DTU 8e Chap 5 Formation of the Solar System

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Neil F. Comins William J. Kaufmann III
Discovering the Universe Eighth Edition
CHAPTER 5 Formation of the Solar System
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Essay Questions for this Lecture

• What are the major similarities and differences
  between inner and outer planets?
• How did the solar system form? How does the
  condensation sequence theory explain the
  similarities and differences between Jovian and
  Terrestrial planets?
• How do we detect other planetary systems around
  distant stars?  What are the strengths and
  limitations of each method?

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WHAT DO YOU THINK?

• 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?

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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

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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

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The solar system exhibits clear patterns of
composition and motion. These patterns are far
more important and interesting than numbers,
names, and other trivia.
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Planets are very tiny compared to distances
between them.
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Sun

• Over 99.9 of solar systems mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each
  second

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Mercury

• Made of metal and rock large iron core
• Desolate, cratered long, tall, steep cliffs
• Very hot and very cold 425C (day), 170C
  (night)

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Venus

• Nearly identical in size to Earth surface
  hidden by clouds
• Hellish conditions due to an extreme greenhouse
  effect
• Even hotter than Mercury 470C, day and night

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Earth
Earth and Moon to scale

• An oasis of life
• The only surface liquid water in the solar
  system
• A surprisingly large moon

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Mars

• Looks almost Earth-like, but dont go without a
  spacesuit!
• Giant volcanoes, a huge canyon, polar caps, and
  more
• Water flowed in the distant past could there
  have been life?

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Jupiter

• Much farther from Sun than inner planets
• Mostly H/He no solid surface
• 300 times more massive than Earth
• Many moons, rings

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Jupiters moons can be as interesting as planets
themselves, especially Jupiters 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

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Saturn

• Giant and gaseous like Jupiter
• Spectacular rings
• Many moons, including cloudy Titan
• Cassini spacecraft currently studying it

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Rings are NOT solid they are made of countless
small chunks of ice and rock, each orbiting like
a tiny moon.
Artists conception
The Rings of Saturn
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Cassini probe arrived July 2004. (Launched in
1997)
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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

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Neptune

• Similar to Uranus (except for axis tilt)
• Many moons (including Triton)

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Pluto and Eris

• Much smaller than other planets
• Icy, comet-like composition
• Plutos moon Charon is similar in size to Pluto

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Space isnt empty
We know space is filled with gas and dust the
raw materials from which planetary systems form!
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and its composition changes
Spectra of exploding and old stars shows heavier
elements being ejected, too
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Clues to Formation of our Solar System?
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Clue 1 Motion of Large Bodies

• All large bodies in solar system orbit in same
  direction in nearly same plane.
• Most also rotate in that direction.

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Clue 2 Two Major Planet Types

• Terrestrial planets are rocky, relatively small,
  close to Sun.
• Jovian planets are gaseous, larger, farther
  from Sun.

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Clue 3 Swarms of Smaller Bodies

• Many icy comets, small dwarf planets, rocky
  asteroids populate solar system in 3 areas

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Notable Exceptions

• Several exceptions to normal patterns need to be
  explained.

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What theory best explains the features of our
solar system?
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According to the nebular theory, our solar system
formed from a giant cloud of interstellar gas.
(nebula cloud)
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Why are there two major types of planets?
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The Laws of Physics

• As gravity causes the cloud to contract, it heats
  up. (Conservation of Energy!)
• As it contracts, it spins faster. (Conservation
  of Angular Momentum!)
• As it spins and contracts, it flattens.
• The temperature varies by distance

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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
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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
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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.

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Tiny solid particles stick to form planetesimals.
Summary of the Condensates in the Protoplanetary
Disk
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Gravity draws planetesimals together to form
planets. This process of assembly is called
accretion.
Summary of the Condensates in the Protoplanetary
Disk
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Accretion of Planetesimals

• Many smaller objects collected into just a few
  large ones.

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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.

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The gravity of rock and ice in jovian planets
draws in H and He gases.
Nebular Capture and the Formation of the Jovian
Planets
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Moons of jovian planets form in miniature disks.
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Summary of Key Ideas
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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.

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Formation of the Solar System

• The Sun formed at the center of the solar nebula.
  After about 100 million years, the temperature at
  the protosuns 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.

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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.

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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.

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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.

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Key Terms
moon (natural satellites) orbital
inclination planet planetesimal protoplanet protop
lanetary disks (proplyds) protosun solar
nebula solar system terrestrial planet
accretion albedo asteroid asteroid belt average
density comet core-accretion model crater gravitat
ional instability model meteoroid microlensing
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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.

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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.

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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.

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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.