© 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 Read Chapter 5 : “Light” Homework: MasteringAstronomy Assignment Chapter 5 Due Friday Midterm: - Next Tuesday, Sept Covers Chapters Multiple Choice

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light Last Time:

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as a Wave f is frequency  is wavelength For light: f = c c = speed of light = 300,000 km/s colorOur eyes recognize f (or ) as color. Last Time:

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as photons Light as a particle.... E = hf Each Photon has an Energy Each Photon has an Energy: Last Time:

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

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.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Warm, Opaque Objects Glow by Thermal Emission of Light Cool Warmer Hot Hotter Cool Warmer Hot Hotter Red & Faint White & Bright

© 2005 Pearson Education Inc., publishing as Addison-Wesley Warm, Solid Objects Glow by Thermal Emission of Light Cool Warmer Hot Hotter Cool Warmer Hot Hotter Red & Faint White & Bright

Thermal Emission At Infrared Wavelengths Thermal Emission At Infrared Wavelengths

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

© 2005 Pearson Education Inc., publishing as Addison-Wesley 1. Warm objects emit Infrared light and radio waves Examples: Warm embers of fire, electric stove Hotter objects emit more light energy per unit surface area (per second), proportional to Temperature 4 : “Thermal Emission” from Warm, Opaque Objects

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

© 2005 Pearson Education Inc., publishing as Addison- Wesley Warm, Solid Objects Emit Light: “Thermal Emission” Examples: Electric StoveElectric Stove Filaments Filaments Fireplace CoalsFireplace Coals Light bulb filamentLight bulb filament Warm human bodyWarm human body

© 2005 Pearson Education Inc., publishing as Addison-Wesley 1. 1.Hotter objects emit more light energy per unit surface area per second. Energy emitted = 6x10 -8 T 4 (Joules per m 2 per sec) 2. 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

© 2005 Pearson Education Inc., publishing as Addison-Wesley “Spectrum” of Light carries information about the Planets and Stars By studying the spectrum of an object, we can learn its: 1Chemical Composition 2Temperature (from thermal emission intensity and wavelength. 3Velocity (from Doppler effect) Separate light into its different wavelengths (spectrum).

© 2005 Pearson Education Inc., publishing as Addison-Wesley Spectrum from a Typical Planet, Comet, or Asteroid Spectrum reveals: Spectrum reveals: 1Chemical Composition – from abs. of wavelengths 2Temperature – from peak wavelength of IR light 3Velocity – from Doppler effect Reflected visible light from Sun Thermal Emission (IR) Absorption by molecules in gases in atmosphere

© 2005 Pearson Education Inc., publishing as Addison-Wesley Formation of the Solar System. Formation of the Solar System Clouds of Gas and Dust in the Milky Way Galaxy

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

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

© 2005 Pearson Education Inc., publishing as Addison-Wesley Interstellar Gas and Dust in Dust and Gas InInterstellar Clouds ! Light absorbed from distant stars along mid-plane. Milky Way Galaxy our Milky Way Galaxy

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Dark Clouds in the Milky Way Centaurus A HST Milky Way

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Interstellar Medium (ISM) Dust & Gas 98% is H and He

© 2005 Pearson Education Inc., publishing as Addison-Wesley Dark Clouds in our Galaxy Dense gas and dust. 1% (by mass) is “rocky/icy” dust particles that could eventually make terrestrial planets.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Absorption of Light by Dust Dust 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

© 2005 Pearson Education Inc., publishing as Addison-Wesley Gas Clouds contain hydrogen, helium, carbon,nitrogen, oxygen and complex molecules

© 2005 Pearson Education Inc., publishing as Addison-Wesley Small Dust particle: Only a few thousand atoms Dust is Made of Atoms

© 2005 Pearson Education Inc., publishing as Addison-Wesley Interstellar Dust Grain: C, O, Si, H 2 0 ice, Si-O. Large Dust Particle: 10,000’s of Atoms!

© 2005 Pearson Education Inc., publishing as Addison-Wesley Stars are continuously forming in the galaxy. Basic Observation

© 2005 Pearson Education Inc., publishing as Addison-Wesley Dense Clouds Floating in our Milky Way Galaxy Gravity pulls atoms closer together

© 2005 Pearson Education Inc., publishing as Addison-Wesley Collapse of the Solar Nebula

© 2005 Pearson Education Inc., publishing as Addison-Wesley A Young Star Forming, surrounded by a protplanetary disk Artists Rendering

© 2005 Pearson Education Inc., publishing as Addison-Wesley As the nebula contracts due to its self-gravity, it heats up, spins faster, and flattens. Cloud Contracting Due to Self-Gravity

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

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

Protoplanetary Disks… Solar System size Star and planet formation Measured Sizes: AU Masses: – M sun

© 2005 Pearson Education Inc., publishing as Addison-Wesley Protoplanetary Disks of Gas & Dust Theory of Planet 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 = M JUP Disk Lifetime ~ 3 Million years Observations  Models of Planet Formation

© 2005 Pearson Education Inc., publishing as Addison-Wesley 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 1969 Greenberg et al 1978 Wetherill & Stewart 1993 Kokubo & Ida 2000 Lissauer 1987 Rafikov 2003 Chambers, Thommes 2002 Goldreich, Lithwick, Sari 2004 Analytical and N-body:

© 2005 Pearson Education Inc., publishing as Addison-Wesley Building the Planets Inward of 3 AU At 3 AU is “Snow Line” : Hotter than 0C Only rocks & metals condensed inward. Too hot for gases (H, He) to stick to rocks. Hydrogen compounds (H 2 O, NH 3, CH 4 ) are gases.  Only rocky planets inward of 3 AU.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Building the Planets Beyond 3 AU - Cold! - Hydrogen compounds (ices, H 2 O, NH 3, CH 4 ) 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

© 2005 Pearson Education Inc., publishing as Addison-Wesley Building the Planets accretion -- small grains stick to one another via electromagnetic force until they are massive enough to attract via gravity to form...

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

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

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

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