TransNeptunian Objects and Pluto

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Trans-Neptunian Objects and Pluto

• Astronomy 311
• Professor Lee Carkner
• Lecture 21

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What is required for a gas giant moon to have
differentiation?

• Nearness to the sun
• A large number of collisions
• A close elliptical orbit
• A strong magnetic field
• A 32 resonance with a near-by moon

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Why does Titan have an atmosphere?

• Because its orbit is very close to Saturn
• Because it is highly oblate
• Because it outgases a lot of nitrogen
• Because it collects molecules from the solar wind
• Because it is very cold

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Observing Tonight

• 830-930 pm
• If clear (check web page)
• Only need to come if you missed the previous
  sessions
• Probably the last session
• Also
• Open house at planetarium this Saturday
  (730-930 pm)

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Pluto -- God of the Underworld

• Pluto is so named because it is at the grim,
  scary edge of the solar system
• Pluto was discovered at Lowell Observatory and
  its first 2 letters commemorate Percival Lowell

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The Discovery of Pluto

• In the late 1800s it was believed that Neptunes
  orbit was being perturbed by a 9th planet
• Many astronomers tried to determine its position,
  including Percival Lowell
• The position turned out to be a coincidence,
  Pluto is too small to effect Neptunes orbit

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The Discovery of Pluto
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Observing Pluto

• No spacecraft has ever visited it
• But will not get to Pluto until 2015
• The best information comes from HST

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New Horizons in the Kuiper Belt
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Pluto Facts

• Size 2300 km
• Smaller than the 7 largest moons
• Orbit 39.5 AU
• Description Very small, very cold, very distant

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Plutos Orbit

• Plutos orbit is much more eccentric and much
  more inclined than any planet
• Eccentricity
• Most other planets e
• Inclination
• Plutos orbit carries it inside the orbit of
  Neptune
• Pluto is tipped on its side like Uranus

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Composition of Pluto

• Pluto has a density of 2000 kg/m3
• Pluto is probably composed of ice and rock
• Spectra of Pluto reveal the presence of methane,
  nitrogen and carbon monoxide ice
• The temperature on Pluto is only 50 K so the
  atmosphere cant escape

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Spectra of Pluto Showing Methane Ice
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Features of Pluto

• HST can see regions of different brightness on
  Plutos surface
• The other bright regions may be areas where
  impacts have gouged out fresh ice

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Plutos Moons

• Charon is only slightly smaller than Pluto (1190
  km compared to 2300 km)
• Their sizes are closer than any planet and moon
• They have very similar densities, masses and
  sizes
• Could be described as a double dwarf planet
• Each is about 50 km in diameter

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Is Pluto a Planet?

• Pro
• Spherical
• Has a moon
• Helps draw attention to the TNOs

• Con
• Eccentric orbit
• Found near many similar objects
• Have to make everything larger a planet

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What Makes Something a Planet?

• Planets used to be obvious
• Needed new definition when rest of solar system
  was discovered with telescopes
• The International Astronomical Union (which has
  authority over these things) removed Pluto from
  the list of planets in 2006
• Important thing is that Pluto is a
  Trans-Neptunian object

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Small, Icy Bodies

• Small icy bodies in the outer solar system
  (beyond Jupiter) have no good name
• Giant comets?, Iceteroids?
• Even if some are inside the orbit of Neptune
• Most are only recently discovered and not well
  characterized or organized

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Discovering TNOs

• At this distance the original protoplanetary disk
  was thin and the planetesimals moved slowly, so
  no planet could form
• About 1000 total now known
• Total population of large TNOs may be 70000
  (larger than 100 km)

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Known TNOs as of 10/2007
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Discovering TNOs
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• Centaur inside the orbit of Neptune
• Resonant a an orbital resonance with Neptune
• Classical Kuiper Belt in the 40-50 AU range
• Scattered Disk large distances and eccentricities

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Classical Kuiper Belt

• Most of the objects have nearly circular orbits,
  low inclinations and are not effected by
  Neptunes gravity
• About 2/3 of known TNOs are here
• Probably formed in place from the leftover
  material at the edge of the solar nebula

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Resonant Objects

• There are special places in the Kuiper belt where
  objects are safe from Neptunes gravity
• Examples
• 23 resonance TNO makes 2 orbits for every 3
  orbits of Neptune
• 12 resonance TNO makes ½ orbit for every orbit
  of Neptune
• Theory Neptune formed closer to the Sun and then
  migrated outwards

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TNOs and Resonance
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Scattered Disk Objects

• Highly inclined, highly elliptical, large
  distances from the Sun, etc
• Can be hard to find due to their odd orbits

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Eris

• The largest TNO currently known is called Eris
• Larger than Pluto
• Semi-major axis of 68 AU, but is currently at 97
  AU due to high eccentricity
• Part of the scattered disk
• Orbit is also highly inclined (44 deg)
• Has a small moon, Dysnomia

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Large KBO Size Comparisons
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Centaurs

• Some TNOs are inside the orbit of Neptune
• Sometimes show a coma or tail like comets
• Have a wide range of orbital parameters
• Centaurs are thought to be former Kuiper belt
  objects that have been ejected inward into the
  gas giant region

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The Oort Cloud

• He computed the orbits of long period comets and
  found
• They should spend most of their time far from the
  Sun

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Diagram of the Oort Cloud
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Population of the Oort Cloud

• They are too far away to see, so we only have
  indirect methods of studying them
• These bodies probably formed in the gas giant
  region and were ejected out to the Oort cloud by
  a close encounter with a large planet

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Tentative Origin of the TNOs

• The gas giants and TNOs gravitationally interact
  with each other
• Some TNOs are flung very far out and form the
  Oort cloud
• Some TNOs are swept up in Neptunes resonances as
  Neptune migrates out and form the Resonant TNOs
• Some TNOs form between 40-50 AU and are not much
  affected by gravitational interaction and form
  the Kuiper belt

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Next Time

• Read Chapter 14.2
• Quiz 3 next Monday
• Final exam the Monday after (November 12) at 3 pm

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Summary Pluto

• Description small, cold , distant
• Pluto resembles a large TNO more than a planet
• Has a closely orbiting large moon Charon
• Properties
• Thin atmosphere
• Very cold (50 K)
• Bright surface features possibly composed of
  fresher ice

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Summary TNOs

• Past the orbit of Neptune the solar system is
  made up of many small icy bodies
• About 1000 found in the last 15 years
• Are organized into many different classes based
  on orbits
• Theories on their origin and evolution still
  under development