How Planets Work

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How Planets Work
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Congratulations!

• You are now the proud owner of an Acme planet,
  the finest planets in the galaxy. Whether it's a
  rocky terrestrial planet, a gas giant, or an ice
  planet, your Acme planet is guaranteed to give
  you billions of years of satisfaction.

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• Your planet comes with a durable factory finish.
  You can keep the finish pristine for eons by
  following a few simple rules. Protect the surface
  from harsh abrasives like large impacting
  asteroids. Avoid sources of excessive heat, like
  exploding supernovae. Melting or vaporizing your
  planet voids the warranty.

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• If your planet is stored in a moderate
  temperature setting, it may become coated with a
  thin film of liquid water. This is normal and
  does not constitute a malfunction.
• Cooler parts of your planet may become coated
  with frozen water, this too is normal and does
  not constitute a malfunction.
• Under some circumstances, your planet may become
  covered with a thin greenish film called life.
  Connoisseurs consider this the best part.

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Important Points

• Bulk Density
• Shape of Planets
• Planetary Interiors
• Atmospheres
• Oceans
• Coordinates
• Names
• What is a Planet?

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Bulk Density

• Bulk density Mass/Volume
• gm/cc kg/m3
• 1 gm/cc 1000 kg/m3
• Volume determined from dimensions
• Mass determined from gravitational pull
• Effect on other bodies
• Effects on spacecraft

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Bulk Density

• lt1 gm/cc (1000 kg/m3)
• Small body Probably ice with a lot of void space
• Giant planet Mostly gases.
• Saturn, 0.7 gm/cc (700 kg/m3) is light enough to
  float.
• 1-3 gm/cc (1000 - 3000 kg/m3)
• Small bodies Mixtures of ice and rock
• Large planets Mostly gases with dense cores.
• 3-5 gm/cc (3000 - 5000 kg/m3)
• Rock, possibly with a denser core of metal.
• gt5 gm/cc (5000 kg/m3)
• Rock, with a large dense core of metal.

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Why Are Planets Round?

• Planets are round because of gravity. Gravity
  pulls everything inward toward the center of a
  planet
• Large hills would flatten under their weight and
  material would flow into depressions
• Earth in proportion to its size is smoother than
  a billiard ball
• Depending on materials, non-round objects can be
  up to a few hundred kilometers across.

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How Gravity Rounds Planets
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Equatorial Bulges

• Rotating planets bulge at the equator due to
  centrifugal force
• The earth's equatorial bulge is 1/298 of its
  diameter
• Radius at the equator is about 11 kilometers more
  than at the poles.
• Chimborazo (Ecuador) and Huascaran (Peru) are the
  farthest points from the center of the Earth, not
  Mount Everest.
• Jupiter and Saturn have 10 bulges

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Planetary Interiors

• Gravity pulls dense materials toward centers
• Satellites and close flybys allow detection of
  deep masses of material
• Moment of inertia is a measure of how material is
  concentrated in a planet
• Shows up in precession of planets
• Affects exact size and shape of equatorial bulge

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Atmospheres

• Atoms in space follow the same laws of physics as
  planets
• If they hit escape velocity, they can escape a
  planet
• Can a planet retain an atmosphere?
• Escape velocity (Determined by mass)
• Temperature (Determines speed of atoms)
• Stripping

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Oceans

• Its never too cold for solids, never too hot for
  gases
• Liquids exist only in a restricted temperature
  range
• At low pressures, its easier for liquids to
  evaporate
• Oceans cannot exist without atmospheres
• Oceans, lakes and seas are probably rare in the
  Universe
• Only known on Earth and Titan

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Sea Level

• How do we know where sea level would be under
  Mount Everest?
• We use a mathematical description of earths
  shape called the geoid
• Now determined from satellite data
• On waterless planets, we use the average radius
  of the planet
• Earths average elevation is 2.5 km below sea
  level

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Latitude and Longitude

• Latitude defined by rotation axis
• Free-rotating planets have arbitrary zero
  longitude
• Usually defined by longitude facing Earth at some
  specified time
• Satellites locked to their parent planets
• Zero longitude is the longitude facing the planet
• Usually 0,0 is directly under the planet
• N,E are positive S, W are negative

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Names

• Set by a commission of the International
  Astronomical Union
• Names (so far) are recognized by the various
  spacefaring nations.
• Features cannot be named after living people or
  people dead less than three years.
• Features cannot be named for political figures
  after 1800.
• Features cannot be named for any religious
  figures from Christianity, Judaism, Islam,
  Hinduism, Buddhism or Confucianism.

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Planetary Geographical Features

• Latin names. Latin is traditional, apolitical,
  and the closest thing to a universal language in
  history
• Linea (Line) Line or band
• Macula (Spot) Dark spot
• Mons (Mountain) Mountain, plural Montes
• Planitia (Plain) Low plain, basin
• Planum (Plain) Plateau
• Regio (Region) Region
• Rupes (Cliff) Cliff or scarp
• Vallis (Valley) Valley

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What is a Planet?

• Ancient Sun, Moon, Mercury, Venus, Mars,
  Jupiter, Saturn
• Copernicus and Kepler Earth in, Sun and Moon out
• Uranus (1781), Ceres (1801)
• New Class created Minor Planets
• Neptune (1846), Pluto (1930), Kuiper Belt (1992)
• 2007 New Class created Dwarf Planets

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What is a Planet?

• Planet
• Round because of gravity
• Massive enough to clear its neighborhood of other
  objects
• Dwarf Planet
• Round because of gravity
• This is not Over Yet
• How round is round enough?
• What else is out there?

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Clearing the Vicinity
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Important Points

• Bulk Density
• Shape of Planets
• Planetary Interiors
• Atmospheres
• Oceans
• Coordinates
• Names
• What is a Planet?