© 2005 Pearson Education Inc., publishing as Addison-Wesley The Planets Lecture 9 Geoff Marcy Light & Origin of the Solar System

© 2005 Pearson Education Inc., publishing as Addison-Wesley

The Planets

Lecture 9

Geoff Marcy

Light&

Origin of the Solar System


• Read Chapter 5 : “Light”

• Homework: MasteringAstronomy

Assignment Chapter 5

Due Friday

• Midterm: • - Next Tuesday, Sept. 30 - Covers Chapters 1-5 - 30 Multiple Choice


LightLast Time:


Light as a Wave

• f is frequency• is wavelength

• For light: f = c

c = speed of light

= 300,000 km/s• Our eyes recognize

f (or ) as color .

Last Time:


Light as photons

Light as a particle . . . .

E = hf

Each Photon has an Energy:

Last Time:


Energy Levels of Atoms• Electron can only have specific

energies in an atom.• Electrons can absorb energy,

from collisions or light.• The energy “excites” them

to higher energy levels.• Then they emit light when they

lose energy (drop to lower energy level).

• Only photons whose energies (colors) equal the difference in electron energy levels can be emitted or absorbed.

HydrogenAtom

Absorption of Light by Atoms & Molecules

• Atoms absorb photons whose energies (i.e. wavelengths) match

the energy difference

between the current energy level

and a higher energy level.

• The resulting spectrum has

all wavelengths (all colors),

but is missing wavelengths

that were absorbed.

• You can determine what types of atoms are in an object

by the emission & absorption lines in the spectrum.


Warm, Opaque Objects Glow by

Thermal Emission of Light

Cool Warmer Hot HotterRed & Faint White & Bright


Warm, Solid Objects Glow by

Thermal Emission of Light

Cool Warmer Hot HotterRed & Faint White & Bright

Thermal EmissionAt Infrared Wavelengths


Warm, Solid Objects Glow by

IR Thermal Emission of Light

Brighter ==> Warmer

Human skin is an emitter of infrared “thermal” radiation with wavelengths greater 3 microns. This energy may be recorded to yield a quantitative temperature map of the skin. The skin temperatures are determined mostly by the flow of blood nearby and by the heat conducted from within the body. An image in the infrared yields information about pathological conditions within the body.

Infrared Thermal Emission from the Earth


1. Warm objects emit Infrared light and radio waves Examples: Warm embers of fire, electric stove.

2. Hotter objects emit more light energy per unit surface area (per second),

proportional to Temperature 4:

“Thermal Emission” from Warm, Opaque Objects


1. Warm objects emit Infrared light and radio waves Examples: Warm embers of fire, electric stove.

2. Hotter objects emit more light energy per unit surface area (per second).

3. Hotter objects emit bluer photons (having shorter wavelength, and higher average energy.)

average increases as 1/ T (using kelvin Temp scale)

“Thermal Emission” from Warm, Opaque Objects


Warm, Solid Objects Emit Light:“Thermal Emission”

Examples:

• Electric Stove Filaments

• Fireplace Coals

• Light bulb filament

• Warm human body


1. Hotter objects emit more light energy per unit surface area per second. Energy emitted = 6x10-8 T4

(Joules per m2 per sec)

2. Hotter objects emit bluer photons (with a higher

average energy.) “Wien Law”

max = 2900 m / T (T in degrees Kelvin)

Summary

“Thermal Emission” from Opaque Objects


“Spectrum” of Light carries information

about the Planets and Stars

By studying the spectrum of an object, we can learn its:1 Chemical Composition2 Temperature (from thermal emission intensity and wavelength.3 Velocity (from Doppler effect)

Separate light into its different wavelengths (spectrum).


Spectrum from a Typical Planet, Comet, or Asteroid

Spectrum reveals:1 Chemical Composition – from abs. of wavelengths2 Temperature – from peak wavelength of IR light 3 Velocity – from Doppler effect

Reflected visible light from Sun Thermal Emission (IR)

Absorption by moleculesin gases in atmosphere


. Formation of the Solar System

Clouds of Gas and Dust in the Milky Way Galaxy


Circular Orbits (elliptical, but nearly circles)

All planets lie in one flat plane (the Ecliptic).

They orbit & spin in same direction (counter clockwise)

Inner Planets: small, rocky Outer Planets: large, made of gas and ice

Overall Properties of our Solar System

How did our Solar System Form ? ? ?


The Origin of the Solar System

• Four characteristics of our Solar System must be explained by a formation theory.

• What is the basic idea behind the theory?


Interstellar Gas and Dust in

Dust and GasIn

InterstellarClouds !

Light absorbedfrom distant starsalong mid-plane.

our Milky Way Galaxy


The Dark Clouds in the Milky Way

Centaurus AHST

Milky Way


The Interstellar Medium(ISM)

Dust & Gas

98% is H and He


Dark Clouds in our GalaxyDense gas and dust. 1% (by mass) is “rocky/icy” dust particles that could eventually make terrestrial planets.


Absorptionof Light by DustDust clouds: Opaque in visible (“Optical”) light. Lower opacity in infrared.Dust scatters visible light more efficiently than infrared ==> To Study the Milky Way Galaxy: use IR !

Visible Light Infrared Light


Gas Clouds containhydrogen, helium, carbon,nitrogen, oxygen

and complex molecules


Small Dust particle:Only a few thousand atoms

Dust is Made of Atoms


Interstellar Dust Grain:

C, O, Si, H20 ice, Si-O.

Large Dust Particle: 10,000’s of Atoms!


Stars are continuously forming in the galaxy.

Basic Observation


Dense CloudsFloating in our Milky Way Galaxy

Gravity pulls atoms closer together


Collapse of the Solar Nebula


A Young Star Forming,surrounded by a protplanetary disk

Artists Rendering


As the nebula contracts due to its self-gravity, it heats up, spins faster, and

flattens.

Cloud ContractingDue to Self-Gravity


Flattening of the Solar Nebula• As the nebula collapses, clumps of gas collide & merge.• Their random velocities average out into the nebula’s direction

of rotation.• The spinning nebula assumes the shape of a disk.



Protoplanetary Disks…

Solar System size

Star and planet formation

Measured Sizes: 100-1000 AU

Masses: 10-3 – 10-1 Msun


Protoplanetary Disks of Gas & Dust

Theory ofPlanet Formation:

Dust collides, sticks

and grows pebbles/rocks Gravity helps attract

more rocks Gravity attracts gas

Formation of Planetary Systems

Observations Thermal Emission (Infrared)

from Dust

Hubble Space Telescope

Pictures of protoplanetary disks.

Mass of Disk = 10-100 MJUP

Disk Lifetime ~ 3 Million years

Observations Models of Planet Formation


Theory of Rocky Planet Formation Inward of 2 AU

Planetesimals (km-sized comets & asteroids)

• Growth of rocks (planetesimals) by collisions and sticking together • Friction circularizes orbits• Big planetesimals gravitationally stir small rocks• Mergers among planetesimals: They grow to Earth-Size

Safronov 1969Greenberg et al 1978Wetherill & Stewart 1993Kokubo & Ida 2000

Lissauer 1987Rafikov 2003Chambers, Thommes 2002Goldreich, Lithwick, Sari 2004

Analytical and N-body:


Building the Planets Inward of 3 AU

At 3 AU is “Snow Line” : Hotter than 0COnly rocks & metals condensed inward.Too hot for gases (H, He) to stick to rocks. Hydrogen compounds (H2O, NH3, CH4 ) are gases. Only rocky planets inward of 3 AU.


Building the Planets Beyond 3 AU

- Cold!- Hydrogen compounds (ices, H2O, NH3, CH4) condensed.- Planetesimals made of ROCK and ICE !- Gases (H, He, hydrogen compounds) gravitate to rocks: - Form Planets made of rock, ices, and gases! Jupiter, Saturn, Uranus, Neptune


Building the Planets

accretion -- small grains stick to one another via electromagnetic force until they are massive enough to attract via gravity to form...


Building the Giant Planets• Gas-giant planets form by gravitationally attracting H and He

gas. More gas acquired: More gravity. Attraction of Gas is a “runaway” ! Jupiters form their own “miniature” solar nebula.

• Moons formed out of the mini-nebula.


Summary:Origin of the Solar System

Theory – our Solar System formed from a giant, swirling cloud of gas & dust.

Depends on simple principles of Physics:• Dust particles collide, stick together &

grow.

• Law of Gravity: gravitational attraction of particles and gas

• Conservation of angular momentum to flatten the protoplanetary disk.


Theory Explains:Orderly Motions in the Solar System

• The Sun formed in the center of the gas-dust protoplanetary disk.• The planets formed in the protoplanetary disk.• This explains:

– all planets lie along one plane (in the disk)– all planets orbit in one direction (the spin direction of the disk)– the Sun rotates in the same direction– the planets would tend to rotate in this same direction– most moons orbit in this direction– most planetary orbits are near circular (viscous smoothing of orbits)


Origin of the Solar System

Our theory explains the properties

of the Solar System

1. Planets in orderly motions:

circular orbits, flat plane, orbit same direction.

There are two types of planets.– small, rocky terrestrial planets– large, hydrogen-rich Jovian planets

Asteroids & comets exist in certain regions of the Solar System

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© 2005 Pearson Education Inc., publishing as Addison-Wesley The Planets Lecture 9 Geoff Marcy Light & Origin of the Solar System