ASTRONOMY 202 Spring 2007: Solar System Exploration
Instructor: Dr. David Alexander
Web-site: www.ruf.rice.edu/~dalex/ASTR202_S07
Class 22: Formation of the Solar System [3/14/07]
Announcements
The Nebular Theory
The Two types of Planets
Age of the Solar System
Extrasolar Planets
Chapters 7&8
Now Playing:
Sun Hits the Sky - Supergrass
Announcements
Web Project
Window is open to choose web project – window closes March 26
See class web-site for details and some news sites
- science.nasa.gov is a good one for solar system
Observing
This week is lousy but we will try to get a night next week and I will also try to get the solar telescopes
working
Homework #5
Homework #5 is now available online
Happy Birthday Albert Einstein (1879 – 1955)
"Reality is merely an illusion,
albeit a very persistent one."
"Anyone who has never made a
mistake has never tried anything
new."
“Wisdom is not a product of
schooling but of the lifelong
attempt to acquire it."
"The most incomprehensible
thing about the world is that
it is comprehensible."
The origin of the solar system
Several ‘theories’ have been promoted to explain the origin of the
solar system.
None have been as successful as The Nebular Theory.
The basis of the Nebular theory can be traced to Immanuel Kant, who in 1755 suggested
that the solar system formed from the collapse of an interstellar cloud of gas and dust.
This was independently put forward by Laplace in 1795.
The Latin for cloud is nebula, hence The Nebular Theory.
1 – 5 light years
~200 AU
The Solar Nebula
Modern observations show that stars, perhaps with planetary systems
around them, are forming out of low density interstellar clouds.
NGC 3603
Orion Nebula
Protoplanetary disks in Orion
The Collapse of the Solar Nebula
The formation of the solar system as we know it is a result of the
conservation of energy (gravitational to kinetic to thermal) and the
conservation of angular momentum.
• Heating
As the nebula collapses, the density and
temperature rapidly increases.
• Spinning
The collapsing sphere rotated faster and faster
as it collapsed.
• Flattening
Collisions in the spinning, collapsing nebula
result in the sphere flattening to form a disk.
• Emptying the Nebula
The three process of heating,
spinning, and flattening explain
the tidy layout of the solar
system.
A plasma wind from the newly formed stars
sweeps the bulk of the hydrogen and helium gas
from the solar system.
The clearing out of the gas early in the history
of the solar system was crucial to determining
the final nature of the solar system.
The Two Types of Planets
To understand this aspect of the solar system we need to know the
composition of the solar nebula and then how this tenuous loosely bound
collection of gas and dust interacted to form the various planets.
Galactic Recycling
The production of elements in the Big
Bang produced Hydrogen, Helium and
small amounts of Lithium.
The heavier elements were produced in
the cores of massive stars early in the
history of the Universe.
The scattering of this material throughout
the Universe provided the basic building
blocks for other stars, planets and us.
The Solar System is 98% H and He, with
the other 2% left over for everything else.
The Two Types of Planets
Condensation: The importance of the frost line.
In the outer parts of the collapsing nebula gravity needed a hand.
The cool temperatures in the outer reaches of the nebula allowed solid
particles to condense out of the gas.
• Hydrogen and Helium gas
98% by mass, do not condense at nebular temperatures
• Hydrogen compounds
Form ices of methane (CH4), ammonia (NH3) and water
(H2O) below 150 K. Made up about 1.4% of mass.
• Rock
Mostly silicon-based minerals making up about 0.4% nebular mass.
Rock is gaseous above 500 – 1300 K.
• Metals
E.g. iron, nickel, aluminium making up ~0.2% of the mass.
Gaseous metals present above 1600 K.
The Two Types of Planets
Condensation: The importance of the frost line.
Different ‘seeds’ for condensation form at different parts of the collapsing nebula.
The Frost Line is defined by the temperature at which the H, He and H compounds
could condense out, i.e ~150 K.
In the Solar System, the frost line lies between the orbit of Mars and Jupiter.
The Two Types of Planets
Accretion
Once condensation had formed the ‘seeds’ of the clumping material,
collisions between them resulted in the seeds growing to a point were
gravity started to take over. The growth process is know as accretion.
The larger amounts of building materials in the outer nebula resulted in
planetesimals beyond the frost line being much bigger than the metallic
and rocky planets of within the frost line. This in turn allowed for the
accumulation of the H and He gas resulting in the gas giant planets.
The path from the production of the basic building blocks through galactic recycling
through the processes of condensation and accretion results in a solar system which
looks very much like our own.
Forming the Solar System
The Age of the Solar System
We can determine the age of the
solar system by measuring the
ages of the rocks within it using
a process known as radiometric
dating.
The oldest Earth rocks solidified 4
billion years ago.
Lunar rocks have yielded an age of
~4.5 billion yrs.
Radioactive isotopes in rock
undergo spontaneous change
(radioactive decay) from one
isotope to another or one
element to another. By
measuring the amounts of the
different isotopes and elements
we can determine how long it
has been since the rock
solidified.
Extra-Solar Planets
We are now able to investigate
some rudimentary aspects of
the star system formation
process around other stars.