FORMATION OF THE SOLAR SYSTEM

1. FORMATION OF THE SOLAR SYSTEM We’ll concentrate on our planetary system now. At the end of the course we will discuss how we find planets around other stars.

2. WHEN, WHAT, HOW? • WHEN: AGES VIA RADIOACTIVE DECAY • Many heavy elements have UNSTABLE NUCLEAR ISOTOPES. • Remember: isotopes of the same element have different numbers of neutrons • Such nuclei can FISSION into lighter ones. • Common is loss of 4He nuclei -- alpha particles. Other PARENT NUCLEI lose neutrons, protons, electrons or positrons • Typically the resulting DAUGHTER NUCLEI are still unstable, so there is a DECAY CHAIN to a final, STABLE NUCLEAR ISOTOPE.

3. Radioactive Potassium turns into Argon • Relative amounts of different isotopes can be measured in rocks and used to date them • Half-life of 40K is 1.25 billion years

4. Half-Life and Nucleochronometers • N(t) = N0 2-t/t_half So, 1/2 gone after 1 half-life; 3/4 gone after 2 half-lives; 7/8 gone after 3 half-lives, etc. • Important NUCLEOCOSMOCHRONOMETERS: 235U231Th207Pb (7.3x108yr), 187Re187Os • 238U234Th206Pb (4.5x109yr) 40K40Ar • These lead to oldest: --earth rocks of 3.9 Gyr --moon rocks of 4.4 Gyr --meteorites of 4.55 Gyr. • THE AGE OF THE SOLAR SYSTEM IS 4.55 Gyr This value agrees with calculations for the evolution of a star with the mass and composition of the Sun.

5. Practice Question Answers • True: a CCD is more linear and preferred over film. • False: Jupiter is 11.2 Earth radii but 318 Earth masses, not about 100. • True: oldest rocks on Earth ~4 Gyr, oldest on moon from Apollo ~4.4 Gyr • False: While most large telescopes are reflectors, they spend most of their time taking spectra, not pictures. • False: The earth’s magnetic field is generated in its liquid outer core. (Mantle is rocky and plastic.) • False: Twice the wavelength means 1/2 the energy: E = hf = hc/ • True: liquids, solids & dense gases give continuum thermal spectrum

6. More practice answers • True:this situation is stimulated emission • B: 293 K is earth or room temp. and thermal emission peaks @ m = 0.29cm-K / 293 K = 2.910-1 cm / 2. 9102 = 1.010-3 cm = 1.010-5 m = 10 m which is in the IR, but some is emitted at every wavelength • D: 500 atoms after 1 half-life of 30 yrs, 250 after 2, 125 after three half-lives, or 90 years • E: angular momentum conservation means flattening, gravity means condensation, collisions meant extra flattening as vertical energy is lost. • A: getting above atmosphere means less turbulence and less absorption by water vapor • D: LA/LB=(RA/RB)2(TA/TB)4= 22(1/2)4=4/16=1/4

7. WHAT MUST BE EXPLAINED? • 99.9% of SS MASS in the SUN • 99% of SS ANGULAR MOMENTUM in the Planets • PLANETS are relatively ISOLATED • ECCENTRICITIES of planetary ORBITS are SMALL • INCLINATIONS also SMALL • Planetary ORBITS are ALL PROGRADE • MOST planetary SPINS are PROGRADE • MOST BIG MOONS have PROGRADE, nearly EQUATORIAL ORBITS • TERRESTRIAL vs. JOVIAN DICHOTOMY • ASTEROIDS are like pieces of planets, but primitive • The KUIPER BELT has asteroid sized, but icy bodies • COMETS are primitive, icy & DISTANT (Oort cloud)

8. HOW CAN THIS BE UNDERSTOOD?CONDENSATION WITHIN A NEBULA! • Start with a COLLAPSING, ROTATING CLOUD of GAS and DUST: a piece of a MOLECULAR CLOUD • Gravity pulls in faster along rotation axis; angular momentum fights collapse in the equatorial plane. • PROTOSTAR forms at the center of a flattened nebula; center gets most mass, little of the angular momentum. • Dust grains stick, settle towards disk plane. • Both Size (adhesion) and Gravity enforce pre-planetary Republicanism: the rich (big) get richer (bigger) • PLANETESIMALS grow via ACCRETION of gas and dust; have eccentric and somewhat inclined orbits • Then they COLLIDE: typically bigger ones grow, smaller are smashed; some of these fragments are accreted later, others escape: disk flattens more

9. Summary: Details to Follow

10. Stars Form in Molecular Clouds • Large scale recycling: gas and dust clouds collapse to form stars (and planets) • Old stars lose eject of their gas into the interstellar medium from which new stars (as in Orion nebula) form

11. Condensation in a NEBULA Naturally Explains: • More mass to the center (Sun) • More angular momentum to the outside • Gaps between planets larger further from the Sun • Prograde orbits, low inclinations and low eccentricities for planets • Collisions USUALLY give prograde spins (but can give exceptions, like Venus and Uranus) • Mini-solar systems with disks around Jovian planets producing moons near equatorial planes

12. Evidence for Flattened Nebulae around Young Stars • HST pictures of nearby stars with flattened rotating disks probably en route to making planetary systems • Protoplanetary Disk Formation

13. Materials in the Solar Nebula • Hydrogen and Helium: 98% -- gases that never condense to solids • Hydrogen compounds: 1.4%-- form ices at T < 150 K (H2O, CH4, NH3) • Rock: 0.4%--condense to solids between 500K and 1300K depending on mineral chemistry (SiO2, Al2O3, Mg2SiO4, etc.) • Metal: 0.2%--Fe, Ni, Al, etc. condense to solids at very high T: 1000--1600 K (most refractory)

14. Temperatures in the Solar Nebula • Only metals survive in the hottest part • Then rocks can live in the areas where terrestrial planets form • Different ices can condense at different cold temperatures, making up much of outer solar system

15. Temperature Gradient Yields Differentiation • Hottest near new star; only refractory elements condense there Inner cores stay rocky with metals Outer cores accrete more gas with prograde spin: yields many outer planet moons • T Tauri wind blows gas from inner SS  INNER and OUTER SS differences • Also roughly gives moon distributions & orbits To some degree, explains nature and location of asteroids and comets

16. Frost Line is Key to Difference between Terrestrial & Jovian Planets • Outside: hydrogen compounds can condense along w/ rocks and metals and planets can grow big • Inside: only rocks and metals can condense so planets stay small • Biggest cores can attract some of the H and He gas via gravity Temperature distribution applet

17. Terrestrial Planet Formation • Dust grains sink to disk plane, can stick to each other (cm-m) • Slow collisions cause growth via more surface & more gravity; orbits are altered and cross more • Randomly some grow much bigger, then: Disk Flattening

18. T Tauri Phase and Inner Planet Formation

19. Outer Planet Formation • Beyond the ice line rocky cores would form too, but • These cores would collect mass from larger volumes so they could grow bigger • Their gravity could be strong enough to also attract ices and some (Uranus, Neptune) or a lot (Jupiter, Saturn) gas to them • The clouds of gas attracted to them had a lot of angular momentum and could flatten to form disks that fragmented into the bigger moons -- like mini-solar systems • Smaller moons are usually captured asteroids

20. Alternative Model: Possible Outer Planet Formation from Direct Disk Instabilities Debris Ejection to Kuiper Belt and Oort Cloud explains asteroid & comet locations

21. Important Exceptions to Rules:Peculiar Spins and Tilts • Most planets have prograde spins but Venus doesn’t • Most have spin axis nearly perpendicular to orbital plane but Uranus doesn’t • Major collisions were much more common in the first 108 yr of SS life • They could cause angular momentum oddities like these

22. Very Important Exception: Our Big Moon • Our Moon’s diameter = 0.27 Earth’s • Our Moon’s mass = Earth’s/81 • Almost certainly formed via a massive rocky planetesimal striking earth after metals sunk to center: only hypothesis that explains lower density but big size

23. Summary in a Figure

24. Summary: Time-Line of SS Formation

25. Peer Instruction Question • Planet Goola has a mass 8 times that of the Earth and a radius 2 times that of the Earth. Goola is most likely to be • 1) a terrestrial planet • 2) a jovian planet • 3) an asteroid • 4) One can’t really tell from this information

26. Peer Instruction Question • Planet Goola has a mass 8 times that of the Earth and a radius 2 times that of the Earth. Goola is most likely to be • 1) a terrestrial planet • 2) a jovian planet • 3) an asteroid • 4) One can’t really tell from this information