observed planetary properties represent initial conditions or dynamical evolution?

Early History of Earth

q Earth grows by collisions. Early Earth was hot.

q             MkT = GMM/R  -> Earth’s initial temperature was high

q Molten Earth cooled and condensed--- heavier material (Iron) settled to core; Lighter material (silicates) in upper mangle and crust

q Impact hypothesis of the Moon (collision with Mars size planet).  Sped up Earth’s rotation and stripped early atmosphere

q Origin of water:

o   Volcanic steam

o   Comets? D/H ratio

q Snow-line (4-5 Aus)

o   Objects that formed beyond the snow line should have composition similar to comets

o   Meter-sized planetismals  could be the source of water (Carbonaceous chondrites have similar D/H ratios to the Earth)

q Amount of water on planet might depend on its formation history.  Because of the role of large late collisions, planets at the same distance from the Sun may have different properties.

q Plate tectonics:

o   Need for formation of land

o   Source of co2

o   Requires sufficient mass and water to lubricate plate motion

o   Venus has Earth-like mass but the lack of water is thought to explain the lack of plate tectonics.

o   Can a tidally locked planet have plate tectonics?