Solar System Formation Solar System Formation Question: How did our solar system and other planetary systems form? “Comparative planetology” has helped us understand • Compare the differences and similarities among the objects in our solar system • Figure out what physical processes could have led to them • Then construct a model of how our solar system formed based on this ------- • This model must explain our own solar system… • …but might or might not explain other planetary systems • If not, modify the model to accommodate discrepancies • In other words, carry out the scientific process ------- • Let’s look at the solar system characteristics comparative planetology has to work with… Solar System Formation -- Characteristics of Our Solar System 1. Large bodies have orderly motions and are isolated from each other – All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane Solar System Formation -- Characteristics of Our Solar System 1. Large bodies have orderly motions and are isolated from each other – All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane – The Sun and most of the planets rotate in this same direction as well Solar System Formation -- Characteristics of Our Solar System 1. Large bodies have orderly motions and are isolated from each other – All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane – The Sun and most of the planets rotate in this same direction as well – And most moons orbit their planet in the direction it rotates Solar System Formation -- Characteristics of Our Solar System 2. Planets fall into two main categories Small, rocky “terrestrial” planets near the Sun Large, hydrogen-rich “jovian” planets far from the Sun Solar System Formation -- Characteristics of Our Solar System 2. Planets fall into two main categories Solar System Formation -- Characteristics of Our Solar System 3. Swarms of asteroids and comets populate the solar system – Asteroids are concentrated in the asteroid belt Solar System Formation -- Characteristics of Our Solar System 3. Swarms of asteroids and comets populate the solar system – Asteroids are concentrated in the asteroid belt – Comets populate the regions known as the Kuiper belt and the Oort cloud Solar System Formation -- Characteristics of Our Solar System 4. Several notable exceptions to these general trends stand out – Planets with unusual axis tilts – Surprisingly large moons – Moons with unusual orbits Summary of Characteristics of Our Solar System 1. Large bodies in the solar system have orderly motions and are isolated from each other – All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane – The Sun and most of the planets rotate in this same direction as well – Most moons orbit their planet in the direction it rotates 2. Planets fall into two main categories – Small, rocky terrestrial planets near the Sun – Large, hydrogen-rich jovian planets farther out • The jovian planets have many moons and rings of rock and ice 3. Swarms of asteroids and comets populate the solar system – Asteroids are concentrated in the asteroid belt – Comets populate the regions known as the Kuiper belt and the Oort cloud 4. Several notable exceptions to these general trends stand out – Planets with unusual axis tilts – Surprisingly large moons – Moons with unusual orbits any successful theory of solar system formation must account for these Solar System Formation – The Nebular Theory • The nebular theory is the best current explanation of our solar system • It is not a new idea… …the philosophers Emanuel Swedenborg and Immanuel Kant suggested it in the 1700s • And like all scientific theories, it is still being refined and improved Solar System Formation – The Nebular Theory • It starts with cold interstellar clouds of gas and dust • These clouds are mostly hydrogen and helium from the Big Bang • But they contain heavier elements that were not formed in the Big Bang • Astronomers call these “metals” (even though they’re not necessarily metallic elements) • Where did these heavier elements come from? • They came from stars! Solar System Formation – The Nebular Theory • Stars make heavier elements from lighter ones through nuclear fusion Solar System Formation – The Nebular Theory • Stars make heavier elements from lighter ones through nuclear fusion • The heavy elements (the “metals”) mix into the interstellar medium when the stars die Solar System Formation – The Nebular Theory • Stars make heavier elements from lighter ones through nuclear fusion • The heavy elements (the “metals”) mix into the interstellar medium when the stars die • And then new stars form from the enriched gas and dust • And the cycle continues Solar System Formation – The Nebular Theory • And at the same time stars are forming… …planetary systems can form • Here’s how it works… Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space • The cloud begins to collapse …WHY would this happen?... Local density increase Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space • The cloud begins to collapse -- local density increase • As it collapses it begins to spin faster …WHY would this happen?... Conservation of angular momentum Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space • The cloud begins to collapse -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out …WHY would this happen?... Collisions and conservation of angular momentum Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space • The cloud begins to collapse -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out – collisions and conservation of angular momentum • At the same time, it begins to heat up in the center …WHY would this happen?... Conversion of gravitational potential energy into thermal energy Solar System Formation – The Nebular Theory • A large cloud -- a nebula perhaps 1 light year across -- floats in space • The cloud begins to collapse -- local density increase • As it collapses it begins to spin faster -- conservation of angular momentum • And as it spins faster, it flattens out -- collisions • At the same time, it begins to heat up in the center -- conversion of potential to thermal energy • And when it gets hot enough, a star forms in the center • And in the disk around the forming star, planets can form • What type of planets can form depends on what the cloud is made of… Solar System Formation – The Nebular Theory • This is what our own cloud—the solar nebula—was made of • But how do we know this? Solar System Formation – The Nebular Theory • This is what our own cloud—the solar nebula—was made of • But how do we know this? This is how… • …the absorption line spectrum of the Sun • It tells us the composition of the gas on the surface of the Sun Solar System Formation – The Nebular Theory • This is the composition of the Sun’s surface gas – its atmosphere • We think the solar nebula had the same composition • But a skeptic might say, is it reasonable to say this? Solar System Formation – The Nebular Theory • After all, the solar nebula collapsed 4.6 billion years ago • The Sun’s been making new atoms with nuclear fusion ever since • That’s how it generates the energy that gives us sunlight • Wouldn’t this change the composition of the Sun’s atmosphere? • The answer has to do with where the new atoms are being made… Solar System Formation – The Nebular Theory • The sunlight-generating fusion reactions happen in the Sun’s core • The core is in the Sun’s center, far from the surface, and held in by intense gravity • So the surface layers should be essentially unchanged • And their composition should be very similar to the solar nebula the Sun formed from Solar System Formation – The Nebular Theory • So it seems reasonable that the Sun’s atmosphere is similar to the nebula it formed from Solar System Formation – The Nebular Theory • The key to the nebular theory is the condensation temperature of these materials • That’s the temperature at which they condense into solid form • The nebula was initially very cold, so everything except H and He was in solid form • But it heated up as it collapsed… • …and the temperature was different at different distances from the center Solar System Formation – The Nebular Theory • This image shows a graph of a modeled temperature profile of the solar nebula… …along with an artist’s rendition of the nebula • The temperature was hottest in the center, and went down away from the center • There was a mixture of metals, rocks, and hydrogen compounds throughout the nebula • These could only be solid where the temperature was below their condensation temperature • So different chemical components of the nebula condensed at different distances • A mixture of solid rock and metal existed out to about 4.5 AU from the center • At 4.5 AU, the temperature dropped low enough for hydrogen compounds to condense, too • The boundary between where they