Our Solar System and Its Origin

6.4 The Formation of Planets

Our Goals for Learning• Why are there two types of planets?• Where did asteroids and comets come

from?• How do we explain the existence of our

Moon and other “exceptions to the rules”?

• When did the planets form?

Four Unexplained Features of our Solar System

1) Why do large bodies in our solar system have orderly motions?

--> 2) Why are there two types of planets?

3) Where did the comets and asteroids come from?

4) How can we explain the exceptions to the ‘rules’?

Why are there two types of planet, when all planets formed from the same nebula?

As gravity causes cloud to contract, it heats up

Conservation of energy

Inner parts of disk are hotter than outer parts.

Rock can be solid at much higher temperatures than ice.

Inside the frost line: too hot for hydrogen compounds to form ices.

Outside the frost line: cold enough for ices to form.

Fig 9.5

Tiny solid particles stick to form planetesimals.

Gravity draws planetesimals together to form planets

This process of assembly is called accretion

Gravity of rock and ice in jovian planets draws in H and He gases

Moons of jovian planets form in miniature disks

Why are there two types of planets?

1. Outer planets get bigger because abundant hydrogen compounds condense to form ICES.

2. Outer planets accrete and keep H & He gas because they’re bigger.

Four Unexplained Features of our Solar System

√ Why do large bodies in our solar system have orderly motions?

√ Why are there two types of planets?

--> 3) Where did the comets and asteroids come from?

4) How can we explain the exceptions the the ‘rules’ above?

Comets and asteroids are leftover planetesimals. • Asteroids are rocky because they formed inside the frostline. • Comets are icy because they formed outside the frostline

Outflowing matter from the Sun -- the solar wind -- blew away the leftover gases

Four Unexplained Features of our Solar System

√ Why do large bodies in our solar system have orderly motions?

√ Why are there two types of planets?

√ Where did the comets and asteroids come from?

--> 4) How do we explain the existence of our Moon and other “exceptions to the rules”?

Earth’s moon was probably created when a big planetesimal slammed into the newly forming Earth.

Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus

Review of nebular theory

Fig 6.27

Four Features of our Solar System - Explained

√ Why do large bodies in our solar system have orderly motions?

√ Why are there two types of planets?

√ Where did the comets and asteroids come from?

√ How do we explain the existence of our Moon and other “exceptions to the rules”?

When did the planets form?

We cannot find the age of a planet, but we can find the ages of the rocks that make it up

We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it

The decay of radioactive elements into other elements is a key tool in finding the ages of rocks

Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old.

What have we learned?• Why are there two types of planets?• Planets formed around solid “seeds” that condensed from

gas and then grew through accretion. In the inner solar system, temperatures were so high that only metal and rock could condense. In the outer solar system, cold temperatures allowed more abundant ices to condense along with metal and rock.

What have we learned?• Where did asteroids and

comets come from?• Asteroids are the

rocky leftover planetesimals of the inner solar system, and comets are the icy leftover planetesimals of the outer solar system.

• How do we explain the existence of our Moon and other “exceptions to the rules”?

•Most of the exceptions probably arose from collisions or close encounters with leftover planetesimals, especially during the heavy bombardment that occurred early in the solar system’s history. Our Moon is probably the result of a giant impact between a Mars-size planetesimal and the young Earth.

What have we learned?

• When did the planets form?

• The planets began to accrete in the solar nebula about 4.6 billion years ago, a fact we determine from radiometric dating of the oldest meteorites.