Making a Solar System
Learning Objectives! What are our Solar System s broad features? Where are asteroids, comets and each type of planet? Where is most of the mass? In what direction do planets orbit and rotate?! How does the Nebular Theory explain our Solar System?! If the Solar System forms from a ball (sphere) of gas, why do the planets orbit in a flat plane (the ecliptic)?! What is the source of energy of the protosun? What key factor in the Nebular Theory guides which planets form at different distances from the Sun? What is the Frost Line?! How do planets form from planetesimals? How do planetesimals relate to the Kuiper and Asteroid Belts? The Oort Cloud? The Period of Heavy Bombardment?! Are there problems with the Nebular Theory?
What is the Age of the Solar System?! Earth: oldest rocks are 4.4 billion years!moon: oldest rocks are 4.5 billion years! Meteorites: oldest are 4.6 billion years!sun: fusion models based on energy output and how much of the Sun s hydrogen has been fused so far estimate an age of 4.6 billion years (next lectures)!the age of the Solar System is around 4.6 billion years
Solar Nebular Theory! Proposed by the German philosopher Immanuel Kant! The Solar System formed from a spinning cloud of gas, dust, and ice! Mostly hydrogen and helium! 4.6 billion years ago
Solar Nebular Theory! In these clouds are small clumps that become gravitationally unstable! The gas and dust has mass (thus gravity)! Gravitational attraction between all particles pull them towards the center of the cloud, and the cloud contracts
But..! Not all of the mass falls in directly. Why?! The gas cloud (which is three-dimensional, of course so think of it as a ball) is spinning slightly. This causes the formation of a flattened structure as it collapses! Forms a pancake-like disk, concentrated at the center, with a rotation in the original direction of spin
At the center of this mess! the Sun begins to form! As gas and dust collapse into the cloud s middle, the energy from the gravitational fall turns to heat! Stuff at the center starts to give off light and heat! This is the protosun Protosun forms in first 2-3 images
But we re not finished yet! This is not yet the Sun as we know it. Its energy is still coming entirely from gravitational contraction! The protosun becomes so hot and dense that nuclear fusion begins...the Sun! Initial contraction to fusion starting takes about 100 million years Protosun forms in first 2-3 images
Planet Formation in the Disk! Heavy elements clump! Form dust grains! Dust grains collide, stick together! Form planetesimals! Like asteroids & comets! Big planetesimals attract small ones! Collisions build up inner planets, outer planet cores
Why are the planets different, according to the Nebular Theory? Temperature (and so distance from the protosun) is the key factor in the Nebular Theory
Why are the Planets Different?! Temperature is the key factor! Inner Solar System: Hot! Light elements (H, He) and ices vaporized! Blown out of the inner Solar System by the solar wind! Only heavy elements (iron etc.) left! Outer Solar System: Cold! Too cold to evaporate ices to space! Rock & ice seeds grew large enough to pull gasses (H, He) onto themselves
Formation of the Inner Planets! The inner Solar System was too hot for ices and light gases to exist. Thus, planetesimals consisted entirely of heavy elements (they were just rocks)! Planetesimals run into each other, then coalesce to form protoplanets! Protoplanets accrete (attract) more planetesimals until most of the matter in the inner Solar System is swept up
Formation of the Inner Planets! Computer models show it takes a few hundred million years to form ~four inner planets! The Sun would have begun fusion by then
Formation of the Outer Planets! The process is initially very similar to the formation of the inner planets! Since it is colder at this distance, ices can exist, and planetesimals consist of rock and ices. This leads to larger protoplanets, which ultimately become the rocky, icy cores of the outer planets! Finally, because hydrogen and helium haven t been cleared from the area, the outer planets gravitationally attract huge amounts of these gases. The outer planets become much bigger than the inner planets
Heavy Bombardment! There were billions of planetesimals in the early Solar System! Many collided with the young planets! Look at the craters on the Moon and Mercury! The period of heavy bombardment! Lasted for about the first 800 million years of the Solar System (after which most planetesimals had hit something or gone into stable orbits)
Fates of the Planetesimals! Between Mars and Jupiter! Remain as the asteroids! Near Jupiter & Saturn!Ejected from the Solar System! Near Uranus & Neptune!Ejected to the Oort Cloud! Beyond Neptune!Remain in the Kuiper Belt
Results! Planetesimals collect to become the planets! Asteroids and comets are left-over planetesimals! The fossils of Solar System birth! The Solar System continues to change, but more slowly! The outer planets are still contracting! Earth and Venus are still volcanically active! Some impacts from left-over planetesimals continue
The The Constellation Orion Orion Nebula
Disks around Young Stars are Common
Discovered Exoplanets Mass (MJupiter) J S U N Me V E Ma Distance of Orbit from Parent Star (AU)
An example of an extrasolar multi-planet system - Kepler 90 Mercury Venus Earth Mars
Next Time Our Beacon: The Sun