Chapter 7 - The Planetary

Our Solar System

• planets are large, spherical isolated bodies that orbit a star

  • must clear debris from its own path

• a planetary system is a system of a star, planets, moons, and smaller bodies

Nebular Hypothesis

• a rotating cloud of interstellar gas gradually collapses and flattens to form a disk

• the sun forms at the center and planets form from the disk

• modern theory calculates conditions required for a cloud to collapse under the force of gravity

  • self-gravity is the attraction between parts of an object tha tpull outer layers toward the center

  • self-gravity is opposed by structural strength, gas pressure, or radiation pressure

  • in a stable object, inward and outward forces are balanced

• Evidence

  • Astronomical Images

    • young stars are seen to be surrounded by rotating disks of gas and dusts

  • Motion of Planets

    • evidence of our own planets support the nebular hypothesis

      • all planets orbits the sun in the same direction and in the same flat plane

      • we can deduce that the planets must have formed from the same flat, rotating disk of material

  • Meteorites

    • contain hints to the solar system’s formation as well

    • many are mixtures of smaller rocks or pebbles, suggesting formation through a process of aggregation

Thought Experiment: Temperature of a Balloon

• make the circumference at the widest part of the balloon

• cold - shrink

• warm - expand

• when temp drops, outward pressure decreases, balloon shrink sunder self-gravity

• illustration of a system maintaining hydrostatic equilibrium

  • ensures stars and planets are stable systems

7.2 The Solar System Began with a Disk

Solar System Beginnings

• a collapsing cloud of interstellar gas formed a protostar and protoplanetary disk

• protostar - a large ball of gas; not hot enough to be a star

  • protoplanetary disk: flat, orbiting disk of gas and dust

Growth of Particles

• within the disc, small articles will collide and stick

• small particles are blown into larger ones by gas motions

• this leads to larger particles (~1km in size) called planetesimals

• at a size of 1km, gravity takes over and pulls nearby objects into the planetesimal

• through growth driven by gravity, planetesimals combine to form planets

• today’s remaining planetesimals are asteroids and comets

  • asteroids - rock and metal

  • comets - ice and rock

What an astronomer sees

• notice the brown clumps that are too dense to see through these are sites of star formations

• notice the jets of materials being ejected by newly formed stars

Visual Summary: The Collapsing Cloud

1. as gravity causes the collapse of a slowly rotating clump, it rotates faster

2. rotation slows collapse perpendicular to but not parallel to the axis, so the clump flatters

3. eventually the clump collapses from the inside out, and an accretion disk and protostar form

Formation of a Flat Disk

• most of the as lands on a accretion disk, which continues the rotation

• the angular momentum of the interstellar cloud is conserved and ends up in the accretion disk

• material in the accretion disk either becomes part of the protostar, forms planets, or is ejected back into interstellar space

Definition of Angular Momentum

• angular momentum is associated with rotating objects. It depends on

  • rotational velocity

  • mass

  • mass distribution

Conservation of Angular Momentum

• conserved quantity. It cannot change unless an external force is applied

• the figure skater’s angular momentum does not change, but as she pulls in her arms, she rotates faster

• p=mv

• L=Iw

• I=mr²

• L=mr²v/r

• L=mvr {Angular Momentum (Point Mass)}

Angular Momentum on a Sphere

I = 2/5mr²

L = Iw = 2/5mr²w

L = 2/5 mr²v/r

L=2/5mrv (v=2pir/p)

L=4/5(pimr/P)

Angular momentum deends on an object;s mass, size, and speed. If a giant giant gas intitially rotaes slowly, what will happen to tits

• speed up

calculate orbital angular momentum, we need to know Jupitaer’s mass, velocity and size of orbit

• 1.94 × 10^43 m²/s

for a uniform sphere, spin angular momentum is give by 4pimR²/5P

• for the sun

  • 1.14 × 10^42 m/s

7.3 The inner disk and outer disk formed at temperatures

Conversion of Energy

• gas the fas cloud shrinks, its gravitational potential energy is converted to kinetic energy, radiative energy, and thermal energy. Energy is conserved

• thermal energy is greater in the inner portion of the disk

• gravitational energy is converted to heat more in inner disk than outer disk

• the inner disk is closer to protostar, which heat sup the surrounding material

Disk Composition

• the composition of dust grains depends on temperature

• inner disk - only refractory materials can form or remain

• outer disk - can also have volatile materials such as ices and organic material

• Refractory - does no melt at high temperature

• Volatile - can melt or evaporate at moderate temperatures

• Organic - contains carbon-hydrogen bonds

Atmospheres

• the primary atmosphere is the gas initially gathered from the disk

  • primarily hydrogen and helium (low-mass gases)

  • the process of gathering atmosphere is called core accretion-gas capture

• Secondary atmospheres occur around some low-mass planets because he initial atmosphere is lost

  • the low-mass planets do not have enough gravity to keep the initial atmosphere form escaping

  • volcanoes emit heavy gases from the planetary interiors that the planet can hold on to for a very long time

  • comets bring water and other volatiles to planets, which evaporate and add to the secondary atmosphere