Honey, I Shrunk the Solar System

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Honey, I Shrunk the Solar System

• or
• Pluto We Barely Knew Ye as a Planet

Image credit JPL
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The Way it Was
Image from JPL
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And Then There Were Eight
Image from JPL
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From Where Did the Word Planet Come?

• The word planet is derived from the Greek word
  for wanderer and was traditionally applied to
  any heavenly body that moved with respect to the
  stars. In this sense the Sun and Moon were also
  planets.
• Dictionary says that a planet is any one of the
  nine large bodies that orbit the Sun.
• But some objects have been found that are larger
  than Pluto- so are they planets?

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Who Discovered the First Planets?

• Ancient cultures knew that some objects were not
  fixed in the sky like the stars.
• The Greeks knew of five such objects Mercury,
  Venus, Mars, Jupiter and Saturn
• By 800 B.C.E. Babylonian astronomers had records
  of planetary motion for Venus, Jupiter and Mars.

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The Solar System Until 1781Mercury, Venus,
Earth, Mars, Jupiter and Saturn
Images from NASA
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The Solar System Grows What to Name a New
Planet?

• March 13, 1781 William Herschel discovers what he
  thinks is a comet, but he has discovered a new
  planet- the seventh in our Solar System.
• Herschel wanted to name the new planet George
  after King George III of England.
• It was decided to continue with the Roman god
  names that had been used for the other planets,
  thus it was named Uranus.
• This set the standard for the convention of using
  Roman god names for the planets.

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Uranus- The First New Planet

• Distance 19.1 AU Doubled the size of the Solar
  System
• Diameter 4 Earth diameters

Image courtesy of NASA
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Another New World Neptune

• The orbit of Uranus was not as expected.
• John Couch Davies, a 24 year old Cambridge grad,
  thought that this might be caused by another
  unknown planet
• In 1845 he submitted his calculations to the
  Astronomer Royal of England. English star charts
  not good enough.
• At nearly the same time French astronomer Urbain
  Jean Joseph de Verrier did the same calculations.
  The Berlin Observatory was given his data and
  the planet was found the first night due to
  better star maps.

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Neptune

• Following the Roman god theme the planet was
  named Neptune, for the sea god since it was blue
• 30 AU from the Sun
• Diameter 3.9 Earth diameters

Image by Hubble Space Telescope
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A Ninth Planet?

• Speculations about a ninth planet date back to
  the late 1800s.
• Percival Lowell urged that a special camera be
  built to look for Planet X.
• In 1929 the camera was finished and installed at
  Lowell Observatory in Flagstaff, AZ
• Clyde Tombaugh discovered Pluto on February 18,
  1930 after looking at over one million stars
• Name Pluto suggested by Venetia Burney, an 11
  year old girl. Pluto was the Roman god of the
  underworld.

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Clyde Tombaugh 1906-1997
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Finding Pluto
Pluto images by Nathan Twining Observatory
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Here it is!
Pluto images by Nathan Twining Observatory
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Pluto

• 39.5 AU from Sun
• Diameter about 0.18 Earth diameter (about 1400
  miles)
• Pluto and Charon essentially a double system
• NASA New Horizons mission will reach Pluto in
  2015

Image by Hubble Space Telescope
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The Arguments for and Against Planethood for Pluto

• Pluto very small
• Doesnt fit into any other categories of planets-
  terrestrial or gas giants
• Orbit strange- tilted 17 from plane of the solar
  system
• May be typical of thousands of icy objects found
  far from the Sun
• It is round like a planet and it orbits the Sun.

Image by JPL
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The Kuiper Belt
John Hopkins University
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Orbital Paths of Planets and Pluto
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Orbit of UB 313 (Known as Xena)
NASA
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Kuiper Belt Object Sizes
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The International Astronomical Union Resolution

• A planet is for the first time defined
  scientifically. A planet orbits a star, has
  sufficient gravity to become round, and has
  cleared the neighborhood around its orbit.
• A Dwarf Planet orbits a star, is not a satellite,
  has sufficient gravity to become round and has
  not cleared the neighborhood of its orbit. Pluto
  is the prototype of this class and currently
  includes Ceres and UB 313. Others will be
  decided upon later.
• A third class, Small Solar System Bodies, was
  defined as all other objects except satellites.
  This includes most asteroids, most comets and
  most trans Neptune objects.

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So Long Planet Pluto and Hello Dwarf Planet
Pluto!
International Astronomical Union