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10/5/2011 Origins and Formation of the Solar System Theory of Formation • What must this theory explain? – Motions of solar system objects – Division between small, rocky terrestrial planets and massive, gas-giant Jovian planets – Presence of asteroid belt, Kuiper Belt, Oort cloud, etc. – Exceptions to the above (Earth’s Moon, tilt of Uranus, etc.) • Theory shouldn’t be specific to our solar system. Our Solar System Our Solar System • Sun • Planets – Moons – Dwarf Planets • Asteroids • Comets The Early Universe • Big Bang – ~92% Hydrogen – ~8% Helium – Very little of anything else • First galaxies and stars form 150 million years after big bang. – Massive stars produce heavy elements – Supernovae seed heavy elements into ISM • New stars form with greater abundances of heavy elements. (Sun is Population III) Different Theories • Nebular Theory: Solar system formed through the gravitational collapse of a large cloud of gas. – Immanuel Kant, 1755 and Pierre-Simon Laplace, 1795 • Close Encounter Hypothesis: Planets are debris from a close encounter between the Sun and another star. – Doesn’t account for orbital motions or Terrestrial/Jovian division Nebular Theory Nebular Theory Proplyds in the Orion Nebula Collapse • Large, irregular cloud of gas – Initially stable • Outside stimulus starts collapse – Collision between clouds – Shock from nearby supernova – Etc. • Begins runaway gravitational collapse – Gravity pulls the same in all directions Heating, Spinning, Flattening • Solar nebula starts as a large, diffuse cloud • Ends up as a small, flat, spinning disk – How? • Heating – Collisions between inwardly falling particles convert kinetic energy to random motions (heat). • Spinning – Conservation of angular momentum • Flattening – Collisions between particles flatten the disk. Testing the Model • Our model should work for other solar systems too, so we should search for evidence of heating, spinning and flattening elsewhere. • Protostellar Disks and Herbig-Haro Objects • Computer simulation • Accretion Disks Protostellar Disks Herbig-Haro Objects Computer Simulation Formation of Planets • What do we have now? – Hot, spinning disk, most matter at center (Sun) – Protoplanetary Disk • Composition of protoplanetary disk most likely homogeneous – Why the division between terrestrial and jovian planets? • Gravity too weak to form planets • Temperature is the key. Condensation • Condensation: the process by which solid or liquid particles form in a gas • Inner regions of the protoplanetary disk are warmer • Protoplanetary Disk is made of four different materials – Hydrogen and Helium gas (98%) – Hydrogen compounds (e.g. water, methane, ammonia) (1.4%) – Minerals and metals (0.6%) Condensation • Different materials condense at different temperatures – Hydrogen and Helium: never condense in interstellar space – Hydrogen compounds: begin to condense around 150 K – Minerals and Metals: begin to condense around 500 K – 1500 K • Heavier materials condensed out of the disk nearer to the Sun, lighter materials could only condense outside the frost line. Frost Line • Inside the frost line, only rocky and metallic materials can condense. – Mostly metals near Mercury’s orbit – Metals and minerals between the orbits of Venus and Mars – Water and carbon-rich materials form near the asteroid belt (between Mars and Jupiter) – Hydrogen compounds outside the frost line (between Mars and Jupiter) • More hydrogen compounds than rocky materials Accretion • How did these small seeds that condensed out of the protoplanetary disk grow into planets? – Accretion • Seeds are on nearly circular orbits – Collisions are gentle – Particles stick together through electrostatic forces • Seeds grow into Planetesimals – Pieces of planets Accretion • Planetesimals grow rapidly at first – Hundreds of kilometers in a few million years • Larger planetesimals have gravitational effect on smaller particles and each other – Create crossing orbits – Increases collisions • Collisions between planetesimals are destructive more often than not – Only the largest avoid being shattered and continue to grow Evidence for Accretion • Meteorites – Rocky with embedded metallic grains – Meteorites from asteroid belt contain more carbon and water-containing minerals. Jovian Planets • Condensation proceeded the same as in the inner regions of the solar system – Ices (condensed hydrogen compounds) mean more material is available • Larger planetesimals have stronger gravity – Pull in gases that terrestrial planets cannot Halting Accretion • What stopped the formation of the planets? • Solar Wind – Most of the material in the solar nebula never became part of any planet • Gas-giants stopped flow of gas into the inner solar system • Eventually all material has either been accreted or removed from the solar system. Halting Accretion Asteroids and Comets • How did asteroids and comets form? – Location tells the story • Asteroids – Located mainly between Jupiter and Mars – No stable orbits • Comets – Spend most of their time in the outer solar system – Icy – Highly elliptical orbits Age of the Solar System • The Solar Nebula Theory describes how the solar system formed. • Radiometric dating can describe when the solar system formed. – Allows us to tell when solid objects last solidified. • Some atoms are known as radioactive isotopes. – Can change into other elements through radioactive decay Age of the Solar System • Radioactive isotopes have a characteristic half-life – Half-life is the amount of time it takes one half of some radioactive material to undergo radioactive decay • By measuring the ratios of radioactive isotopes and their products, we can roughly determine the age of an object. – Must make some assumptions. • Moon rocks are about 4.44 billion years old – Oldest meteorites are about 4.5 billion years old Overview • • • • Solar nebula collapsed under its own gravity Heating, spinning, flattening Sun forms at center of disk Heavier materials condense out of protoplanetary disk, lighter materials outside of the frost line only • Planetesimals form and begin to accrete materials • Accretion stops when there is no more material Next Time… The Sun Quiz 4 1) The shape of the Sun’s spectrum is roughly described by a a) Blackbody spectrum b) Absorption spectrum c) Emission spectrum d) Radiative spectrum 2) What is do we call the bending of light rays in matter due to the change in the speed of light? a) Reflection b) Refraction c) Absorption d) Diffraction