Pluto
Outer World of the Solar System

Feb. 5, 2005, jeb

OUTLINE
• Discovery of Pluto
• Basic Properties of Pluto
• Hubble Space Telescope Images of Pluto
• Orbital Properties
• Pluto's moon, Charon
• Pluto's other properties
• Pluto's origin
• Future exploration of Pluto
• Golden Globe Awards for Pluto
• Other Sources on Pluto

Discovery of Pluto in 1930

• Percival Lowell estimated position of ninth planet from deviations in orbits of Uranus and Neptune
• Discovered 14 years after Lowell's death very close to Lowell's predicted location by American Clyde Tombaugh
  • Lowell's calculations now appear to have been incorrect and predicted location was accidentally accurate
  • Pluto is far too small to have detectable influence on Neptune or Uranus
  • Pluto is named for the mythical Greek god of the underworld
    • note 1st two letters of Pluto stand for Percival Lowell

Hubble Space Telescope Images of Pluto

back to outline

• Pluto is the only planet that has not been visited by spacecraft
• Most revealing images were taken with the Hubble Space Telescope in 1994,
  after the mission to repair the optics in 1993
  • Pluto surface map

Orbital Properties

back to outline

• Semi-major axis of 39.5 AU
• very faint object in Earth's sky
  • highly elongated and tilted orbit
    • eccentricity of 0.25 (largest in Solar System)
      • approaches within about 30 AU of Sun (perihelion)
      • at farthest point, about 50 AU from Sun (aphelion)
    • inclination of 17^o to ecliptic (largest in Solar System)

• Passes inside Neptune's orbit at perihelion
  • Inside Neptune from 1979 to 1999
    • making Neptune the most distant planet from the Sun during this period
• Locked in 3:2 resonance with Neptune
  • Will not ever come close enough to Neptune to collide

Pluto's moon, Charon

back to outline

• Discovered in 1978
  • named for the mythical boatman who ferried the dead across the river Styx into Hades
• HST images much more distinct than ground-based observations
• Inclined orbit (118^o)
  • orbital period of 6.4 days
    NOTE - this is identical to the rotation period of Pluto!
  • Furthermore, Charon rotates once for each revolution
    • just as the Earth's Moon does
• by chance, a series of several eclipses (1985-1991) occurred
  • provided detailed info on size and orbits of Pluto and Charon
  • about one-half the radius of Pluto
    600 km vs. 1137 km (Pluto)
    medium sized moon
    • most closely matched planet and moon in the Solar System
      • "double planet"
  • about one-sixth the mass of Pluto
    M_PLUTO = 0.0021 M_EARTH

Pluto's other properties

back to outline

• Radius = 1137 km (more like a moon than any other planet)
• Density (2060 kg/m³) midway between Jovian and Terrestrial
  • Density similar to Triton (Neptune's moon)
  • Probably mostly water-ice compostion
  • Surface temperature of about 50K (solid methane)

Pluto's origin

back to outline

• Escaped moon?
• Icy asteroid
  • and Charon?
    • more icy asteroids beyond Pluto?
• Do Triton and Pluto have a similar origin?
  • They are similar
    • density, mass, radius
    • Pluto: 2060 kg/m³, 0.002 M_E, 1137 km
    • Triton: 2100 kg/m³, 0.0035 M_E, 1350 km
  • Did both once move in independent orbits around the Sun?
  • Charon?