Making a Solar System

! 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?

Learning Objectives

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: all about 4.6 billion years old!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 gravitationallyunstable

! 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

! 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

But..

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 The protosun (a type of star known as a T Tauri protostar) forms in images 2–3

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 50 million years

The protosun (a type of star known as a T Tauri protostar) forms in images 2–3

Planet Formation in the Disk! Heavy elements clump

!Form dust grains! Dust grains collide and

stick together!Form planetesimals!Like asteroids & comets

! Bigger planetesimals attract smaller ones !Collisions build up the inner

planets, the outer planet cores

Temperature (and so distance from the protosun) is the key factor in the Nebular Theory

Why are the planets different, according to the Nebular Theory?

! 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 outer space! Rock & ice “seeds” grew large enough to

pull gasses (H, He) onto themselves

Why are the Planets Different?

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

! Computer models show it takes a few hundred million years to form ~four inner planets

! The Sun would have begun fusion before then

Formation of the Inner 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

Formation of the Outer 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

! Near Jupiter & Saturn

! Near Uranus & Neptune

! Beyond Neptune

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko.

2014 MU69 (Ultima Thule)

Results! Planetesimals collect to become the planets! Asteroids and comets are the left-over

planetesimals!The “fossils” of the birth of the Solar System

! The Solar System continues to change, but more slowly!The outer planets are still contracting!The Earth & Venus are still volcanically active!Some impacts from remnant planetesimals

continue

TheConstellation

Orion

The Orion

Nebula

Disks around Young Stars are Common

Zooming in on Protoplanetary Disks with ALMA

www.eso.org/public/czechrepublic/images/eso1611a/ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)

Next Time

Our Beacon: The Sun