Download Chapter 6 Lecture 4

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.
Fig 9.5
Inside the frost line: too hot for hydrogen compounds to form ices.
Outside the frost line: cold enough for ices to form.
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
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.