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Diameter: 1,700 km
Distance from Sun: 7,375,927,931 km (farthest) 4,436,824,613 km (closest)
Surface area: 16,647,940 km2
Length of day: 153.29 hours
Gravity: 0.66 m/s2
Circumference: 7,323 km
Mass: 1.3090 x 1022 kg
Length of Year: 247.7 years
Surface temp: -229 C


Pluto orbits beyond the orbit of Neptune (usually).  It is much smaller than any  of the official planets and now classified as a “dwarf planet“. Pluto is  smaller than seven of the solar  system’s moons (the Moon,  Io,  Europa,  Ganymede, Callisto,  Titan and  Triton).

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after  many  other suggestions) perhaps  because it’s so far from the Sun that it is in perpetual darkness and  perhaps because “PL” are the initials of Percival Lowell.

Pluto was discovered in 1930 by a fortunate accident.  Calculations which later turned out to be in error had predicted a planet beyond  Neptune, based on the motions of Uranus and Neptune.  Not knowing of the error,  Clyde W. Tombaugh      at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto  anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too  small  to account for the discrepancies in the orbits of the other planets.  The search for Planet X continued but  nothing was found.  Nor is it likely that it ever will be: the discrepancies  vanish if the mass of Neptune determined from the  Voyager  2 encounter with Neptune  is used.  There is no Planet X. But that  doesn’t mean there aren’t other objects out  there, only that there isn’t a  relatively large and close one like Planet X was assumed to be. In fact, we now  know that there are a very large number of small objects in the Kuiper  Belt beyond the orbit of Neptune, some roughly the same size as Pluto.

Pluto HST image

Pluto has not yet been visited by a spacecraft. Even the  Hubble Space Telescope can  resolve only  the largest  features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006.  If all goes well it should reach Pluto in 2015.

Fortunately, Pluto has a satellite, Charon. By  good fortune, Charon was discovered (in 1978) just before its orbital  plane moved edge-on toward the inner solar system. It was therefore  possible to observe many transits of Pluto over Charon and vice versa.  By  carefully calculating  which portions of which body would be covered at  what times, and watching  brightness curves, astronomers were able to  construct a rough map of  light and dark areas on both bodies.

In late 2005,  a team using the Hubble Space Telescope  discovered two  additiona tiny moons orbiting Pluto.  Provisionally designated    S/2005  P1 and S/2005 P2, they are now known as Nix and Hydra.  They are estimated to be  between 50 and 60 kilometers in diameter.

Pluto with moons

Pluto’s radius is not well known. JPL’s value of 1137 is given with an error  of +/-8, almost one percent.

Though the sum of the masses of Pluto and Charon is known pretty well (it can  be  determined from careful measurements of the period and radius of  Charon’s orbit and  basic physics)       the individual masses of Pluto and  Charon are difficult to determine because that requires determining their  mutual motions around the center of mass of the  system which requires much  finer measurements — they’re so small and far away  that even HST has difficulty.  The ratio of their masses is probably somewhere  between 0.084 and 0.157; more observations are underway but we won’t get really  accurate data until a  spacecraft  is sent.

Pluto is the second most contrasty body in the Solar System (after Iapetus).

There has recently been considerable controversy about the classification of  Pluto.  It was classified as the ninth planet shortly after its discovery and  remained so for 75 years.  But on 2006 Aug 24 the IAU decided on a new definition of “planet” which does not include Pluto.  Pluto is now classified as a “dwarf planet”, a class distinct from “planet”.  While this may  be controversial at first (and certainly causes confusion for the name of this  website) it is my hope that this ends the essentially empty debate about Pluto’s  status so that we can get on with the real science of figuring out its physical  nature and history.

Pluto has been assigned number 134340 in the minor planet catalog.

Pluto’s orbit is highly eccentric.  At times it is closer to the Sun than  Neptune (as it was from January 1979 thru February 11 1999).  Pluto  rotates in the opposite direction from most of the other planets.

Pluto is locked in a 3:2 resonance  with Neptune; i.e. Pluto’s  orbital period is exactly 1.5 times longer than  Neptune’s.      Its orbital inclination is also much higher than the other  planets’. Thus though it appears that Pluto’s orbit crosses Neptune’s,  it really doesn’t and they will never collide.

Like Uranus, the plane of Pluto’s  equator is at almost right angles to the plane of its orbit.

The surface temperature on Pluto varies between about -235 and -210 C (38 to  63 K).  The “warmer” regions roughly correspond to the regions that appear  darker in optical wavelengths.

Pluto’s composition is unknown, but its density (about 2 gm/cm3) indicates  that it  is probably a mixture of 70% rock and 30% water ice much like  Triton.  The bright areas of the surface seem to be covered with ices  of nitrogen with  smaller amounts of (solid) methane, ethane and carbon  monoxide. The composition of the darker areas of Pluto’s surface is unknown  but may be  due to primordial organic material or photochemical  reactions driven by cosmic rays.

Little is known about Pluto’s atmosphere, but it probably consists primarily  of  nitrogen with some carbon monoxide and methane.  It is extremely  tenuous, the surface pressure being only a few microbars. Pluto’s  atmosphere may exist as a gas only  when Pluto is near its perihelion;  for the majority of Pluto’s long year, the atmospheric gases are frozen  into ice.       Near perihelion, it is likely that some of the atmosphere  escapes to space perhaps even interacting with Charon. NASA mission  planners  want to arrive at Pluto while the atmosphere is still unfrozen.

The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties  between Pluto and Triton suggest some  historical connection between them.  It was once thought that Pluto may have once been a satellite of  Neptune’s, but this now seems unlikely. A more popular idea is that  Triton, like Pluto, once moved in an  independent orbit around the Sun and  was later captured by Neptune.  Perhaps Triton, Pluto and Charon are the  only remaining members of a large class of similar objects the rest of  which were ejected into the Oort cloud.  Like  the Earth’s Moon, Charon  may be the result of a collision between Pluto and another body.

Pluto can be seen with an amateur telescope but it is  not easy.  There are several Web  sites that show the current position of Pluto (and the other planets) in the  sky, but much more detailed charts and careful observations over several days  will be required to reliably find it. Suitable charts can be created with many  planetarium  programs.




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