Transcript ASTR100 Class 01 - University of Maryland Department of

ASTR100 (Spring 2008)
Introduction to Astronomy
The Formation of Planets
Prof. D.C. Richardson
Sections 0101-0106
Qu ickTime™ and a
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What features of our solar system
provide clues to its origin?
Motion of Large Bodies
 All large bodies
in the solar
system orbit in
the same
direction and in
nearly the same
plane.
 Most also rotate
in that direction.
Two Major Planet Types
 Terrestrial
planets are
rocky, relatively
small, and close
to the Sun.
 Jovian planets
are gaseous,
larger, and
farther from the
Sun.
Swarms of Smaller Bodies
 Many rocky
asteroids and
icy comets
populate the
solar system.
Notable Exceptions
 Several
exceptions to
normal patterns
need to be
explained.
What theory best explains the
features of our solar system?
The Nebular Theory
 According to the
nebular theory,
our solar system
formed from a
giant cloud of
interstellar gas.
(nebula = cloud)
Orion Nebula
The Nebular Theory
 We can see
stars forming
in other
interstellar
clouds, lending
support to the
nebular theory.
Proplyds in the
Orion Nebula
What caused the orderly patterns of
motion in our solar system?
Conservation of Angular Momentum
 The rotation speed
of the cloud from
which our solar
system formed
increased as the
cloud contracted.
Conservation of Angular Momentum
 Rotation of
contracting cloud
speeds up for same
reason a skater
speeds up as she
pulls in her arms…
Collisions
 Collisions between
particles in the cloud
caused it to flatten
into a disk.
Collisions
 Collisions between
gas particles in a
cloud gradually
reduce random
motions.
Collisions
 Collisions between
gas particles also
reduce up and down
motions.
Collisions
 The spinning cloud
flattens as it shrinks.
Disks Around Other Stars
 Observations of disks around other stars
support the nebular hypothesis.
Why are there two major types of
planets?
Conservation of Energy
 As gravity causes the
cloud to contract, it
heats up.
Disk Temperature
 Inner parts of disk
are hotter than outer
parts.
 Rock can be solid at
much higher
temperatures than
ice.
The Frost Line
 Inside the frost line: too hot for hydrogen
compounds to form ices.
 Outside the frost line: cold enough for ices to
form.
Formation of Terrestrial Planets
 Small particles of rock and metal were
present inside the frost line.
 Planetesimals of rock and metal built
up as these particles collided.
 Gravity eventually assembled these
planetesimals into terrestrial planets.
Formation of Terrestrial Planets
 Tiny solid particles
stick to form
planetesimals.
 Gravity draws
planetsimals
together to form
planets.
 This process of
assembly is called
accretion.
Accretion of Planetesimals
 Many smaller objects collected into just a few
large ones.
Formation of Jovian Planets
 Ice could also form small particles
outside the frost line.
 Larger planetesimals and planets were
able to form.
 Gravity of larger planets was able to
draw in surrounding H and He gases.
Formation of Jovian Planets
 The gravity of rock
and ice in jovian
planets draws in H
and He gases.
Formation of Jovian Moons
 Moons of jovian planets form in miniature
disks.
The Solar Wind
 Radiation and
outflowing matter
from the Sun—the
solar wind—blew
away the leftover
gases.
Where did asteroids and comets
come from?
Asteroids and Comets
 Leftovers from the accretion process.
 Rocky asteroids inside frost line.
 Icy comets outside frost line.
Heavy Bombardment
 Leftover
planetesimals
bombarded other
objects in the late
stages of solar
system formation.
Origin of Earth’s Water
 Water may have
come to Earth by
way of icy
planetesimals from
the outer solar
system.
How do we explain the existence of our
Moon and other exceptions to the “rules”?
Giant Impacts
 Earth’s Moon was
probably created
when a giant
planetsimal slammed
into the newly
forming Earth.
 Other large impacts
may explain other
exceptions, like
Venus’ rotation and
Uranus’ tilt.
Movie 1
Movie 2
Captured Moons
 The unusual “irregular” satellites of some
planets may be captured planetesimals.
Review of
nebular
theory
Fig 6.24
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.
Thought Question
Suppose you find a rock originally
made of potassium-40, half of which
decays into argon-40 every 1.25 billion
years. You open the rock and find 3
atoms of argon-40 for every 1 atom of
potassium-40. How old is the rock?
1. 1.25 billion years.
2. 2.5 billion years.
3. 5 billion years.
4. It is impossible to determine.
Thought Question
Suppose you find a rock originally
made of potassium-40, half of which
decays into argon-40 every 1.25 billion
years. You open the rock and find 3
atoms of argon-40 for every 1 atom of
potassium-40. How old is the rock?
1. 1.25 billion years.
2. 2.5 billion years.
3. 5 billion years.
4. It is impossible to determine.