Download Properties of the Planets & Formation of the Solar

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 5
Formation of the Solar System
WHAT DO YOU THINK?
1.
2.
3.
How old is the Earth? How do we
know???
How old are the Sun and other planets?
Were they created during the “Big
Bang”? How do we know?
Have any Earthlike planets been
discovered orbiting Sunlike stars? How
can we tell?
In this chapter you will discover…
how the solar system formed
 why the early solar system was much
more violent than it is today
 how astronomers define various types of
objects in the solar system
 how the planets are “grouped”

In this chapter you will discover…
how moons formed throughout the solar
system
 the “debris” in the solar system
 that disks of gas and dust, as well as
planets, have been observed around a
growing number of stars
 that newly forming stars & planetary
systems are being observed

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
Space isn’t empty…
We know space is filled with gas and dust – the
raw materials from which planetary systems
form!
…and its composition changes
Spectra of exploding and old stars shows
heavier elements being ejected, too
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

Two Major Planet Types
Terrestrial
planets are
rocky,
relatively
small, and
close to the
Sun.
 Jovian planets
are gaseous,
larger, and
farther from

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)
Detecting Planets around
OTHER stars!
KEPLER
COROT
WASP
BETA Pictoris – a Hint at Stars with
Planetary Disks?
Planet Detection

Indirect: Measurements of stellar
properties revealing the effects of orbiting
planets

Direct: Pictures or spectra of the planets
themselves
Indirect: Gravitational Tugs

The Sun and
Jupiter orbit
around a common
center of mass.

The Sun 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 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).
Inferring planets that
aren’t seen!
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 4-day
orbital period

Short period means
small orbital
distance
1st 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.
Direct Detection
Special techniques for concentrating or eliminating
bright starlight are enabling the direct detection of
planets.
Direct Detection
The Process of Science

Observation: TMR-1 “detected” in 1998
The Process of Science
Observation: TMR-1 “detected” in 1998
 Hypothesis: It is a planet,
connected by a disk to
a double (binary star)

The Process of Science
Observation: TMR-1 “detected” in 1998
 Hypothesis: It is a planet ?
 Critical Tests: Spectra

The Process of Science
Observation: TMR-1 “detected” in 1998
 Hypothesis: It is a planet ?
 Critical Tests: Spectra
 Result: A background star!

The Process of Science
How do extrasolar planets
compare with our solar system?
Measurable Properties

Orbital period, distance, and shape

Planet mass, size, and density

Composition (by spectra)
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.”

See “SuperWASP”
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.
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, with 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.
Summary of Key Ideas
Formation of the Solar System



Hydrogen, helium, and traces of lithium, the three
lightest elements, were formed shortly after the creation
of the universe. The heavier elements were produced
much later by stars and are cast into space when stars
die. By mass, 98% of the observed matter in the
universe is hydrogen and helium.
The solar system formed 4.6 billion years ago from a
swirling, disk-shaped cloud of gas, ice, and dust, called
the solar nebula.
The four inner planets formed through the accretion of
dust particles into planetesimals and then into larger
protoplanets. The four outer planets probably formed
through the runaway accretion of gas and ice onto rocky
protoplanetary cores over millions of years, but possibly
by gravitational collapse in under 100,000 years.
Formation of the Solar System


The Sun formed at the center of the solar nebula. After
about 100 million years, the temperature at the
protosun’s center was high enough to ignite
thermonuclear fusion reactions.
For 800 million years after the Sun formed, impacts of
asteroid-like objects on the young planets dominated the
history of the solar system.
Comparative Planetology



The four inner planets of the solar system share many
characteristics and are distinctly different from the four
giant outer planets.
The four inner, terrestrial planets are relatively small,
have high average densities, and are composed
primarily of rock and metal.
Jupiter and Saturn have large diameters and low
densities and are composed primarily of hydrogen and
helium. Uranus and Neptune have large quantities of
water as well as much hydrogen and helium.
Comparative Planetology



Pluto, once considered the smallest planet, has a size,
density, and composition consistent with the known
Kuiper Belt Objects (KBOs).
Asteroids are rocky and metallic debris in the solar
system, larger than about a kilometer in diameter, and
found primarily between the orbits of Mars and Jupiter.
Meteoroids are smaller pieces of such debris. Comets
are debris that contain both ice and rock.
Planets Outside Our Solar System




Astronomers have observed disks of gas and dust
orbiting young stars.
At least 250 extrasolar planets have been discovered
orbiting other stars.
Most of the extrasolar planets that have been discovered
have masses roughly the mass of Jupiter.
Extrasolar planets are discovered indirectly as a result of
their effects on the stars they orbit.
Key Terms
accretion
albedo
asteroid
asteroid belt
average density
comet
core-accretion model
crater
gravitational instability
model
meteoroid
microlensing
moon (natural
satellites)
orbital inclination
planet
planetesimal
protoplanet
protoplanetary disks
(proplyds)
protosun
solar nebula
solar system
terrestrial planet
WHAT DID YOU THINK?


Were the Sun and planets among the first generation of
objects created in the universe?
No. All matter and energy were created by the Big Bang.
However, much of the material that exists in our solar
system was processed inside stars that evolved before
the solar system existed. The solar system formed
billions of years after the Big Bang occurred.
WHAT DID YOU THINK?


How long has Earth existed, and how do we know this?
Earth formed along with the rest of the solar system,
about 4.6 billion years ago. The age is determined from
the amount of radioactive decay that has occurred in it.
WHAT DID YOU THINK?


What typical shape(s) do moons have, and why?
Although some moons are spherical, most look roughly
like potatoes. Those that are spherical are held together
by the force of gravity, pulling down high regions. Those
that are potato-shaped are held together by the
electromagnetic interaction between atoms, just like
rocks. These latter moons are too small to be reshaped
by gravity.
WHAT DID YOU THINK?


Have any Earthlike planets been discovered orbiting
Sunlike stars?
Not really. Most extrasolar planets are Jupiter-like gas
giants. The planets similar in mass and size to Earth are
either orbiting remnants of stars that exploded or, in the
case of Gliese 581, a star much less massive and much
cooler than the Sun.