Pluto and the Kuiper Belt External Links page.

Pluto: Discovery and Orbit Discovery: Not visible with the naked eye; Predicted in the 1800's 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!) What is it? A planet? A comet-like "Trans-Neptunian Object"? In 1999 it was reconfirmed as a planet, but the number of known objects similar to Pluto in the Kuiper Belt keeps growing... 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).

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 [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; Why? Planet 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; the New Horizons mission is scheduled for launch in 2006, so it can reach Pluto by 2015.

Pluto's "Moon" Charon: The boatman who ferries souls across the river Styx, discovered in 1978. Properties: 1/6 of Pluto's mass, 1/2 of Pluto's radius! Orbits: Since the masses are so close, Pluto and Charon, only 20,000 km apart, orbit around the common center of mass [once every 6.4 days], locked in rotations that 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; We might need to understand Kuiper Belt objects better, and why relatively many of them are binaries.

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 2004, about 1000 are known (the total number is much larger, but unknown). History: Predicted by Kenneth Edgeworth in the 1940's and Gerard Kuiper in the 1950's; 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 Special Objects: The largest known Kuiper Belt objects are 2002 LM60 "Quaoar" (about 1300 km in diameter, larger than Charon, in an almost circular orbit at 43 AU) and 2003 VB12 "Sedna" (possibly 1500 km in diameter, in a very elliptic orbit that reaches 900 AU), but there may be other large ones; Some are in binary pairs, probably formed by gravitational attraction. 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. 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.