Pluto and the
Kuiper Belt

Pluto: Discovery and Orbit

  • Discovery: Not visible with the naked eye; Predicted in the 1800s on the basis of inaccurate measurements of the position of Uranus and Neptune, then observed (by chance) with a telescope in 1930 close to the position predicted by Percival Lowell.
  • Origin of name: The Roman god of the dead and the underworld (or Percival Lowell!)
  • Orbit: 40 AU on average, but sometimes closer than Neptune due to its eccentricity; tilted 17.2°! Period 248 yr, locked in a 3:2 resonance with Neptune (that prevents them from colliding).

What is it? Classified as a planet after its discovery, when no other Trans-Neptunian Object was known; The status grew increasingly uncertain after the 1992 discovery of the Kuiper Belt and other large objects in it; In 1999 the IAU reconfirmed it as a planet, but after an object larger than Pluto (2003 UB313, Eris) was discovered, in 2006 the IAU voted to reclassify it as a "dwarf planet", and in 2008 the term "plutoid" was introduced.

  Pluto: Appearance and Exploration

  • Appearance: Our best images are blurred ones from the HST; Covered with nitrogen frost, with markings from complex molecules.
  • Size and rotation: 1/5 Earth radii [about 2300 km across]; We know size and mass [0.0025 Earth masses and 0.06 of its gravity] because of Charon eclipses; Very tilted, retrograde rotation.
  • Physical properties: We know something about its composition from the density and from images [density 2.3 times that of water; made mostly of water ice], and about its atmosphere from eclipses; Similar to Triton.
  • Exploration: Not reached yet, but the New Horizons spacecraft, launched in 2006, has passed the orbit of Uranus and should reach Pluto by 2015 [it is the fastest spacecraft yet, at > 83,000km/h after its 2007 Jupiter encounter].

Pluto's Moons

  • Charon: By far the largest moon; Named after the boatman who ferries souls across the river Styx, discovered in 1978; It has 1/6 of Pluto's mass, about 1/2 of Pluto's radius [about 1200 km across], is only 20,000 km from Pluto, and has active geysers on its surface.
  • Orbit: The masses of Pluto and Charon are close, so they both orbit around the common center of mass [once every 6.4 days], with their rotations locked so that they always show the same side to each other!
  • Origin: Do Pluto's and Charon's strange orbital characteristics hint at an impact? Simulations indicate Charon may have formed like our Moon, but we need to understand Kuiper Belt objects better, and why relatively many of them are binaries.
  • Other moons: Nix and Hydra (45–160 km in diameter) were discovered in 2005; Two more moons have been discovered in 2011 and 2012; There may be more.

  The Kuiper Belt

  • What is it? A swarm of icy/rocky objects beyond the orbit of Neptune, between 30 and 50 AU or so from the Sun, where many comets come from (Kuiper Belt Objects or Trans-Neptunian Objects); As of 2013, thousands of individual ones are known, but the total number is much larger.
  • History: Predicted by Kenneth Edgeworth in the 1940s and Gerard Kuiper in the 1950s; Discovered in 1992 (until then, only Pluto was known); Currently studied by the Spacewatch project and others from Earth.
  • Origin and evolution: Probably formed from leftover material beyond the orbit of Neptune after planet formation; We don't know much about its evolution, but looking at orbits of different types of objects in it will give us clues on possible past events that affected the Solar System.

Kuiper Belt Objects

  • Size and shape: We can estimate the size from their brightness if we make a guess for their albedo, like for asteroids; Shape can be estimated from their brightness variations if they rotate.
  • Large objects: The largest known Kuiper Belt object is Eris (a.k.a. 2003 UB313, or "Xena" the "10th planet", roughly 2400 km across), then Pluto, 2003 VB12 "Sedna" (about 1600 km, in a very elliptic orbit that reaches 900 AU), 2002 LM60 "Quaoar" (about 1250 km, in an almost circular orbit at 43 AU), and there may be other large ones; Some are in binary pairs, probably formed by gravitational attraction.
  • Other special ones: The oddest one is Haumea (2003 EL61), the fifth dwarf planet, football-shaped and spinning once every 3.9 hr, with two known ice moons.
  • Plutinos: The ones that, like Pluto, are in a 2:3 resonance with Neptune.
  • Centaurs: There are also ambiguous cases, intermediate between comets or Kuiper Belt objects and asteroids; Some have been found inside the orbit of Neptune; Chiron, 170 km across, is between Saturn and Uranus.

"Pluto started out as the ninth planet, a supported fulfillment of Percival Lowell's prediction of Planet X. Let's simply retain Pluto as the ninth major planet. After all, there is no Planet X. For 14 years, I combed two-thirds of the entire sky down to 17th magnitude, and no more planets showed up. I did the job thoroughly and correctly... Pluto was your last chance for a major planet."
– Clyde Tombaugh, discoverer of Pluto, in a 1994 letter to Sky & Telescope

"It's pretty clear, if we discovered Pluto today, knowing what we know about other objects
in the Kuiper Belt, we wouldn't even consider it a planet"

– Michael Brown, California Institute of Technology.

page by luca bombelli <bombelli at>, modified 24 oct 2013