Before heaven and earth had taken form all was vague and amorphous. Therefore it was called the Great Beginning. The Great Beginning produced emptiness and emptiness produced the universe. Huai-Nan Tzu, 2nd cent. BCE

Assignment: STUDY FOR THE EXAM!

In Class: Question to Ponder Who was the first person in space? a) Alan Shepard b) John Glenn c) Yuri Gagarin d) Neil Armstrong e) Jim Lovell Guessing the Origin of the Solar System "Origins" questions are always problematic in astronomy. We only get to see the current product. Must guess how it came to be. Constrain models of the formation of the solar system based on how it looks today. What We Know About the Solar System Most of the mass is in the center (a.k.a. the Sun). The remaining mass orbits as planets, asteroids, comets, etc. Massive planets on the outside, low mass planets on the inside. All of this stuff orbits in the same direction. All of this stuff orbits in the same plane. The Nebular Hypothesis First put forth by Kant (1750's) and Laplace (1790's). States that Sun and planets formed from the gravitational collapse of a giant cloud of diffuse material. Such clouds are common in the spaces between the stars. If the cloud were initially rotating, even only a tiny bit, the collapse would amplify the rotation and a star surrounded by an orbiting disk of the material would result. This orbiting disk might agglomerate into the planetery system we have today. A Problem with the Nebular Hypothesis: Chemical Composition If the Sun and planets formed simultaneously from the same cloud, they should have the same chemical composition. Sun's composition is 90% Hydrogen, 10% Helium, and bits of everything else. Outer planets have the same composition (roughly). Inner planets are deficient in Hydrogen and Heliuim; made mostly of denser materials -- iron, silicates, carbon, and oxygen. Temperature As a Differentiating Influence So the solar system is differentiated. Crunchy in the inside and fluffy on the outside. Differentiation driven by temperature effects. Planets formed from condensation of nebular gases. Heavier elements condense at higher temperatures, so iron and silicon become solids throughout the nebula (i.e., both near the SUn and far from it). However, lighter elements, like water, ammonia, and methane don't condense until the temperature is pretty low. Hydrogen and Helium are always gaseous. As a result, planets in the inner solar system form from iron, silicon, and other heavy stuff. There isn't much of this, so they end up small. Outewr planets form from iron, silicon, and other heavy stuff, plus the ices of water, methane, and ammonia, as well as other species that only turn solid at low temperatures. There's a lot more of this stuff, so these planets get bigger. The outer planets get big enough that their gravitational tugs are sufficient to hold atmospehres of Hydrogen and Helium. There's lots of Hydrogen and Helium around, so the outer planets get enormous.