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Lecture 37 The Formation of the Solar System How did the solar system come about? Condensation sequence Are there others? Apr 24, 2006 Astro 100 Lecture 37 1 The Solar System Brief History of Time (assumes T = 13.7 Gyr) • 3 minutes: Formation of He, D • 1-2 billion yrs: Milky Way Earliest Stars • 2-8 Gyr: MW Disk supernovae: C,N,O,Fe.. • 9 Gyr: Collapse of solar gas cloud • 13.699 Gyr: People evolve (1 million BP) • 13.699999 Gyr: Astronomy (1000 BP) Apr 24, 2006 Astro 100 Lecture 37 2 1 Some important features of Solar System • 1) Radioactive dating of old SS rocks gives same age: Earth: 3.9 Gyr; Moon 4.5 Gyr; Meteorites 4.6 Gyr • 2) Most orbital and rotation planes confined to ecliptic plane; motions counterclockwise • 3) Extensive satellite and ring systems around Jovian planets • 4) Planets have more heavy elements than Sun; divided into two classes based on density (composition): Apr 24, 2006 Astro 100 Lecture 37 3 Planet Classes Type Planet Mass Terrestrial Mercury 0.06 5.4 Venus 0.82 5.2 Earth 1.00 5.5 Mars 0.11 3.9 Jupiter 318 1.3 Saturn 95 0.7 Uranus 14 1.3 Neptune 17 1.7 Jovian Density gm/cm3 Chief Material Rock, Iron Hydrogen, Helium Ices • Note: terrestrial planets all closer to the Sun Apr 24, 2006 Astro 100 Lecture 37 4 2 Models • (1) suggests planets formed at same time as sun: 5 Gyr consistent with sun being ~2x luminosity of zero-age main sequence G2 star. • (2,3) suggests we associate planetary and satellite/ring systems with remnants of dusty disks seen around forming stars (eg T Tauri stars): The Nebular Model. • (4) suggests that what planets condensed out of depended on how far they were away from sun: The Condensation Sequence The different densities of the planets is now explained by condensation temperatures in Nebular Model: • Nebular Dust temperature should increase to center of nebula: – More of remnant heat from collapse retained at center (more opaque there) – After sun turns on, dust reaches "equilibrium temperature": Apr 24, 2006 Astro 100 Lecture 37 5 Equilibrium Blackbody Temperature • Energy absorbed/area from sun (flux) = Energy emitted as Blackbody – Solar Energy Absorbed/ Area = Lum(Sun)/(4π Dist2) – Energy Emitted/ Area = constant x T(equil)4 => T4 (equil) = constant × Lum(Sun) / Dist2 • Condensation Sequence: what materials are solid at each position in nebula based on freezing point: Atom Molecule Freeze Dist Hydrogen (H) Helium (He) H2 10 K > 100 AU Oxygen (O) Carbon (C) Nitrogen (N) H2O CH4 NH3 200- 300 > 10 AU Iron (Fe) Silicon (Si) Magnesium(Mg) FeS 700 > 1 AU 1000 > 0.5 AU Apr 24, 2006 (Mg,Fe)SiO4 Astro 100 Lecture 37 6 3 Formation Scenario Formation stages: • I. Solid particles collide, stick together, sink toward disk ("planetesimals"): – Ices don't condense inside orbit of Mars: – Terrestrial planetesimals mostly rock, metals; not very massive – Jovian planetesimals mostly Icy materials + rock; much more massive • II. Coolest, most massive (J,S) collect H,He by gravity • III. More Collisions => heating, differentiation of interiors • IV. Flushing of remnants by solar radiation, wind; planetary collisions, ejection • V. Evolution of atmospheres Apr 24, 2006 Astro 100 Lecture 37 7 Other Solar Systems Until 14 years ago, our solar system was the only one known. Since then, 97 more (163 planets) found around nearby solar-type stars by doppler effect: planet- star system is like spectroscopic binary, with acceleration of planet on star seen as cyclic variation of positions of absorption lines in stellar spectrum (like "single-line spectroscopic binary"). Need very high accuracy: doppler effects of like 10 meters/sec. Check out this website: http://planetquest.jpl.nasa.gov/index.html • Example: 47 Ursa Majoris (follow Big Dipper pointers backwards to 5th mag star) – – – – Distance: 17 pc (55 ly) Spectral Type: G0V Luminosity: 1.3 Lsun Planet (47 UMa b): M > 2.4 M(Jup) Period: 1103 days Velocity amplitude: 45 m/s Apr 24, 2006 Astro 100 Lecture 37 8 4 Comparative Solar Systems • A problem: this method most sensitive to massive planets close in, which shouldn't exist according to simple formation theory! • Latest fix to theory suggests that it is possible for planet orbits to spiral in towards star after formation, if the nebula is very thick, or the TTauri flushing occurs late. • But: why didn't this happen to our SS? Apr 24, 2006 Astro 100 Lecture 37 9 Our Solar System Figure 14.2, p440, Arny Apr 24, 2006 Astro 100 Lecture 37 10 5 Interiors of the Planets Figure 14.6, p445, Arny Apr 24, 2006 Astro 100 Lecture 37 11 ProtoPlanetary Systems in Orion Apr 24, 2006 Astro 100 Lecture 37 12 6 Nebular Model Figure 14.11, p450, Arny Apr 24, 2006 Astro 100 Lecture 37 13 Some Solar Systems Apr 24, 2006 Astro 100 Lecture 37 14 7