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Trans-Neptunian Objects and Pluto. Astronomy 311 Professor Lee Carkner Lecture 21. Gas Giant Moons. Kinetic energy of launch equals potential energy at peak PE = KE mgh = ½mv 2 h = ½mv 2 /mg = ½v 2 /g Gravity on Io g = GM/R 2 g = [(6.67X10 -11 )(8.94X10 22 )]/(1.82X10 6 ) 2

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trans neptunian objects and pluto
Trans-Neptunian Objects and Pluto

Astronomy 311

Professor Lee Carkner

Lecture 21

gas giant moons
Gas Giant Moons
  • Kinetic energy of launch equals potential energy at peak
    • PE = KE
    • mgh = ½mv2
    • h = ½mv2/mg = ½v2/g
  • Gravity on Io
    • g = GM/R2
    • g = [(6.67X10-11)(8.94X1022)]/(1.82X106)2
    • g= 1.8 m/s2
  • Final height
    • h = [(½)(6002)]/1.8 = 100000 m = 100 km
pluto god of the underworld
Pluto -- God of the Underworld
  • Pluto is the God of the Dead in Roman mythology
  • Pluto was discovered at Lowell Observatory and its first 2 letters commemorate Percival Lowell
the discovery of pluto
The Discovery of Pluto
  • In the late 1800’s it was believed that Neptune’s 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 Neptune’s orbit

No spacecraft has ever visited it

      • But will not get to Pluto until 2015
  • The best information comes from HST
pluto facts
Pluto Facts
  • Size: 2300 km
    • Smaller than the 7 largest moons
  • Orbit: 39.5 AU
  • Description: Very small, very cold, very distant
composition of pluto
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 can’t escape
features of pluto
Features of Pluto
  • HST can see regions of different brightness on Pluto’s surface
  • The other bright regions may be areas where impacts have gouged out fresh ice
pluto s moons
Pluto’s Moons
  • Pluto’s largest moon Charon was discovered as a small bulge in a high resolution image (1978)
    • Their sizes are closer than any planet and moon
  • They have very similar densities, masses and sizes
  • Two other smaller moons Nix and Hydra were discovered by HST in 2005
    • Each is about 50 km in diameter
is pluto a planet




Eccentric orbit

Not largest TNO

Is Pluto a Planet?
pluto s orbit
Pluto’s Orbit
  • Pluto’s orbit is much more eccentric and much more inclined than any planet
    • Eccentricity =
      • Most other planets e<0.1
    • Inclination =
  • Pluto’s orbit carries it inside the orbit of Neptune
  • Pluto is tipped on its side like Uranus
small icy bodies
Small, Icy Bodies
  • Small icy bodies in the outer solar system (beyond Jupiter) have no good name
  • Lets call all of them “Trans-Neptunian Objects” or “TNOs”
  • They are all similar to Pluto (but usually much smaller)
  • Most are only recently discovered and not well characterized or organized
discovering tnos
Discovering TNOs
  • Around 1950 Kuiper and Edgeworth proposed a belt of comets out beyond Neptune
  • In 1992 the first (besides Pluto) TNO was discovered (QB1)
  • Discovered via long exposures with large telescopes (including HST)
  • Total population of large TNOs may be 70000 (larger than 100 km)
  • Resonant: in an orbital resonance with Neptune
  • Classical Kuiper Belt:
  • Scattered Disk: large distances and eccentricities
classical kuiper belt
Classical Kuiper Belt
  • Most of the objects have nearly circular orbits, low inclinations and are not effected by Neptune’s gravity
  • Probably formed in place from the leftover material at the edge of the solar nebula
resonant objects
Resonant Objects
  • TNOs tend to collect on these orbits
  • Examples:
      • Pluto is in this group so they are called Plutinos
      • Marks the edge of the classical Kuiper Belt, few TNOs beyond this point
  • Theory: Neptune formed closer to the Sun and then migrated outwards
    • Swept up TNOs into resonances as it moved out
scattered disk objects
Scattered Disk Objects
  • Some TNOs have very irregular orbits
  • These objects are thought to have been scattered by gravitational interaction with a gas giant (mostly Neptune)
  • Can be hard to find due to their odd orbits
  • 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
  • Has a small moon, Dysnomia
    • formerly called “Gabrielle”
  • Some TNOs are inside the orbit of Neptune
    • Called Centaurs
  • 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
    • Will eventually collide with something or be ejected from the solar system altogether
the oort cloud
The Oort Cloud
  • In 1950 Dutch astronomer Jan Oort postulated a spherical shell of comets surrounding the solar system at about 50,000 AU
  • He computed the orbits of long period comets and found:
    • They should spend most of their time far from the Sun
population of the oort cloud
Population of the Oort Cloud
  • There may be as many as 1 trillion comets in the Oort cloud
  • 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
tentative origin of the tnos
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 Neptune’s 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
next time
Next Time
  • Read Chapter 14.2
  • Quiz 3 next Monday
  • Final exam the Monday after (November 3) at 3 pm
summary pluto
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
summary tnos
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