Solar system

Making a Solar System

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?

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 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 gravitationallyunstable

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

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

But..

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 Collisions build up inner

planets, outer planet cores

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

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 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

The Constellation

Orion

The Orion

Nebula

Disks around Young Stars are Common

Next Time

Our Beacon: The Sun