Chapter 8
Formation of Planetary Systems
Our Solar System and Beyond
Where did the solar system come
from?
• According to the
nebular theory, our
solar system formed
from a giant cloud of
interstellar gas.
• (nebula = cloud)
Evidence from Other Gas Clouds
• We can see
stars forming
in other
interstellar gas
clouds, lending
support to the
nebular theory.
The Orion Nebula with Proplyds
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
must have
increased as the
cloud contracted.
Flattening
•
Collisions between
particles in the
cloud caused it to
flatten into a disk.
The spinning
cloud
flattens as it
shrinks.
Formation of the Protoplanetary Disk
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.
Inner parts of
the disk are
hotter than
outer parts.
Rock can be
solid at much
higher
temperatures
than ice.
Temperature Distribution of the Disk and the Frost Line
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
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.
Tiny solid
particles stick to
form
planetesimals.
Summary of the Condensates in the Protoplanetary Disk
Gravity draws
planetesimals
together to form
planets.
This process of
assembly
is called
accretion.
Summary of the Condensates in the Protoplanetary Disk
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 from ices as well as metals
The gravity of these larger planets was able to
draw in surrounding H and He gases.
Moons of jovian planets form in miniature disks.
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
How do we explain the existence
of our Moon and other exceptions
to the rules?
Heavy Bombardment
• Leftover
planetesimals
bombarded
other objects
in the late
stages of solar
system
formation.
Giant Impact
Giant impact stripped matter from Earth’s crust
Stripped matter began to orbit
Then accreted into Moon
Origin of Earth’s Water
• Water may
have come to
Earth by way
of icy
planetesimals
from the outer
solar system.
Odd Rotation
• Giant impacts
might also
explain the
different
rotation axes
of some
planets.
Captured Moons
•
The unusual moons of some planets may
be captured planetesimals.
Review of
nebular theory
Thought Question
How would the solar system be different if the solar
nebula had only been half as hot?
A. Jovian planets would have formed closer to
the Sun.
B. There would be no asteroids.
C. There would be no comets.
D. Terrestrial planets would be larger.
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.
Radioactive Decay
• Some isotopes
decay into
other nuclei.
• A half-life is
the time for
half the nuclei
in a substance
to decay.
Thought Question
Suppose you find a rock originally made of potassium40, half of which decays into argon-40 every 1.25
billion years. You open the rock and find 15 atoms of
argon-40 for every atom of potassium-40. How long
ago did the rock form?
A.
B.
C.
D.
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago
Dating the Solar System
Age dating of meteorites
that are unchanged since
they condensed and
accreted tells us that the
solar system is about 4.6
billion years old.
Dating the Solar System
•
•
•
Radiometric dating tells us that the oldest
moon rocks are 4.4 billion years old.
The oldest meteorites are 4.55 billion
years old.
Planets probably formed 4.5 billion years
ago.
How do we detect planets around
other stars?
Planet Detection
• Direct: Pictures or spectra of the planets
themselves
• Indirect: Measurements of stellar
properties revealing the effects of orbiting
planets
•
For more check out this website: http://astro.unl.edu/naap/esp/detection.html
Direct Detection
• Special techniques for concentrating or eliminating
bright starlight are enabling the direct detection of a very
few extrasolar planets.
Indirect detection : Gravitational
Tugs
• The Sun and Jupiter
orbit around their
common center of
mass.
• The Sun therefore
wobbles around that
center of mass with
the same period as
Jupiter.
Gravitational Tugs
• Sun’s motion around
solar system’s center
of mass depends on
tugs from all the
planets.
• Astronomers who
measured this motion
around other stars
could determine
masses and orbits of
all the planets.
Astrometric Technique
• We can detect planets
by measuring the
change in a star’s
position in the sky.
• However, these tiny
motions are very
difficult to measure
(~0.001 arcsecond).
Doppler Technique
• Measuring a star’s
Doppler shift can tell
us its motion toward
and away from us.
• Current techniques
can measure motions
as small as 1 m/s
(walking speed!).
First Extrasolar Planet Detected
• Doppler shifts of star
51 Pegasi indirectly
reveal planet with 4day orbital period
• Short period means
small orbital distance
• First extrasolar planet
to be discovered
(1995)
First Extrasolar Planet Detected
• The planet around 51 Pegasi has a mass similar to
Jupiter’s, despite its small orbital distance.
Thought Question
Suppose you found a star with the same mass as
the Sun moving back and forth with a period of
16 months. What could you conclude?
A.
B.
C.
D.
It has a planet orbiting at less than 1 AU.
It has a planet orbiting at greater than 1 AU.
It has a planet orbiting at exactly 1 AU.
It has a planet, but we do not have enough
information to know its orbital distance.
Transits and Eclipses
• A transit is when a planet crosses in front of a star.
• The resulting eclipse reduces the star’s apparent brightness and
tells us the planet’s radius.
• When there is no orbital tilt, an accurate measurement of planet
mass can be obtained.
How do extrasolar planets compare
with those in our solar system?
Measurable Properties
• Orbital period, distance, and shape
• Planet mass, size, and density
• Composition
Orbits of Extrasolar Planets
• Most of the detected
planets have orbits
smaller than
Jupiter’s.
• Planets at greater
distances are harder
to detect with the
Doppler technique.
Orbits of Extrasolar Planets
• Most of the detected
planets have greater
mass than Jupiter.
• Planets with smaller
masses are harder to
detect with the
Doppler technique.
Planets: Common or Rare?
• Observations indicate ~2-5% of sun-like
stars have Jupiter size planets
• The others may still have smaller (Earthsized) planets that cannot be detected using
current techniques.
• Current estimations range between 20 and
50% of sun-like stars having planets.
Surprising Characteristics
• Some extrasolar planets have highly
elliptical orbits.
• Some massive planets orbit very close to
their stars: “Hot Jupiters.”
Hot Jupiters
Do we need to modify our theory
of solar system formation?
Revisiting the Nebular Theory
• Nebular theory predicts that massive
Jupiter-like planets should not form inside
the frost line (at << 5 AU).
• The discovery of “hot Jupiters” has forced a
reexamination of nebular theory.
• “Planetary migration” or gravitational
encounters may explain “hot Jupiters.”
Planetary Migration
• A young planet’s
motion can create
waves in a planetforming disk.
• Models show that
matter in these waves
can tug on a planet,
causing its orbit to
migrate inward.
Gravitational Encounters
• Close gravitational encounters between two
massive planets can eject one planet while
flinging the other into a highly elliptical
orbit.
• Multiple close encounters with smaller
planetesimals can also cause inward
migration.
Modifying the Nebular Theory
• Observations of extrasolar planets have
shown that the nebular theory was
incomplete.
• Effects like planet migration and
gravitational encounters might be more
important than previously thought.
Chapter 12
Asteroids, Comets, and Dwarf Planets
Their Nature, Orbits, and Impacts
Asteroid Facts
• Asteroids are rocky leftovers of planet formation.
• The largest is Ceres, diameter ~1,000 km.
• There are 150,000 in catalogs, and probably over a
million with diameter >1 km.
• Small asteroids are more common than large asteroids.
• All the asteroids in the solar system wouldn’t add up to
even a small terrestrial planet.
Asteroid Orbits
• Most asteroids orbit
in a belt between
Mars and Jupiter.
• Trojan asteroids
follow Jupiter’s
orbit.
• Orbits of near-Earth
asteroids cross
Earth’s orbit.
Origin of Asteroid Belt
• Rocky planetesimals
between Mars and
Jupiter did not
accrete into a planet.
• Jupiter’s gravity,
stirred up asteroid
orbits and prevented
their accretion into a
planet.
Origin of Meteorites
• Most meteorites are pieces of asteroids.
Meteor Terminology
• Meteoroid: A small piece of rock or dust in
space. Not as big as an asteroid.
• Meteor: A meteoroid that is falling through
the Earth’s atmosphere. (shooting stars)
• Meteorite: A rock from space that falls
through Earth’s atmosphere and reaches
Earth..
Comet Facts
• Formed beyond the frost line, comets are icy
counterparts to asteroids.
• The nucleus of a comet is like a “dirty snowball.”
• Most comets do not have tails.
• Most comets remain perpetually frozen in the
outer solar system.
• Only comets that enter the inner solar system grow
tails.
Anatomy of a Comet
• Coma is atmosphere
that comes from
heated nucleus.
• Plasma tail is gas
escaping from coma,
pushed by solar
wind.
• Dust tail is pushed
by photons.
Growth of Tail
Comets eject small particles that follow the comet around in its
orbit and cause meteor showers when Earth crosses the comet’s
orbit.
Meteors in a shower appear to emanate from the same area of sky
because of Earth’s motion through space.
Only a tiny number of
comets enter the inner
solar system; most
stay far from the Sun.
Oort cloud:
On random orbits
extending to about
50,000 AU
Kuiper belt:
On orderly orbits
from 30–100 AU in
disk of solar system
How did they get there?
• Kuiper belt objects are believed to have formed in the their
current location along with the formation of the rest of the
solar system. They orbit in a flat plane, aligned with the
plane of planetary orbits, orbiting in the same direction as
the planets.
• Oort cloud objects were once closer to the Sun, but they
were kicked out there by gravitational interactions with
jovian planets: spherical distribution, orbits in any
direction.
Is Pluto a Planet?
•
•
•
•
•
Much smaller than the eight major planets
Not a gas giant like the outer planets
Has an icy composition like a comet
Has a very elliptical, inclined orbit
Pluto has more in common with comets than with the
eight major planets.
Discovering Large Iceballs
• In summer 2005,
astronomers
discovered Eris, an
iceball even larger
than Pluto.
• Eris even has a
moon: Dysnomia.
Dwarf Planets
• There are many icy
objects like Pluto on
elliptical, inclined orbits
beyond Neptune.
• The largest ones are
comparable in size to
Earth’s Moon.
• If the object is massive
enough for its own
gravity to make it
spherical(ish) then it is
called a dwarf planet.
Kuiper Belt Objects
• These large, icy
objects have orbits
similar to the
smaller objects in
the Kuiper Belt that
become short period
comets.
• So are they very
large comets or very
small planets?
HST’s view of Pluto and moons
9.4 Cosmic Collisions: Small Bodies
Versus the Planets
Our goals for learning:
• Have we ever witnessed a major impact?
• Did an impact kill the dinosaurs?
• Is the impact threat a real danger or just
media hype?
• How do other planets affect impact rates
and life on Earth?
Mass Extinctions
• Fossil record shows occasional large dips in
the diversity of species: mass extinctions.
• The most recent was 65 million years ago,
ending the reign of the dinosaurs.
Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks
but is often found in meteorites.
• Luis and Walter Alvarez found a worldwide
layer containing iridium, laid down 65
million years ago, probably by a meteorite
impact.
• Dinosaur fossils all lie below this layer.
Iridium Layer
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
lower rock layers
Likely Impact Site
• Geologists found a
large subsurface
crater about 65
million years old in
Mexico.
Frequency of Impacts
• Small impacts
happen almost daily.
• Impacts large
enough to cause
mass extinctions are
many millions of
years apart.