Download Our Solar System and Beyond

Chapter 6
Formation of Planetary Systems
Our Solar System and Beyond
6.1 A Brief Tour of the Solar System
Our goals for learning:
• What does the solar system look like?
The solar system exhibits clear patterns of composition and
motion.
These patterns are far more important and interesting than
numbers, names, and other trivia.
Planets are very tiny
compared to
distances between
them.
Sun
• Over 99.9% of solar system’s mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each second
Mercury
• Made of metal and rock; large iron core
• Desolate, cratered; long, tall, steep cliffs
• Very hot and very cold: 425°C (day), –170°C (night)
Venus
• Nearly identical in size to Earth; surface hidden by clouds
• Hellish conditions due to an extreme greenhouse effect
• Even hotter than Mercury: 470°C, day and night
Earth
Earth and
Moon to scale
• An oasis of life
• The only surface liquid water in the solar system
• A surprisingly large moon
Mars
• Looks almost Earth-like, but don’t go without a spacesuit!
• Giant volcanoes, a huge canyon, polar caps, and more
• Water flowed in the distant past; could there have been
life?
Jupiter
• Much farther
from Sun than
inner planets
• Mostly H/He;
no solid surface
• 300 times more
massive than
Earth
• Many moons,
rings
Jupiter’s moons
can be as
interesting as
planets
themselves,
especially
Jupiter’s four
Galilean moons
• Io (shown here): Active volcanoes all over
• Europa: Possible subsurface ocean
• Ganymede: Largest moon in solar system
• Callisto: A large, cratered “ice ball”
Saturn
•
•
•
•
Giant and gaseous like Jupiter
Spectacular rings
Many moons, including cloudy Titan
Cassini spacecraft currently studying it
Rings are
NOT solid;
they are made
of countless
small chunks
of ice and
rock, each
orbiting like a
tiny moon.
Artist’s conception
The Rings of Saturn
Cassini probe
arrived July
2004.
(Launched in
1997)
Uranus
• Smaller than
Jupiter/Saturn;
much larger than
Earth
• Made of H/He gas
and hydrogen
compounds (H2O,
NH3, CH4)
• Extreme axis tilt
• Moons and rings
Neptune
• Similar to Uranus
(except for axis
tilt)
• Many moons
(including Triton)
Pluto and Eris
• Much smaller than other planets
• Icy, comet-like composition
• Pluto’s moon Charon is similar in size to Pluto
What have we learned?
• What does the solar system look like?
— Planets are tiny compared to the distances
between them.
— Each world has its own character, but there
are many clear patterns.
6.2 Clues to the Formation of Our Solar
System
Our goals for learning:
• What features of our solar system provide
clues to how it formed?
• What theory best explains the features of
our solar system?
What features of our solar system
provide clues to how it formed?
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?
According to the
nebular theory, our
solar system formed
from a giant cloud of
interstellar gas.
(nebula = cloud)
What have we learned?
• What features of our solar system provide clues
to how it formed?
— Motions of large bodies: All in same
direction and plane
— Two major planet types: Terrestrial and
jovian
— Swarms of small bodies: Asteroids and
comets
— Notable exceptions: Rotation of Uranus,
Earth’s large moon, and so forth
What have we learned?
• What theory best explains the features of our
solar system?
— The nebular theory, which holds that our
solar system formed from a cloud of
interstellar gas, explains the general features
of our solar system.
6.3 The Birth of the Solar System
Our goals for learning:
• Where did the solar system come from?
• What caused the orderly patterns of motion
in our solar system?
Where did the solar system come
from?
Galactic Recycling
• Elements that
formed
planets were
made in stars
and then
recycled
through
interstellar
space.
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?
Orbital and Rotational Properties of the Planets
Conservation of
Angular Momentum
•
The rotation speed
of the cloud from
which our solar
system formed
must have
increased as the
cloud contracted.
Rotation of a
contracting
cloud speeds
up for the
same reason a
skater speeds
up as she pulls
in her arms.
Collapse of the Solar Nebula
Flattening
•
Collisions between
particles in the
cloud caused it to
flatten into a disk.
Collisions
between gas
particles in a
cloud
gradually
reduce random
motions.
Formation of Circular Orbits
Collisions
between gas
particles also
reduce up
and down
motions.
Why does the Disk Flatten?
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.
What have we learned?
• Where did the solar system come from?
— Galactic recycling built the elements from which
planets formed.
— We can observe stars forming in other gas clouds.
• What caused the orderly patterns of motion in
our solar system?
— The solar nebula spun faster as it contracted because
of conservation of angular momentum.
— Collisions between gas particles then caused the
nebula to flatten into a disk.
— We have observed such disks around newly forming
stars.
6.4 The Formation of Planets
Our goals for learning:
• Why are there two major types of planets?
• Where did asteroids and planets come
from?
• Do we explain the existence of the Moon
and other exceptions to the rules?
• When did the planets form?
Why are there two major types of
planets?
Conservation
of Energy
As gravity
causes the
cloud to
contract, it
heats up.
Collapse of the Solar Nebula
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.
The gravity of these larger planets was able to
draw in surrounding H and He gases.
The gravity of
rock and ice in
jovian planets
draws in H and
He gases.
Nebular Capture and the Formation of the Jovian Planets
Moons of jovian planets form in miniature disks.
Radiation and
outflowing
matter from
the Sun —
the solar
wind — blew
away the
leftover
gases.
The Solar Wind
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?
Captured Moons
•
The unusual moons of some planets may
be captured planetesimals.
Giant Impact
Giant impact stripped matter from Earth’s crust
Stripped matter began to orbit
Then accreted into Moon
Odd Rotation
• Giant impacts
might also
explain the
different
rotation axes
of some
planets.
Review of
nebular theory
Thought Question
How would the solar system be different if the solar
nebula had cooled with a temperature half its current
value?
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.
Thought Question
How would the solar system be different if the solar
nebula had cooled with a temperature half its current
value?
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.
Thought Question
Which of these facts is NOT explained by the
nebular theory?
• There are two main types of planets: terrestrial
and jovian.
• Planets orbit in the same direction and plane.
• Asteroids and comets exist.
• There are four terrestrial and four jovian planets.
Thought Question
Which of these facts is NOT explained by the
nebular theory?
• There are two main types of planets: terrestrial
and jovian.
• Planets orbit in the same direction and plane.
• Asteroids and comets exist.
• There are four terrestrial and four jovian
planets.
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
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.
What have we learned?
• Why are there two major types of planets?
— Rock, metals, and ices condensed outside the
frost line, but only rock and metals condensed
inside the frost line.
— Small solid particles collected into planetesimals
that then accreted into planets.
— Planets inside the frost line were made of rock
and metals.
— Additional ice particles outside the frost line
made planets there more massive, and the gravity
of these massive planets drew in H and He gases.
What have we learned?
• Where did asteroids and comets come from?
— They are leftover planetesimals, according to the
nebular theory.
• How do we explain the existence of Earth’s moon and
other exceptions to the rules?
— The bombardment of newly formed planets by
planetesimals may explain the exceptions.
— Material torn from Earth’s crust by a giant impact
formed the Moon.
• When did the planets form?
— Radiometric dating indicates that planets formed 4.5
billion years ago.
6.5 Other Planetary Systems
Our goals for learning:
• How do we detect planets around other
stars?
• How do extrasolar planets compare with
those in our own solar system?
• Do we need to modify our theory of solar
system formation?
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
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.
Stellar Motion due to Planetary Orbits
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!).
Oscillation of a Star's Absorption Line
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.
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.
Planetary Transits
Direct Detection
• Special techniques for concentrating or eliminating
bright starlight are enabling the direct detection of
planets.
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?
• One in ten stars examined so far have
turned out to have planets.
• The others may still have smaller (Earthsized) planets that cannot be detected using
current techniques.
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.
Thought Question
What happens in a gravitational encounter that
allows a planet’s orbit to move inward?
A. It transfers energy and angular momentum to
another object.
B. The gravity of the other object forces the planet
to move inward.
C. It gains mass from the other object, causing its
gravitational pull to become stronger.
Thought Question
What happens in a gravitational encounter that
allows a planet’s orbit to move inward?
A. It transfers energy and angular momentum to
another object.
B. The gravity of the other object forces the planet
to move inward.
C. It gains mass from the other object, causing its
gravitational pull to become stronger.
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.
What have we learned?
• How do we detect planets around other
stars?
— A star’s periodic motion (detected through
Doppler shifts) tells us about its planets.
— Transiting planets periodically reduce a star’s
brightness.
— Direct detection is possible if we can block the
star’s bright light.
What have we learned?
• How do extrasolar planets compare with those in
our solar system?
— Detected planets are all much more massive
than Earth.
— Most have orbital distances smaller than
Jupiter’s, and have highly elliptical orbits.
— “Hot Jupiters” have been found.
• Do we need to modify our theory of solar system
formation?
— Migration and encounters may play a larger
role than previously thought.