Learning Objectives What are the features of our Solar System? Where are asteroids, comets and each 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 guides the types of planets that form at each distance 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?
What is the Age of the Solar System? Earth: oldest rocks are 4.4 billion years Moon: oldest rocks are 4.5 billion years Mars: 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 Sunʼs hydrogen has been fused so far estimate an age of 4.5 billion years (next lecture) Age of Solar System is around 4.6 billion years old
Solar Nebula 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 Nebula 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 toward the center of the cloud, and the cloud contracts
But.. Not all 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 fall towards the middle of the cloud of dust, their gravitational potential energy is turned into heat The stuff nearest the center starts to give off light and heat. This is the protosun Protosun forms in ﬁrst 2-3 slides
But weʼre not ﬁnished yet… Because this is not yet the Sun as we know it. Its energy is still coming entirely from gravitational contraction Eventually, it becomes so hot and dense, fusion begins (hydrogen converts to helium, and energy is released) Initial contraction to hydrogen ignition takes about 100 million years Protosun forms in ﬁrst 2-3 slides
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 Collisionsbuild up inner planets, outer planet cores
Why are the Planets Different? Temperature (and thus distance from the center of the cloud) is the key factor!
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 almost all matter in 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 havenʼt been cleared from the area, the outer planets gravitationally attract huge amounts of these gases. Planets become much bigger than inner planets
Heavy Bombardment There were billions of planetesimals in the early Solar System Many collided with the young planets Look at the Moon & Mercury Period of heavy bombardment Lasted for about the ﬁrst 800 million years of the Solar System (after which most of the 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 Most stuff goes into the planets Asteroids and comets are left-over planetesimals “Fossils” of Solar System birth The Solar System continues to evolve, but slower Outer planets still contracting Earth and Venus are still volcanically active Impacts from left-over planetesimals continue