Chapter 7:The Birth and

Evolution of Planetary Systems

Where did the solar system come from? How was it

made?

“Facts” that must be accounted for in any theory of solar system

formation• All the major planets orbit in almost the same plane

• All the planets orbit in the same direction• Almost all the planets rotate in the same direction

as they orbit• The inner planets are rocky bodies while the outer

planets are gaseous and/or icy bodies• 99% of the mass of the solar system is in the Sun• Most of the angular momentum of the solar system

is in the planets, not the SunLook at ClassAction Solar System Properties Explorer

in the Solar System Characteristics module

We start with a cold cloud of gas and dust

Because of the internal motions of the gas and dust, the cloud almost

always has some slight overall rotation

The cloud starts to collapse due

to gravityAngular momentum causes the cloud’s initial slow rotation to spin faster and flatten out

Angular momentum is what causes a skater to “spin-up”

Angular momentum depends on both the velocity, V, and the size, R. If R decreases, V must increase. It is also what causes the pizza dough to flatten out when tossed

The “Spin-up” causes the cloud to flatten out

Angular momentum keeps stuff from falling straight in. Instead, it spirals down onto a disk. This is the pizza toss effect

At this point we have something that looks like a star

surrounded by a disk of gas and dust

The protostellar Sun is getting its energy from gravitational collapse, not from fusion like “normal”

stars.

The temperature in the protoplanetary disk falls off as

you get farther from the protosun

Check out planet Formation Temperature Plot on ClassAction website Solar System Characteristics module

The solar nebula is composed mostly of hydrogen and

helium

The most common things to condense will be hydrides of carbon (CH4…methane), nitrogen (NH3…ammonia) and oxygen (H2O…water). These condense at fairly low temperatures. Elements like silicon and iron condense at higher temperatures.

What is found at different distances from the protosun depends on temperature and

abundance

Condensation begins to form dust grains

The dust grains are tiny: about the size of particles in smoke. They are also charged with static

electricity

The dust grains quickly start

sticking togetherClose to the protosun the

grains are exclusively silicon, iron and other heavy elements: “rocky” materials. Farther out there are more grains of “icy” materials than rocky ones. Static electricity also plays an important part in making the grains stick together

Accretion is a snowball effect that builds larger and larger

objects

Eventually Planetesimals are formed

Close to the Sun the planetesimals look like asteroids

Far from the Sun the planetesimals are a mix

of ice and rock

Planetesimals merge to

form protoplanets

The larger the planetesimal, the stronger its gravity is. The stronger its gravity, the more it attracts stuff and the more violent the collisions become.

The gas giants form a large core of ice and rock and then grow by

sweeping up large amounts of gas

When the gasses get blown away, the condensation

phase ends

The Solar Nebula Stage

Condensation starts and planetesimals begin growing

The Accretion Stage

Planetesimals grow bigger by collisions. There may be hundreds of moon sized protoplanets form in the inner solar system. The outer planets have grabbed up the last of the gas

The accretion stage was a violent period with planet

smashing collisions

Finally, we have a new star and new planets

The new planets at this stage are nothing like the planets we see today. They will evolve over time to become the eight planets we see now

The gas giants were like mini solar systems, forming a

system of moons

Finding extra-solar planets

Our theory was designed to explain the formation of our solar system. How does it match up with other planetary systems around other stars?

We have seen lots of disks around forming stars

confirming some of the nebular theory

Actually seeing a planet has only recently been

done

Newton’s 3rd Law applies to the Sun and planets

If the Sun tugs on Jupiter, keeping it in orbit, then Jupiter tugs on the Sun, making it orbit. The two actually orbit a common point just outside the surface of the Sun

Watch ClassAction Extrasolar Planet module Influence of Planets on the Sun animation

The Doppler Effect technique detects the motion of a star

caused by a planet

Watch ClassAction Extrasolar Planet module Radial Velocity Graph animation

The transit method measures a planet directly if it passes in

front of its star

The planet will be a dark spot passing across the face of the star. The dimming of the light from the star may be tiny but it is measurable if the planet is large enough.

OGLE detects gravitational microlensing caused by a

planet

The Doppler method is the most prolific but it finds large mass planets close to their

star

Visit http://exoplanet.eu

So what do we do about our solar nebula model?

The basic modification is that things move, sometimes they move a lot: Migration theory

Our model predicted small rocky planets close to the star

We are finding large gas giants close to their star!