Dating the UniverseLecture 6: Formation of the Solar SystemAstro 202Prof. Jim Bell ([email protected])Spring 2008But first...Graded Paper 1 returned today...Paper 2 is due at beginning of class on Feb. 14Don’t wait until the last minute!Paper 3 to be posted online next Tuesday 2/12How did the Solar System Form?!Observational Constraints–Planetary motions–Planet, asteroid, comet compositions–Meteorite and cosmic dust ages–Other solar systems?!The Solar Nebula Model–Formation of the Terrestrial Planets–Formation of the Giant Planets• Chemical Condensation ("Lewis") Model!Planetary EvolutionAstro 202 4Observational Constraints!For a theory of the formation of our solar system to be viable, it must be consistent with our presently-available data!Theories must fit 3 classes of constraints:(1) Motions: The observed spin and orbital motions of the planets, asteroids, and comets(2) Composition: The measured or inferred compositions of planets, asteroids (meteorites), and cometsAstro 202 5"Motion" Constraints!All of the planets orbit the Sun in roughly the same plane (the ecliptic), which is very close to the Sun's equatorial plane!The orbits of the major planets are nearly circular!Planets, asteroids, and most comets circle the Sun counter-clockwise as viewed from above (exceptions: some comets)!The Sun and most of the planets generally spin in this same counterclockwise direction and have very small tilts relative to the plane of the solar system (exceptions: Venus, Uranus, Pluto)!Nearly all the planets and asteroids spin with the same period, roughly 20±10 hours per "day" (exceptions: Venus, Pluto)!The Sun contains 99.9% of the mass of the solar system, but the planets contain 99.7% of the conserved angular momentumAstro 202 6Top viewEdge onAstro 202 7Compositional Constraints!The compositions of the planets vary with distance from the Sun–The inner planets are dense and rocky/metal-rich–The outer planets are primarily gas (H, He) and ices–Asteroids in the main belt are transitional–Comets and KBOs are primarily ices!Most of Jupiter and Saturn, and the atmospheres of Uranus and Neptune, have approximately the same composition as the Sun! Many meteorites appear to be chemically different (more "pristine") than planets and moonsAstro 202 8Astro 202 9Age Constraints!The primitive (unaltered) carbonaceous and ordinary chondrite meteorites ALL have ages of approximately 4.5 billion years!Tiny fractions of some very primitive meteorites and cosmic dust particles have ages even slightly older than this!The oldest rocks from the Moon are about 4.4 billion years old!More details in next 4 lectures...Astro 202 10Major Characteristics of our Solar System: Summary(1) Large bodies have orderly motions(2) Planets fall into two main categories • Small, rocky inner planets • Giant, gas- and ice-rich outer planets(3) Swarms of asteroids and comets abound!(4) There are notable exceptions to the trends• Tilt of Uranus, tilt of Pluto's orbit, spin of Venus, orbit of Triton, ...Astro 202 11Historical Models for the Formation of the Solar System!Catastrophic Models–A large comet collided with the Sun and ejected material that cooled to form the planets [Buffon, 1745]–Another star made a close pass by the Sun, tidally pulling material out that cooled to form the planets [Bickerton, 1880]!Nebular Models–Sun and planets formed from a vast nebula of gas [Descartes, 1644] that swirled around and condensed into planets [Kant, 1755]–Nebula contracted into a fast spinning disk, shedding "rings" of material that formed the planets [Laplace, 1796]!All have problems with the basic physics or matching some of the constraints, but nebular models "best"Astro 202 12The Solar Nebula Theory!The leading model today is called the solar nebula theory–Previous generations of stars lived and died before our solar system was formed ("cosmic recycling")–The violent death of a previous star or stars formed an enormous cloud of hydrogen, helium, tiny amounts of heavier elements, and interstellar dust particles in this part of our galaxy–This cloud collapsed under its own gravity and spun into a disk–Most of the mass fell into the center of the disk and formed the Sun–A tiny fraction of the leftovers formed the planets, moons, etc.–Some day (maybe 5 billion years from now) the Sun will die and redistribute most of this matter back into an interstellar cloud...Astro 202 13Cosmic Recycling...Ours may not be the first solar system to occupy this part of the galaxy...Astro 202 14The Solar Nebula TheoryAstro 202 15The "Standard Model":From gas clouds to gas giants..."accretion"Astro 202 16The Violent Young SunOriginal nebula may have had ~2 solar masses of material; half of it lost by the young Sun's strong "bipolar outflow"Astro 202 17How long did this take?!FAST according to computer models–Contraction, spin-up of original cloud of gas takes only a few million years–Condensation of dust and settling of the dust into a rotating disk may have taken only ~ 100 years!–Clumping of dust by self-gravity into 1-5 km planetesimals may have taken only ~ 1000 years!–Accretion of planetesimals into ~1000 km objects may have occurred over only ~1 million years–Growth of a few objects to even larger Moon, Mars, or Earth sizes in only about 100 million yearsAstro 202 18The Cosmic Calendar(from Carl Sagan’s Cosmos)Putting the age of the solarsystem into perspective...Our galaxy formsAstro 202 19Implications...!The original catastrophic theories assumed that the Sun formed first and the planets formed later, by accident–Implies that planets are rare!Nebular theories assume that the Sun and planets form together–Implies that planetary systems are commonAstro 202 20Testing the Solar Nebula Theory!How do we decide if this is right? (we weren't there!)–Do the models rely on the laws of physics and chemistry or do they require miracles or ad hoc assumptions?–Do the models accurately predict the present nature and distribution of the planets?–Are there clues in the compositions of the planets or comets or meteorites that could help to support this model?–Can we look elsewhere in the galaxy for evidence for this kind of process of solar system formation?Astro 202 21Compositional Evidence• The solar nebula theory predicts a specific chemical condensation sequence as the hot gaseous nebula cooled...• In general, this sequence is consistent with the observed changes in planetary