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1Class 15Formation of the Solar System What does a successful model needs to explain? The three key properties of the planetary system The basic properties of the Sun The Solar Nebula Model Age of the Solar SystemI : What do we need to explain? Three basic properties of the planetarysystem… Size/composition of the terrestrial planets versus theJovian planets Directions and orientations of planetary orbits Sizes of terrestrial planet orbits versus Jovianplanetary orbits Even more fundamental fact that needsexplaining… 99% of the angular momentum ofthe solar system is associated with theplanetary orbits (mostly Jupiter, in fact) ratherthan the rotating Sun.2 Basic properties of the Sun… Contains 99.9% of the mass in the Solar System Slowly rotates (in same direction as planetaryorbits); but carries only tiny fraction of the solarsystem’s angular momentum Composed mostly of hydrogen and helium3Composition of interstellar gas in our part of the GalaxyBigBangStars4 Brainstorm possible scientific ideas forhow solar system formed…II : The Solar Nebula model Basics of the idea… Start with an interstellar gas cloud that startscollapsing under the action of gravity Due to conservation of angular momentum, cloudflattens into a rotating disk as it collapses Large quantities of gas flow inwards through thisaccretion disk, and form the Sun Planets form in the remaining disk… terrestrialplanets form in the hot inner part of the diskwhereas Jovian planets form in the cooler outer partsof the disk A mini-version of this scenario plays out around eachJovian planet, producing the satellite system5Likely temperature distribution in the Solar Nebula6Mount Fuji (Japan) Dependence of temperature on distance fromproto-Sun is important for planet formation… Inner solar system (inside the “snow line”) Too warm for ices to freeze Only the “rock-forming” materials condense intosolid particles Successive collisions : dust ⇒ gravel ⇒ boulders ⇒planetesimals ⇒ planets78 Jovian planet formation (beyond “snow line”) Core accretion model: Due to freezing of ices, there’s a lot more solid materialavailable than in inner solar system Collisions built up a rock+ice object several timeslarger than Earth (took several million years) Once core is large enough, gravitationally attracts thesurrounding gas to form thick atmospheres If correct, cannot form Neptune and Uranus in currentlocations… would need to have formed them closer inand be thrown out by Jupiter Disk instability model: Hypothesizes that the solar nebula was gravitationallyunstable… clumps could spontaneously form Once large clump formed, it gravitationally pulls inmore gas … does not need an initial rock/ice core Uranus & Neptune could form in current locationKuiper belt is a dusty/icy debris ring, gravitationally thrown tothe outer solar system by the massive planets (mainly Jupiter)9III : How long ago did theSolar System form? Current estimate: 4.56 billion years ago Determine this by radioactive dating ofmeteorites… Basics of radio-active dating: If the balance of protons-neutrons is wrong, anatomic nucleus is unstable In a given time (the half-life), half of these unstablenuclei will split (decay) and form more stable nuclei87Rb → 87Sr (half-life 47.0 billion years)238U → 206Pb (half-life 4.5 billion years)14C → 14N (half-life 5730 years) Can estimate the age of a rock by how much of aparticular unstable isotope is still present Not a simple exercise…


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