Introduction to the Solar System

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Introduction to the Solar System

• Edward M. Murphy
• Space Science for Teachers
• 2005

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The Solar System
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The Terrestrial Planets
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The Jovian Planets
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The Jovian Planets, Earth and Sun
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Properties of the Planets
Name Distance (AU) Period (years) Diameter (Earth1) Mass (Earth1) Density
Mercury 0.39 88 days 0.38 0.055 5.4
Venus 0.72 224 days 0.95 0.82 5.3
Earth 1.0 1.0 1.0 1.0 5.5
Mars 1.52 1.88 0.53 0.11 3.9
Jupiter 5.20 11.86 11.2 317.8 1.3
Saturn 9.54 29.46 9.4 94.3 0.7
Uranus 19.18 84.07 4.0 14.6 1.2
Neptune 30.06 164.8 3.88 17.2 1.6
Pluto 39.44 248.6 0.17 0.0025 2.1
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The Sun
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Orion Molecular Cloud
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Orion Molecular Cloud

• The closest and best studied giant molecular
  cloud is the Orion molecular cloud.
• Distance 1500 LY.
• Diameter 100 LY.
• Mass 430,000 MSun.
• It includes the Orion Nebula and the Horsehead
  Nebula.

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Orion Molecular Cloud
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Visible Light
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Visible Light
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Infrared Light (0.1 mm)
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Radio (CO at 2.6 mm)
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Orion Nebula

• The Orion Nebula is a young cluster of hot stars
  (ages 3x105 1x106 years).
• Most of the young stars are hidden from view by
  the dust in the molecular cloud.
• Infrared radiation, with its longer wavelength,
  can penetrate this dust to show us the interior
  of the cloud.

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Orion Nebula

• Visible light images of the central cluster in
  the Orion Nebula reveal the four bright stars
  (called the Trapezium).
• Infrared images reveal more than 500 stars in
  this cluster.
• Star formation is not very efficient. Only a few
  percent of the gas in the cloud is converted into
  stars.

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Orion Nebula

• Distance to the Trapezium Cluster is about 1530
  LY.
• It consists of about 1800 stars
• The average age of the stars is less than 1
  million years old, some are as young as a few
  hundred thousand years.
• The largest star is an O6 has a mass of 50 Msun.

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Ground Based Image
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HST Image
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IR Ground Image
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Formation of Planets

• Observations show that at least 50 of protostars
  are surrounded by disks of material.
• These disks a a typical mix of interstellar
  material
• Hydrogen and helium in gaseous form
• Ice and dust grains solid particles contain
  most of the heavy elements.
• These disks typically contain 1-10 of the mass
  of our Sun.

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Protostars in Orion
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Formation of Planets

• Early on, the dust grains in the disk collide and
  begin to stick together.
• These small clumps stick to other clumps.
• Eventually, this accretion of material creates
  planetesimals, objects the size of small moons.
• Planetesimals are large enough that their gravity
  begins to attract more material, including other
  planetesimals.

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Formation of Planets

• Planetesimals have been constructed from the
  solid particles in the disk and are made mostly
  of elements other than hydrogen and helium.
• Billions of these objects will continue to
  collide and grow until all that is left is a
  handful of large planets and some leftover
  planetesimals asteroids and comets.
• This processof growth by accumulation, which
  builds planets, is called accretion.

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Formation of Planets

• Although most of the material was swept up by 100
  million years, the intense bombardment of the
  planets continued for a billion years.
• Many smaller objects are destroyed by collisions,
  with the resulting debris swept up by the large
  planets (fragmentation).

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Formation of Planets

• Even today, the Earth sweeps up many tons of
  material every day (mostly tiny dust grains).
• When you see a meteor streak across the sky you
  are watching accretion happen!

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Formation of Planets

• The temperature of the dust disk surrounding the
  star indicates its distance from the star.
• The inner disk is hot (1000 K at 0.1 AU).
• The middle disk is cool (100 K at 1 AU).
• The outer disk is cold (30 K at 10 AU).

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• The local temperature determines what materials
  existed as solids
• 500-2000 degrees Metals and rocks
• 100-500 degrees Water ice (and metal and rocks)
• 20-100 degrees Methane ice (and water ice and
  metal and rocks)

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Composition of the Planets
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• The temperature in the solar nebula determines
  the typical composition of the planets and their
  satellites
• Close to the Sun, objects are made of metal and
  rock
• Far from the Sun objects are made mostly of ice
  with some rock and metal

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HST Image of Jupiter
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Formation of the Giant Planets

• The Jovian planets started as icy/rocky seeds
  from the accumulation of solid planetesimals
• They grew in an area rich in ice
• Cores grew quickly and became sufficiently
  massive to attract and retain gas from the
  nebula.
• The Jovian planets have slushy cores left over
  from this early stage.

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Age of the Solar System

• Using the radioactive decay of certain elements,
  we can determine the ages of bodies in the solar
  system.
• The decay of radioactive elements is a
  statistical process.
• The half-life is the time it takes for ½ of the
  atoms of the original parent element to decay
  into the daughter element.
• Over time, the ratio of parent to daughter
  element decreases (that is, there is less of the
  parent element and more of the daughter element
  as rocks get older).

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Radioactive Decay
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Radioactive Decay Reactions Used to Date Rocks
Parent Daughter Half-Life (Gyr)
Samarium-147 Neodymium-143 106
Rubidium-87 Strontium-87 48.8
Thorium-232 Lead-208 14.0
Uranium-238 Lead-206 4.47
Potassium-40 Argon-40 1.31
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Age of the Solar System

• Lets use the decay of Potassium-40 to Argon-40 as
  an example.
• The ½ life of Potassium-40 is 1.3 billion years.
• Imagine that we have a newly formed rock that
  contains some Potassium-40. Argon is a noble
  gas, and does not form minerals in rocks. All
  the Argon-40 in the rock must come from the decay
  of Potassium-40.
• After 1.3 billion years, there will be ½ as much
  Postassium-40 in the rock. There will be as much
  Argon-40 as Potassium-40.
• After 2.6 billion years, there will be ¼
  (1/2x1/2) as much Potassium-40 as when the rock
  was born. There will be 3 times as much Argon-40
  as Potassium-40.

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Age of the Solar System

• After 3.9 billion years (3 half lives), there
  will be 1/8 as much Potassium-40 as when the rock
  was born. There will be 7 times as much Argon-40
  as Potassium-40.
• If we take a rock today, the ratio of the amount
  of parent to daughter isotope can tell us the age
  of the rock.
• We can use the decay of multiple elements to
  cross-check the results.

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Radioactive Decay
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Age of the Solar System

• We have directly analyzed rocks from Earth, the
  Moon, Mars, and meteorites.
• The oldest Earth rocks are about 4.3 billion
  years old. However, Earth is geologically active
  and this represents a minumum age.
• The oldest Moon rocks are about 4.4 billion years
  old. The Moon was also active, so this
  represents a minimum age for the Moon.

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Age of the Solar System

• Most meteorites contain parts that are 4.56
  billion years old.
• Since the meteorites come from many parts of the
  solar system, they likely date the formation of
  the whole solar system.
• Therefore, the solar system is 4.56 billion years
  old.

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Planets Around Other Stars

• Planets around other stars are difficult to
  detect because they are very faint compared to
  the star and very close to the star.
• However, we can use the fact that the star orbits
  around the center of mass of the system to detect
  the presence of planets.

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Planets Around Other Stars

• The positional displacement of the star due to
  the orbit of the planet is very small. Consider
  the Sun and Jupiter. The Sun is 1050 times more
  massive than Jupiter. Therefore, its orbit
  around the center of mass is 1050 times smaller.
  From a distance of 10 pc (32.6 LY), the maximum
  displacement of the Sun is 0.0008 arcseconds.

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Wobble of Sun Due to Jupiter
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Planets Around Other Stars

• Currently, positional shifts of this size are too
  small to detect from ground based telescopes
  because of the blurring effect of the Earths
  atmosphere.
• NASA is launching two new missions (Full sky
  Astrometric Mapping Explorer and the Space
  Ineterferometry Mission) that should be able to
  measure these displacements from spacecraft.

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Doppler Shift Due to Motion Around the Center of
Mass
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Planets Around Other Stars

• Again consider the Sun and Jupiter. As the Sun
  orbits the center of mass every 12 years, its
  radial velocity changes by about 13 m/s (30 mph).
• Astronomers have developed the technology to
  detect shifts this small.
• In 1995, two groups of astronomers announced the
  discovery of the first planets outside our solar
  system orbiting Sun like stars.

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The Planet Around 51 Pegasi

• Michel Mayor and Didier Queloz discovered that
  the star 51 Pegasi (G5 V star 48 LY from Earth)
  was shifting every 4.2 days.
• From Keplers Law, we see that the distance from
  the planet to the star is 0.0512 AU.
• At this distance, the planet must be very hot
  (1000 K).
• From the radial velocity of the star, the mass of
  the planet turns out to be 0.44 times the mass of
  Jupiter!

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The Planet Around 51 Pegasi
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The Planet Around 51 Pegasi

• For comparison, the nearest planet to our Sun is
  Mercury (period of 88 days, distance 0.5 AU, mass
  1/836 of Jupiter).
• Today, 110 planetary systems have been discovered
  around Sun like stars, including 10 systems with
  multiple planets.

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P4.6 days P0.66 years P2.6 years
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Extrasolar Planets

• About 6 of all Sun like stars appear to have
  these massive planets around them.
• The planets have masses of 0.4 to 17 times that
  of Jupiter.
• Typically they orbit their stars at distances of
  0.07 to 3 AU.

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Extrasolar Planets

• The origin of these hot Jupiters is a mystery.
• Either they formed this close to their parent
  star, in which case we need to radically revise
  the way we think planets form around stars, or
• The planets formed in the outer solar system, but
  migrated inward.

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Extrasolar Planets

• It is important to note that the technique used
  to discover these planets favors the discovery of
  massive planets with short periods.