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Home , Accretion (astrophysics), Nebula

Origin of the

Our theory must explain the data 1. Large bodies in the Solar System have Lecture 7 orderly motions. Formation of the Solar System 2. There are two types of . – small, rocky terrestrial planets Reading: Chapter 9 – large, -rich Jovian planets •Quiz#2 Today: Lecture 60 minutes, 3. & exist in certain regions then quiz 20 minutes. of the Solar System •Homework#1 will be returned on 4. There are exceptions to these patterns. Thursday.

Origin of the Solar System The Solar

• The nebular theory holds that our Nebular Theory Solar System formed out of a nebula which collapsed under its Our Solar System formed from a giant, swirling own . cloud of & . • observational evidence – We observe in the process of Depends on two principles of Physics: forming today. – The are always found within interstellar clouds of gas. •Newton’s Law of Gravity

gravitaional potential energy ! heat newly born stars in the Nebula •Conservation of angular Solar Nebula rotational motion is conserved The cloud of gas from which our own Solar System formed : A Scenario 1. The solar nebular was initially somewhat spherical and a • angular momentum – the momentum involved few light years in diameter. in spinning /circling = mass x velocity x radius – very cold • torque – anything that can cause a change in an – rotating slightly object’s angular momentum (twisting force) 2. It was given a “push” by some event. – perhaps the shock wave from a nearby 3. As the nebula shrank, gravity increased, causing collapse. 4. As the nebula “falls” inward, gravitational potential energy is converted to heat. – Conservation of Energy 5. As the nebula’s radius decreases, it rotates faster – Conservation of Angular Momentum

Conservation of Angular Momentum Flattening of the Solar Nebula • As the nebula collapses, clumps of gas collide & merge. • In the absence of • Their random velocities average out into the nebula’s direction a net torque, the of rotation. => Orderly motion total angular momentum of a • The spinning nebula assumes the shape of a disk. system remains constant. Collapse of the Solar Nebula Orderly Motions in the Solar System 1. As the • The formed in the very center of the nebula. – temperature & density were high enough for reactions to nebula begin • The planets formed in the rest of the disk. collapses, • This would explain the following: it heats up, – all planets lie along one plane (in the disk) – all planets orbit in one direction (the spin direction of the disk) spins faster, – the Sun rotates in the same direction – the planets would tend to rotate in this same direction and flattens. – most orbit in this direction – most planetary orbits are near circular (collisions in the disk)

More Support for the Nebular Theory Building the Planets I: Condensation • We have observed disks around other stars. Condensation – elements & compounds began • These could be new planetary systems in formation. to condense (i.e. solidify) out of the nebula…. depending on temperature! " Pictoris

AB Aurigae Colder Building the Planets II: Frost Line Building the Planets III: Accretion So only rocks & metals condensed within 3.5 AU of the Sun… the so-called frost line. Accretion -- small grains stick to one another via Hydrogen compounds (ices) condensed beyond the electromagnetic force (imagine “static electricity”) frost line. until they are massive enough to attract via gravity to form .

Building the Planets IV: Planetesimals Building the Planets V: Jovian planets Planetesimals then will: and their moons • Each gas (Jovian) formed its own “miniature” • combine near the Sun to form rocky planets solar nebula. (gravitational heating and conservation • combine beyond the frostline to form icy planetesimals of angular momentum.) which… • gravitationally capture H/He far from Sun to form gas • Moons formed out of the disk. planets Building the Planets VI: Solar Wind! Solar Wind --- charged particles streaming out from the Sun cleared away the leftover gas