Outer Worlds

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The Outer WorldsUranus, Neptune, and Pluto
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Announcements

• Reading Assignment
• Finish Chapter 17
• 6th homework due on Tuesday, April 24
• Quiz on Tuesday, April 24 (will cover all
  lectures since the last exam and Chapters 14-17)
• Final Activity next Thursday
• Come prepared with 2 multiple-choice final exam
  questions
• Next study-group session is Monday, April 23,
  from 1030AM-1200Noon in room 330.
• Public lecture Tuesday, April 24 730PM (this
  room)
• Movie Night Monday, April 23, 630PM (this room)

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Discovery of Uranus

• Discovered by William Herschel in 1781
  (discovered by chance)
• Herschel was an English musician and amateur
  astronomer
• He used a 6.5 homemade reflector (it took him
  over 200 attempts to make it!)
• Prior to its discovery, Uranus had been seen, but
  not recognized as a planet (it had been
  designated 34 Tauri)
• Herschel made many contributions to astronomy,
  including the discovery of binary star systems,
  and the motion of the solar system in space

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Discovery of Neptune

• The problem
• Uranuss orbit didnt match Newtons Laws, with
  the forces from all known planets at that time
• Proposed solution there must be another planet
  !
• Bodes Law gives the distance to be 39 AU
• Its mass must be about the same as Uranus

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Neptunes Discovery

• John Couch Adams (English student)
• Predicted its location in 1843 (using Newtons
  Laws including the its force on Uranus to explain
  Uranuss orbit)
• George Airy (Astronomer Royal) wouldnt look!
• Wanted a few more details worked out

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Neptunes Discovery

• Urbain Le Verrier (French Professor)
• Predicted its location in 1846 published it
• Airy (now interested) did not have good enough
  star charts
• Johann Galle (German astronomer) did have good
  star charts
• LeVerrier convinced Galle to look for it

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Neptunes Discovery

• Galle found it in 30 minutes !
• Galle is often credited for being the discoverer,
  although Adams and Le Verrier were the first to
  predict its presence
• Instructors note Why did Adams get credit? He
  never published it !

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Uranus and Neptune from Earth

• Uranus is visible with the naked eye under good
  (dark sky) conditions
• Neptune is not visible to the naked eye
• Both can be seen as planets with moderate-sized
  telescopes (rather than a point-like star)
• No surface features are visible without a filter
• Only recently were a ground-based telescope able
  to discern surface features on Uranus using an IR
  filter and adaptive optics

Uranus
Neptune
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Uranus Basic Facts

• Equatorial diameter of 51,800 kilometers
• about 4 Earth diameters
• Mass is about 14.5 Earth masses
• Density 1.3 g/cm3
• Orbital period is 84.01 years
• About the average lifespan of a human
• average distance from the Sun is about 19 AU
• Retrograde rotation
• Rotational period is 17 hours 14 minutes.
• It has 27 moons
• The two largest are Titania and Oberon
• Both are about ½ the size of our moon

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Neptune Basic Facts

• Equatorial diameter of 49,500 kilometers
• Nearly 4 Earth diameters
• A little bit smaller than Uranus
• Mass is about 17 Earth masses
• A little bit more massive than Uranus
• Density 1.7 g/cm3
• Orbital period is about 165 years
• It is about 29.8 AU from the Sun
• Rotational period is about 16 hours
• Has 13 moons
• The largest is Triton which is about 3/4 the size
  of our moon

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Uranuss Composition and Appearance

• The atmosphere of Uranus is composed of
• 83 hydrogen,
• 15 helium,
• 2 methane
• Methane absorbs red light, giving Uranus its
  blue-green color.
• Methane on Uranus is 5 times more abundant than
  on Jupiter and Saturn
• It lacks an appreciable internal heat source,
  thus there are no visible surface features

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Uranuss Atmosphere

• Visibly, Uranus appears as a virtually
  featureless hazy blue ball.
• Lack of internal heat.
• Clouds on Uranus are cold and don't billow up
  above the top haze layer.
• Seen in false color (at the right), Uranus
  reveals atmospheric features
• The atmosphere is arranged into clouds running at
  constant latitudes (like Jupiter and Saturn)

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Uranus has an unusual tilt

• Herschel discovered that moons were orbiting in a
  plane perpendicular to its orbit
• Moons orbit about their parent bodys equator
• Uranuss rotation axis is tilted 98o with respect
  to the plane of its orbit about the Sun
• Thought to be the result of a collision with a
  planet sized body early in the solar system
  formation

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Neptunes Atmosphere

• Neptune's atmosphere is made up of hydrogen,
  helium, and methane.
• Neptune's has clouds and storm systems that
  revolve around the planet, but with wind speeds
  of 300 m/sec (700 mi/hr
• These winds are driven by an internal heat source
  (unlike Uranus)

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Neptunes Clouds

• The clouds seen on Neptune are frozen methane in
  the atmosphere
• Much more cloud activity is seen on Neptune than
  on Uranus
• This is because Uranus lacks a substantial
  internal heat source while Neptune does
• Neptune radiates more than twice the energy it
  receives from the Sun

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The Giant Dark Spot on Neptune

• Similar to Jupiters Giant Red Spot (GRS)
• However, the giant dark spot on Neptune is not as
  long-lived as the GRS on Jupiter
• Disappeared in 1994
• Another re-appeared in the northern hemisphere in
  1995
• Rotates counterclockwise, as does the GRS on
  Jupiter

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Uranus and Neptune contain a higher proportionof
heavy elements than Jupiter and Saturn

• Both Uranus and Neptune may have a rocky core
  (about the size of Earth) surrounded by a mantle
  of water and ammonia (like Windex!)
• Electric currents in the mantles may generate the
  magnetic fields of the planets

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The Origin of Magnetic Fields on Uranus and
Neptune

• Not massive enough to possess a liquid metallic
  hydrogen layer like Jupiter
• H2O in the interior is under enough pressure to
  induce ionization
• Ionization increases its electrical conductivity
• An ionized-water dynamo!

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• The magnetic axes of both Uranus and Neptune are
  steeply inclined from their axes of rotation
• The magnetic and rotational axes of all the other
  planets are more nearly parallel
• The magnetic fields of Uranus and Neptune are
  also offset from the centers of the planets

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Uranus and Neptune each have a system of
thin,dark rings
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Triton (Moon of Neptune)

• Discovered by William Lassell only a month after
  Neptune was discovered
• Orbits Neptune in a retrograde orbit !
• opposite to the rotation direction of Neptune
• The only large moon to do this
• The orbit is also highly inclined relative to
  Neptunes equator
• Density 2066 kg/m3
• More rock than in Saturns moons

Probably a captured asteroid!
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Triton (Moon of Neptune)

• Shows evidence of geological activity
• Lacks large craters it is a very young surface
• Probably tidal heating from Neptune
• Perhaps extreme tidal heating in the past
• It also has a tenuous nitrogen (N2) atmosphere
• 70,000 times less atmospheric pressure than Earth
• about 500 times thinner than the Martian
  atmosphere

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Pluto and Charon

• How was Pluto Discovered?
• What is the nature of the planet and its moon?
• Is Pluto a planet?

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Planet X

• Percival Lowell repeated Adams Le Verrier
  calculations for planet X
• He looked, but never found it
• After Lowell Died, there was a tedious search

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• Lowell hired Clyde Tombaugh who discovered Pluto
  in 1930
• Pluto was named after the lord of Hades (in Greek
  mythology), PL are the initials of Percival
  Lowell

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Plutos Discovery

• Only planet found by an American, and only
  planet found in the 20th century
• Too small to effect the orbits of other planets
• The calculations of Lowell didnt matter

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